AVO MK III, VALVE CHARACTERISTIC METER Service manual

THE “AVO ”

VALVE CHARACTERISTIC METER

Mk. III.

PUB LIS HE D B Y

AVO LIMITED,

AVOCET HOUSE, 91-05 VAUXHALL etUDGE ROAD, LONDON, S.W.l

Telephone: Victoria 3404 (12 lines)

FHE AVq

yn LVECHAKACTe^ c ^

2

Mk [n

FORE WORD

OR more than a quarter of a century we have been engaged in the design and
manufacture of “ AVO ” Electrical Measuring Instruments. Throughout that time we

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have consistently pioneered the design of modern multi-range instruments and have
kept abreast o f and catered for the requirements of the epoch-making developments in
the fields o f radio and electronics.

The success of our steadfast policy of maintaining high standards of performance in
instruments of unexcelled accuracy, and making such instruments available at reasonable
cost, is reflected in the great respect and genuine goodwill which “ AVO ” products enjoy
in every part of the World.

It has been gratifying to note the very large number of instances where the satisfaction
obtained from the performance of one of our instruments has led t o the automatic choice

of other instruments from the “ AVO ” range. This process, having continued over a long
period o f years, has resulted in virtual standardisation on our products by numerous
Public Bodies, The Services, Railway Systems, and Post Office and Telegraph Undertakings
throughout the world.

Our designers have thereby been encouraged to ensure that new instruments or

accessories for inclusion in the “ AVO ” range fit in with existing “ AVO ” apparatus and
serve to extend the usefulness of instruments already in use. Thus, the user who standardises
on “ AVO ” products will seldom find himself short of essential measuring equipment, for,

by means of suitable accessories, his existing equipment can often be adapted to meet
unusual demands.

It is with pleasure that we acknowledge that the unique positi on attained by “ AVO ”

is due in no small measure to the co-operation of so many users who stimulate our Research
and Develo pm ent staffs from time to time with suggestions, criticisms, and even requests
for the produ ction of entirely new instruments or accessories. It is our desire to encourage
and preserve this relationship between thos e who use “ AVO ” Instruments and those who
are responsible for their design and manufacture, and correspondence is therefore
welcomed, whilst suggestions will receive prompt and sympathetic consideration.

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IN DE X

Foreword......................................................................................................................... 3

Introduction ..............................................................

The Basic Method of characteristic checking ... ... ... ... ... 7

The Basic Method of checking diodes and recti fiers

Insulation Testing ... ... ... ... ... ... ... ... ... 8

The Protective Relay

The Valve Panel and Selector Switch .......................................................................... 9

Procedure for setting up valve base connections

Provision for new valve bases .......................................................................... 12

The prevention of Self oscillation o f valves under test

Diagram of Standard base pin connections .................................................. 13

Procedure for valves having internally connected pins

The controls on the front panel, their functions and operations

The Set ~ Control ..................................................................................... 15

The Leakage Switch
The Circuit Selector Switch
The Anode and Screen Voltage Switches
The Heater Voltage Switches

The Negative Grid Voltage Controls .............................................................. 16

The Backing Off Controls

The Meter Switch .................................................................................................. 16

The Set mA/V Control
The Electrode Selector Switch

The Mains Adjustment Panel at the rear of the In stru ment
General Procedure for testing a v a lv e

Mains voltage adjustment and panel set up—cold and hot leakage tests—mutual
characteristic checks and gas tests—diode and rectifier tests made under load.

Instructions for testing specific valve types ... ... ... ... ... ... 21

Multiple diodes and rectifiers—double triodes, double pentodes and double tetrodes—
combined diode and amplifying valves—frequency changers of heptode and hexode types
—frequency changers employing separate electrode assemblies.

The Use of the Links on the Valve Panel of the Instrument

Tuning Indicators (Magic Eyes)

Gaseous Rectifiers................................................................................................. 24

Cold Cathode Rectif iers..................................................................................... 24

Thyratron s............................................................................................................. 24

Neon Indicators .................................................. ... ... ... 24

General Precautions to be observed when using the Valve Characteristic Meter ... 25
Abbreviated Working Instructions for the “ AVO ” Valve Characteristic Meter

Mk. I l l ......................................................................................................................... 26

Circuit diagram of Valve Characteristic Meter

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The “ AVO ” Valve Data Manual and Handbook

This instrument will produce maximum information when used in conjunction with the Valve

Manufacturer’s Graphs and Technical Data, but to enable rapid checks to be made relative to a valve’s

general efficiency, the “ AVO ” Valve Data Manual (civilian valve types) and the “ AVO ” Valve Data
Handbook (service valve types) have been produced.

This instruction book refers throughout to the “ AVO ” Valve Data Manual, a copy of which should
always be kept with the instrument. New editions of this data manual will be published from time to time.
Watch our advertisements in the technical press for farther announcements.

4

Introduction

to

THE “ AVO ” VALVE CHARACTERISTIC METER Mk. Ill

'The problem of designing a Valve Testing Instrument capable o f giving a true and

comprehensive picture of the state of any valve, has always been one of considerable

magnitude, increasing in complexity as new valve types are brought into general use.

For a quick general purpose test necessitating a minimum o f time and technical effort
a mutual conductance figure will give an adequate idea of a valve’s usefulness, and the
original “ AVO ” Valve Tester was designed to test the efficiency o f valves on this basis.

Whilst a Valve Tester must, of necessity, be accompanied by a data book correlating
the results of the Tester with the condition o f the valve in question, a purely empirical
figure, if used as a standard, will always give rise to doubts in the mind of the operator.
The instrument should therefore, produce a figure which can be compared with some
standard quoted by the valve manufacturer, if the operator is to use his instrument with
confidence. For this reason the “ AVO ” Valve Tester used the static zero bias mutual
conductance figure as a basis of comparison, this figure being at that time almost universally
quoted by the valve manufacturer.

In order to reproduce this standard correctly, it was also necessary to reproduce the
stated values of DC anode and screen voltage, a matter of some considerable difficulty
when it is realised that for any stated condition of anode and/or screen volts the correspond
ing electrode currents can vary over very wide limits, and in the case o f valves of low
initial anode current and high slope, the actuation of the control which produces the
milliamp-per-volt reading might easily double the anode current flowing. With DC
methods of testing the inherent internal resistance of the rectifying circuits used could be
such as to give regulation errors which could cause results to be meaningless unless com
plicated thermionic stabilising circuits and a vast array of monitoring meters were used
in all voltage supply circuits. Such complications would n o t only render the Tester of
prohibitive price and size, but would considerably increase the complication o f operation

for the non-technical user.

The problem was overcome by the introduction of the AC method of operation (Patent
No. 480752) by which means the necessary DC test conditions were correctly simulated
and a true mutual conductance figure produced by the application o f AC voltages of
suitable amplitude to all electrodes. This enormously simplified the power supply problem,
rendered regulation errors negligible, and obviated the necessity for voltage circuit
monitoring. The “ AVO” Valve Tester thus fulfilled normal testing needs for a longperiod.

During recent years, however, electronic techniques have become much more precise
and the nature and multiplicity o f valve types have continuously increased. The zero bias
mutual conductance figure is seldom quoted by the valve manufacturers, who, usually now
publish the optimum working point mutual conductance and voltage figures, and in a large
number o f cases give full families of curves, from which, precise operation, under a variety o f
working conditions, can be judged. To cater for present day requirements therefore, a
valve testing device should not only be capable o f producing a working point mutual
conductance figure at any reasonable value of anode, screen or grid voltage recommended
by the manufacturers, but should also be capable, if necessary, or reproducing any one of
the mutual characteristics associated with the valve in question. The instrument thus has

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to simulate the performance of a comprehensive valve measuring set-up o f laboratory

type and yet, at the same time, be sufficiently cheap and simple t o cater for the needs of
the comparatively inexperienced radio test assistant. It is obvious that the very much

wider application of an instrument of this class, would render the regulation difficulties,
already referred to, much more critical.

Investigations were, therefore, put in hand to see whether the AC test method would

reproduce DC conditions not only in respect of the mutual conductance figure taken at
a single discrete point, but at all points on all characteristics from zero bias to cut off.
In other words, it was necessary to determine whether the general function for a DC static
valve characteristic ( λ^ + μιλ^ - Ι^ Vg2 )

la

-------

-

Ra.

would hold when la was measured in terms of DC current, but when Va, Vg2 and, if
necessary, Vg i , were replaced by 50 cycle AC voltages of suitable magnitude and phase.
It was eventually found that a complete co-relation between these two sets of conditions
was held when the grid voltage took the form of a sinusoidal wave form with the positive
half cycle suppressed (in other words, rectified but completely unsmoothed AC), and the

followin g relationships were maintained:—

Va RMS = 1.1 Va indicated D C

Vg2 R M S = 1.1 Vg2 indicated DC

Vgj (mean unsmoothed) = 0.52 Vg^ indicated DC

la (mean DC) = 0.5 indicated la

From the above conditions, therefore, the required relationships were obtained which

formed the basis o f operation of the Valve Characteristic Meter (Patent No. 606707).

Such an instrument, whilst retaining the advantages o f simplicity, size and reasonable
price, resultant upon the elimination of complicated regulated DC supply systems and
universal monitoring, would have the inherent regulation easily obtained from a well-
designed AC transformer. It would enable a valve to be checked at any point on any one
of its many mutual characteristics and if necessary would allow a full family of character

istics to be drawn.

The basic method of characteri stic checking

The fundamental circuit of operation of the instrument is shown in Figure 1. A s in the

original Valve Tester, the process o f obtaining a direct reading mutual conductance figure

is simplified by the introduction of a backing o f f circuit, which balances out the deflection

due to the standing anode current at the desired test conditions prior to the measurement

of mutual conductance. It will be noticed that the current flowing in this backing off circuit
is similar in wave form, but precisely opposite in phase to the anode current, this elim
inating any indesirable ripple that could otherwise become apparent when the meter, after
backing off, was set to a sensitive range. To facilitate the measurement of mutual
conductance of high slope/short grid base valves and valves requiring a long heater
stabilising period, two distinct methods o f measurement have been incorporated.

The basic method of checking diodes and rectifiers

Any simple emission test at low applied voltage must necessarily give rise to a purely
empirical figure for the valve in question, which cannot necessarily be correlated with any
one o f the maker’s characteristics and which, owing to the fact that it relates to the lower
bend portion of the rectifier characteristic may vary very widely for any given type of valve.

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The important function o f a rectifying valve is that it will, under suitable reservoir load
conditions, produce sufficient current to operate the apparatus which it is intended to supply.
This fundamental requirement, therefore, is the basis o f rectifier testing in the Valve
Characteristic Meter. A sufficiently high AC voltage is applied to operate the valve above

the bend in its characteristic, and to ensure that its internal voltage drop is negligible.
With a suitable reservoir condenser in circuit, the DC load is adjusted to correspond to

a number of D C current conditions, i.e. 1mA, 5mA, 15mA, 30mA, 60mA, 120mA and

180mA. The actual current flowing in the load circuit is then indicated on a meter shunted
to correspond with the DC load required. The meter reading will then indicate the com 
parative efficiency of the valve on the basis o f the required DC load. Each ha lf of a full
wave rectifying valve is tested separately thus enabling the two halves to be checked for
matching and any tendency to produce hum by partial half waving to be indicated.

The pre-determined load figures are chosen so that they not only give a sufficiently
wide range of currents to cater for the normal requirements of electronic apparatus, but
also correspond to the DC maximum emission figures usually quoted by manufacturers
in their rectifying valve data.

Signal diode valves are similarly tested, but usually these loads at the 1mA or 5mA
load positions, being normally more than sufficient to cover the rectified signal current
that would be obtained. The basic operating circuit of the diode and rectifier system is

show n in Figure 2.

Insulation Testing

To cover all eventualities, three distinct forms of insulation measurement are catered
for in the Valve Characteristic Meter. Measurements are taken with DC applied voltages,
and direct indication o f the insulation value in megohms is shown on the meter scale.
As an initial test, prior to the application of operating voltages to the valve, the rotation of
a switch enables the insulation figure to be shown, which occurs between each of the valve

o

o
o

o
o

o

__

fe

o
o
o

electrodes taken in order and all the others strapped together. The denomination of the

electrodes between which any breakdown exists will thus be automatically indicated and
further, the continuity of the heater circuit is shown as a zero resistance at the heater (H)

position of the switch.

With the application of heater voltage to directly heated valves, electrode expansion

may be sufficient to cause a breakdown between the heater and an adjacent electrode.
In the same manner cathode distortion may occur in indirectly heated valves causing
similar breakdowns. To show up this condition a test circuit is provided indicating the
insulation resistance between the heater and cathode of a valve and all other electrodes
strapped when heater voltage has been applied.

Finally the very important factor of heater to cathode insulation when the heater is

hot can be tested, the insulation again being shown directly in megohms, the usual cathode
to heater connection being opened for this purpose and the applied voltage being in such
a direction as to make the cathode negative with respect to the heater, thus avoiding false
indications of insulation resistance due to electrode emission.

Protective relay

To prevent damage to the internal components of the valve characteristic meter due
to inadvertent or deliberate shorting of the electrode voltages, a protective relay is incor
porated which operates when damaging overloads of alternating current are taken from

either the anode or screen voltage sources. The relay carries three windings, one in each
of the two high tension supplies, the remaining winding being a “ hol d-o ff ” coil. Operation
of the relay connects a high resistance lamp in series with the transformer primary winding
whilst simultaneously a red warning indicator is illuminated behind the transparent meter
scale and aural warning given. This operation places the instrument in a safety condition
and normal working cannot be restored until the instrument has been switched off, the
fault removed and the instrument switched on again. The relay is entirely self-setting and in

consequence no reset mechanism has been incorporated.

NOTE: The relay does not p r ot e ct the valve when in co rr ect heater voltages are a p p li e d .

It m u st also be str e sse d tha t the relay will no t o p er ate on the p a s s a g e of nor ma l
hea v y current of a DC natu re occurring in a valve anode circuit, an d it wil l n ot

protec t the m ov e m en t i f the l a t te r is w ron gly s e t on a range too low to a c c ommo d a te

the current passin g . This prob l e m can only be d e a lt with b y ensuring th at the m ov e
men t is al w a y s se t to its m a x imum current ran ge when the mag nit ude of the e xpect e d
current is unknown,

THE VALVE PANEL AND SELECTOR SWITCH

The Valve Panel comprises 19 valve holders of the following types :— English 4/5 pin,
7 and 9 pin, 8 pin side contact, B7G, B8A, B8B, (or B8G) (American Loctal), B9A, B9G,
Mazda Octal, B3G, 4 and 5 pin Hivac: American-4, 5, 6 and small 7 pin UX, medium
7 pin UX, and Octal. Provision is made by means o f plug-in adaptors to cater for newly
introduced valve bases. The valve holders are all wired with their corresponding pins,

according to the standard pin numbering, in parallel, i.e., all pins number one are wired

together, all pins number, two, and so on. This wiring combination is associated with the

well-known “ AVO ” Multi-Way Selector Switch which enables any one o f the nine
standard pin numbers to be connected to any one of the electrode test circuits in the Valve
Characteristic Meter, thus enabling any electrode combination to be set up for any normal
valve holder.

It will be seen that the Selector Switch comprises nine thumb control rollers, numbered

from left to right 1—9. This numbering appears on the moulded escutcheon immediately

9

behind the rollers and corresponds to the valve pins in the order of their standard pin
numbering. Thus valves with any number of base connections up to nine can be accom
modated. Further, to accommodate top cap and other external valve connections a socket
panel is provided with nine sockets marked Gl, S, Al, A2, Dl, D2, C, H—, H-f, the
markings corresponding to the valve electrode connections which are made externally
to the valve.

Rotation of the rollers by the finger rim provided will reveal that each roller can be
set in any one of ten positions, the setting in question being indicated in the window opening
at the front o f the escutcheon. The ten positions on the roller are marked as under:—

1234567890

C Η — H + G S A A2 Dl D2 —

The numbers are provided for ease of memorising and no ting base combinations,
but the corresponding electrode denominations are shown by the letter appearing in the

escutcheon window immediately underneath the number, thus:—

(1) C corresponds to Cathode, or to an electrode normally connected to cathode

e.g., G3.

(2 ) H -
(3) H+

(4) G
(5 ) S

(6) A

(7) A2
(8 ) Dl

(9) D2

(0) -

„ Heater normally Earthy or connected to negative L.T.

in the case of a battery valve.
„ the other Heater connection or centre tap.
„ Control Grid
„ Screen Grid or g2 -
„ normal anode of single or multiple valve. In the case of

an Oscillator mixer valve, A represents the Oscillator

anode.

„ second anode of double valves, and in the case of Oscillator

mixer valves, the mixer anode.

„ the first diode anode of half and full wave signal diode

and rectifier valves, diode and rectifier/amplifier

combinations.

„ the second diode anode of signal diode and rectifier valves,

diode and rectifier/amplifier combinations.

„ a disconnected valve pin or to a pin upon which an internal

electrode is anchored. Such pins are marked “ I.C.”

in manufacturers’ literature, or by an asterisk (*) in the

“ AVO ” valve data manual. This switch posi ti on leaves

the particular valve pin completely disconnected.

NOTE: Some in s tru men ts are fit ted with this ro ller pos i t ion marked^

This m a r k i n g is syn o n ym o u s with ^

Procedure for set ting up valve base connections

The standard procedure for setting up a valve ready for test is as follows. From some
suitable source i.e. “ AVO” Valve Data Manual or Handbook, Valve Manufacturer’s
Data Leaflet or published manual o f Valve Data, determine, the pin basing connections

for the valve, in order of their standard pin numbering. Rotate the rollers of the Selector
Switch until the set up number or electrode letter combination appears in the window
reading from left to right in order of the standard pin numbering. In the case of valves
having less than nine pins, the free rollers on the right o f the set up combinations corres
ponding to non-existent valve electrodes should be set at 0. When the valve is inserted in the

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appropriate valve holder, use the universal top cap lead to connect any top cap or side

connection on the valve to its appropriately marked socket, on the Socket Panel immediately

behind the Selector Switch. N ote that the loctal valve holder having only eight normal pins
has its centre lug connected to the ninth roller (corresponding to pin No. 9) to accommodate

valves which have a cathode connection made to this lug.

The accompanying examples show how to correlate the pin basing data and the

equivalent set-up combination for a number of valves in common use.

Valve Type

1. Osram MH4 indirectly
heated triode.

642310000

A G Η—H + C — — — —

Set up Number

British 5-pin base.

2. Osram U50 full wave
rectifier directly heated.

0 2 0 8 0 9 0 3 0

— H--------Dl — D2 — HH

Octal base.

3. Mullard FenA4 indirect
ly heated output pentode.

0 4 5 2 3 1

— G S H — H-f C

British 7 pin base.

4. American 6K8 indirectly 0 2 7 5 4
heated frequency changer. — Η — A2 S G

Octal base. Top Cap Gl.

Base Diag ram

---------

6 0 0

A — —

5. Mullard T DD2A battery
double diode triode.
British 5-pin base.

6 8 2 3 9

A Dl Η —H + D2

Top Cap Gl.

6. Mullard EF50 indirectly 2
heated HF pentode. H—

B9G base.

5 6 1 0

S A G 3 —

11

0 0 0 0

1 4 0 3

C G — H-f-

Provision for New Valv e Bases

To cover the possibility of the introduction of new valve bases not provided for on the

standard panel and also the introduction of valves which may necessitate special conditions
associated with standard valve holders, a plug-in adaptor is available which enables many
non-standard valve holders to be combined in this adaptor and plugged into the octal or

other suitable base on the Valve Characteristic Panel. These adaptors are available for

bases not included on the Valve Panel, and also with a blank valve holder mounti ng panel
in which can be mounted the user’s own valve holder i f he requires any special arrangement
for which we have not catered.

The Prevention of Self Oscillation o f valves under test

It will be realised that the length of wiring and its associated capacity, connected
to the grid and anode pins of any one of the valve holders, can constitute a tuned line
corresponding t o a high resonant frequency often of the order of 100 megacycles per
second or higher. A number of modern valves have sufficiently high slope t o overcome
the inherent losses associated with such a tuned line, and are, therefore, capable of bursting
into oscillation at a frequency determined by the constants o f their associated valve holder
wiring when being tested at or near their maximum working slope. It is quite obvious
that in order to test a valve some wiring must exist between the valve holder and test

circuit. Further, since a multiple test panel is desirable to obviate the necessity o f a vast
number of separate plug-in units, the total amount of wiring associated with any one valve
holder must be a considerable number of inches in length. It is almost impossible to
increase the effective resonant frequency of the lines thus produced to such a high value
that no normal valve will oscillate therewith. The only alternative is to render the line of
comparatively high loss and in extreme cases to stopper the valve in question right on top
of its anode and/or grid connection. Unfortunately, however, since a very large number
of pin combinations have to be accommodated in any one valve holder the presence of such
a resistance in say a heater or cathode circuit could give completely erroneous results, and
this stoppering system could therefore only be very sparsely used.

The problem of se lf oscillation has been almost completely eliminated in the “ AVO ”
Valve Characteristic Meter Mark III, by wiring the Valve Holder Panel in connection
loops of predetermined lengths, so that any valve inserted would tend to oscillate at a
definite frequency dependent on the lo op lengths. These separate inter-connection loops
are then loaded with ferrox cube beads so that oscillation cannot occur when testing valves

with conventional characteristics, irrespective of the Valve Holder and pin combination
used.

In certain circumstances where a newly introduced valve of high efficiency is likely

to be tested in any quantity and shows signs of oscillation, the separate valve holder adaptor

can be employed with considerable advantage. By this means a valve holder can be stoppered
to the maximum extent necessary for the valve in question without references to any other

valves that may be incorporated therein, for when other types of valve are likely to be
used, the adaptor can be set aside and the valve panel used normally. It must be stressed
that this oscillation is unlikely to occur where the valve is tested at anode currents lower
than normal, or at a point on its curve which renders its mutual conductance low. Were
a purely empirical method of testing employed in the Valve Characteristic Meter, therefore,
the problem would in all probability not arise, but since every effort has been made to
actually test the valve under its correct operating conditions o f current and voltage, then
it is on this account working at its normal efficiency and can, unless special precautions are
taken, give rise to the oscillation troubles to which we have referred.

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DI A GR A M O F ST AN D AR D PI N CONNEC TI ONS

(viewed from u ni id e o f base)

BRI T I S H N INE P IN ( B9> BRI T IS H SEVEN PIN ( B 7)

IN T ERN A TI O N A L O C T AL (A 0 8 )

B7A

SUB MIN IATURE8 PI N ( 8 8 D )

HIVAC FOUR PIN (SM4) HIVAC FIVE PIN (SM S )

AMERICAN SM ALL SEVEN PI N (S M 7 )

BRI TI SH 4 / 5 PIN (B 5 & B 4 )

AM E RIC A N FOU R P IN (U X 4 )

AME RI C AN SE V EN P I N ( U X 7 )

B 9C

AME RIC A N FIVE PIN (UX 5)

AM E R ICAN LOCTAL (B B B O R B 8G )

« Λ

•

4 *

3

* 5

• 2 6·

7.

B7G

B9 A

B 3 Q

SAA &7AA

(λ coin WiWfJ

B 8 A

COLOURED

SPOT '

0 0 0 0 0)

<

I 2345^

BS A

B SB

Whilst discussing the problem of oscillation, mention should be made of the rectifier
(which will be seen in the circuit diagram) included in the screen circuit o f pentode and
tetrode valves. This rectifier has been incorporated to obviate a difficulty which can arise
in certain circumstances when testing valves o f the beam tetrode type with alternating
current applied to their electrodes. As the applied electrode voltages approach zero during
a portion o f their operative cycle, the focusing of the beam of such valves is to some extent
upset and the result can be that the screen circuit begins to show an emission in a reverse
direction to normal screen current, with the result that the anode current rises and the

current taken by the screen decreases rapidly and becomes negative. This can cause screen

overheating and besides giving an unstable and erroneous impression of the condition
of the valve, can, if allowed to continue, damage the valve. To obviate this condition,

therefore, the rectifier is included in such a manner that only its low forward resistance is
presented to the screen passing current in the normal direction, thus causing a negligible
variation to standard conditions, but the reverse resistance of the rectifier is operative to

limit screen current of the opposite direction to negligible proportions and thus prevent

the conditions stated above, from coming into effect.

Procedure for Valves having Internally Connected Pins

On certain valves of recent manufacture, particularly the miniature glass type employing

B7G, B8A, B9A, etc. bases, it has become the practice of manufacturers to connect internally,
certain of the valve electrodes to pins which would otherwise be blank and free from
any connection. Although the manufacturers specify the pins on which this is likely to
occur they reserve the right to vary the nature of the internal connections from time to
time as prevailing conditions might demand. This in itself prevents the inclusion o f the
electrode thus internally connected, in the normal selector switch set-up of the vaive.

Valves with internally connected pins present no difficulty when tested on the valve
characteristic meter Mk. I [I but because the valve data manual is used with earlier instru
ments, internally connected valve pins are marked (*) in the Roller Selector Switch number
column. When using the valve characteristic meter Mk. Ill, where the asterisk appears in
the Roller Selector Switch number denoting an internal connection, the appropriate roller
should be set 0, e.g. U81, where the roller selector switch number reads **9 **8 230, set

roller selector switch to read 009 008 230 and follow the normal procedure.

THE CONT R OL S ON THE FRO N T PANE L

THEIR FUN CTIO N S AND OP E RATIONS

All the controls necessary for carrying out the essential valve testing functions are
situated on the front panel of the instrument, and by the manipulation of these controls
and the use of the valve panel already described, the following tests can be undertaken,

1. The direct indication o f insulation resistance between specific electrodes with the
vaive cold. This test will also indicate heater continuity.

2. The direct indication o f insulation resistance between electrodes with the valve
filament hot, including a separate test for the important function of cathode to
heater insulation.

3. The measurement of mutual conductance directly in milliamps/volt over a full

range of applied high tension and bias voltages.

4. The comparative indication o f valve goodness on a coloured scale on the basis
o f mutual conductance reading.

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5. The ability to plot complete sets of mutual characteristics Ia/Vgj, la/Va, Is/Vgi,

Is/ Vs, etc., with a complete range of applied electrode voltages corresponding to

D.C. operating conditions.

6. The testing o f rectifiers under reservoir condenser conditions with a full range of
D.C. loading.

7. The testing o f signal diodes under suitable D.C. load.

8. The testing of the separate sections of multiple valves, the non-operative section
of the valve being maintained at reasonable working electrode voltages.

9. The indication of grid current and valve softness, directly on meter scale.

10. The anode current can, if desired, be read on an external meter of greater sensitivity
and tests carried out on valves which require an anode load.

The separate functions of the controls available are as foll ows:—

The Set ~ Control

This control enables minor adjustments to be made to the input tappings on the mains

transformer after the coarse mains tapping has been set.

The Leakage Switch

This switch serves the dual purpose of putting the instrument in a condition for the
initial setting of the Set ~ control and also indicates the electrodes, i f any, between which
leakage occurs with the valve in a cold condition. It also serves to indicate heater continuity.

The Circuit Selector Switch

This is a five position switch enabling the instrument to be set up in readiness for the

type of test to be undertaken. All the necessary internal circuit connections are made to

satisfy the test conditions required, whilst internal test circuits, not required, are auto
matically removed from the valve.

On position Check (C) the instrument is set up for the initial mains voltage adjustment,

and is suitably connected for the cold electrode leakage test.

At the Check (H) position of the switch, the valve is automatically tested for electrode

leakage, with the heater hot, between the cathode and heater, and all other electrodes
strapped.

At position C/H. INS the valve is automatically tested for cathode to heater insulation

with the valve hot.

With the circuit selector turned to Te st all normal mutual characteristics are measured

in conjunction with the electrode voltage switches and other relevant controls. It will be

noted that in the case of the insulation tests the meter is automatically shunted to the
appropriate sensitivity and the insulation scale can be read directly. On the Test position
of the Circuit Selecto r switch, however, the Meter Swit ch is brought into circuit, thus

enabling the metei range to be suited to the current measurement t o be undertaken.

Also at this setting in conjunction with the D! and Di positions of the Electrode

Selector switch and the appropriate scale of the Meter Switch, signal diodes and rectifying

valves can be checked. At the position gas, the meter is connected in series with the grid,

and gives direct indication of any gas current flowing.

1 5

The Anode and Screen Volt age Switches

As their names imply these switches enable the requisite electrode voltages to be applied
to screens and anodes o f valves for the purpose of carrying out mutual characteristic
measurements. They are calibrated in the equivalent D C voltage settings and, therefore,
no account need be taken of the actual value of AC voltage which appears at the electrodes
o f the valve, which, as already explained, will differ from the equivalent D C value marked

at the switch position.

The Heater Voltage Switches

This dual switch combination is for adjustment of the heater voltage applied to the
valve under test. To enable a very wide range of heater voltages to be obtained the settings
of the two switches are arranged to be additive. Thus, with the left hand switch set at 0

all useful voltages between 0.625 and 7.5 can be applied to the valve by the right hand
switch, whilst with the left hand switch at any figure above 0 the value indicated on the
left hand switch should be added to the indication o f the right hand switch. For example,
with the right hand switch set at 5 and the left hand switch at 80, the heater voltage
applied to the valve will be 85.

The Negative Grid Voltage Controls

The negative grid volts supply comprises two sections: (a) a continu ously variable

control calibrated 0—20 and (b) a five-way switch giving four increments each o f 20 volts.
This arrangement enables any bias voltage down to —100 volts to applied to the valve,
the incremental steps being additive to the setting of the variable control.

The Backing O ff Controls enables the initial anode current reading for the valve to

be neutralised prior to the taking of mutual conductance readings. Two variable controls

are used for this purpose, one fine and one coarse, which provide smooth backing off control
to a maximum of 100mA. The rotation o f the controls in a clockwise direction will cause
the meter needle to approach zero. For normal characteristic tests, both controls should

initially be set fully anti-clockwise.

The Meter Switch is a combination switch to shunt the meter suitably to the current

measurement to be undertaken and also to insert the right value of load when making
tests on rectifiers and diodes. It has two calibration scales. The left hand scale marked
la, with switch positions 2.5, 10, 25 and 100, is used with the Circuit S elector at position

“ t est ” and the Electrode Selector at the position Al, A2 or S to indicate the full scale

deflection of the meter in m A when measuring anode or screen current. The position

mA/V after having the “ backed off” standing anode current is used in conjunction with
the Set mA/V Control for the measurement o f mutual conductance either direct or by
the comparison method using the coloured scale on the meter.

The right hand scale marked D/R with switch positions 1, 5, 15, 30, 60, 120 and 180
represents the load current when making diode or rectifier tests with the Electrode S elector
at Dl or D2. Thus if the valve is rated at say 60 mA per anode, the Meter Selector Switch
should be turned to “ 60 ” on the D/R scale and the comparative goodness o f the valve

with reference to this basic figure will be shown on the coloured scale.

The Se t mA/V Control. This control has two scales 1—10 and 3—30 selected by means

o f an associated toggle switch. When the control is set to the expected mutual conductance
figure for the valve under test, the standing anode current backed-off to zero and the

Meter Switch set t o mA/V, the meter shows the relative go od n ess of the valve under test.

1 6

If required the actual mutual conductance of the valve can now be obtained by rotating
the set mA/V Control until the meter needle covers the calibration point at the centre of

the “ goo d ” portion of the scale (marked 1 mA/V) the mutual conductance of the valve

can now be read directly from the Set mA/V Control.

The Electrode Selector Swit ch marked A[, Αχ, S, Dl and D2 enables separate tests

to be made on multiple valves, and also makes possible the taking o f Screen (or g2 )

characteristics. With this switch turned to “ Aj”, the figures of anode current and mutual
conductance shown on the meter are relevant to the anode designated on the set-up roller

by a· As such the switch is in position for measurements on all single electrode system
valves (triodes, pentodes, etc.). This position also serves for the first half of double valves
(double triodes etc.) and for the triode or pentode section of multiple diode valves (double-
diode-triode, etc.) The same setting of this switch serves for the triode or oscillator section

o f frequency changers.

With the Elect rode Selector Switch at position “ A2", the indicator meter will show

anode current and mutual conductance associated with the second anode o f double valves,
the mixer anode of frequency changers and all anode systems associated with the set up

figure a72. In this condition the first anode is not left floating, but has the normal screen
volts supplied to it via a limiting resistance.

With the Elect rode Selector set to “ S ”, the current meter is inserted in the screen (g{)
circuit of valves and screen current will thus be indicated. When making this test, anode
voltage is automatically applied to all anodes in the valve. Not e that in the case of a double
pentode valve, the current indicated will be the combined current of both screens.

With the Electrode Se lector at position D l, the indicating meter is associated with
the diode anode of a signal diode or rectifying valve (and the first anode of double diode
and full wave rectifiers). This switch position is directly associated with the anode desig

nated on the selector switch roller by Df.

With the Elect rode Selector at position D 2 the indicating meter is associated with
the second anode of double diodes and full-wave rectifiers. In this case the switch

position is associated with the roller switch setting ^2.

The Mains Adjustment Panel at the rear of Instrument

This will be uncovered by the removable plate at the back o f the instrument and the

following will be exposed to view.

(a) The coarse setting for the applied 50/60 ~ mains voltage marked 100/115, 200/215,

220/230, 240/250, the setting being made by means of the plug on this small sub-
board, to the tapping most nearly corresponding to the nominal mains voltage.

(ό) The fuse holder cap which when unscrewed reveals a small cartridge fuse which

may be thus easily replaced if blown. The correct value for this fuse is 3 amp.

GENER AL PROCEDURE FOR TE STING A VALVE

1. After having set the coarse mains voltage plug at the rear of the instrument
to suit the supply voltage, connect mains lead to supply noting that red and black leads
are live and neutral. The green or yellow lead is the Earth connection. Switch on and
the indicator lamp should light up. The valve to be tested should not be inserted at this
stage. A l l o w a few moments for the instrument to warm up.

17

2. Turn the Circuit Selector switch to position Check (C)andLea k ageswitch to position
11 ~ The instrument needle should now rise and assume a position near the black
region of the insulation scale denoting zero ohms. Ro t a te the Set ~ control until the
meter needle assumes its nearest point to the red line in the middle of this black scale

marking. With a correct setting of the initial mains voltage adjustment rotation of the
Set ~ control should enable the needle to be moved on either side of the red line. If
this is not the case and rotation of the Set ~ control does not enable the needle to reach

its setting mark from either direction, then the initial mains setting should be moved to

the next appropriate tapping. This tapping should be higher than the one chosen if the
needle always appears to the right of the red mark and lower if to the left.

3. Having set up the accuracy of the instrument to conform to the applied mains

voltage, refer to the “ AVO ” Valve D at a Manual, or alternatively to the maker’s charac
teristic data for the valve and set up the appropriate valve holder connections on the
Valve Panel selector switch as already explained.

Set the Heate r Voltag e Switches to their correct value for the valve and insert it in the
appropriate valve holder (NOTE-Heater voltages in parenthesis should be ignored as they
relate to valve tester Type 160 o n l y ) , wi thout moving the Circuit Selector switch from its

position Check (C). Rotate the Lea kag e switch through its various electrode positions
starting with the extreme counter clockwise position marked “ H At position “ H ” the
meter should show a short, thus indicating heater continuity. Thereafter any reading

obtained on the insulation scale of the meter will show an electrode insulation breakdown

corresponding to the electrode indicated by the Leakag e switch setting. (Thus a reading on
the meter of 1 megohm when the Leakage switch is set to position “ Gj ” and position
“ S ” will indicate that a cold insulation breakdown of 1 me gohm is occurring between the
grid and screen electrodes of the valve.) It will be noted that wherever electrode leakage
occurs, indication of this will be shown at two positions of the Leakage switch, because,
obviously, leakage must occur between two points. In the case of breakdown to heater from
any other electrode, such leakage indication will only occur at one switch setting subsequent
to the initial selector setting, which should automatically show zero ohms to denote
heater continuity.

4. Having ensured that no cold leakage path o f any magnitude is present in the valve
to be tested turn the Circuit Sel ector switch to Ch eck (H ). A ll o w a few moments for the
valve heater to warm up and note whether any meter deflection occurs. Such a deflection
would denote in megohms the amount of insulation breakdown that occurs between

cathode and heater strapped and all other electrodes of the valve when heater voltage is
applied. Note that if, for any reason, the Circuit Selector switch is turned back to Check

(C) there will, in all probability, be an indication of an apparent cold electrode insulation

breakdown between a number of the valve electrodes. This need not be the cause and the
reading will be found generally to disappear after a few moments. The reason for such
an indication is obvious when it is realised that the valve cathode has been heated during
the C heck (H) test. When returning to the Check (C) position, therefore, the cathode is
hot and still emitting. What appears to be a temporary electrode breakdown, therefore,
is in fact the indication o f emission which disappears as the heater or cathode cools.

5. Turn Circuit Sele ctor switch to C /H. INS when any cathode to heater insulation

breakdown which occurs with the heater ho t will be shown on the insulation resistance
scale of the meter. No set rule for the rejection o f a valve on this score can be laid down,
but it will be realised that in many circuits where an appreciable potential exists between
heater and cathode such as, for instance, in cathode follower circuits or DC valve amplifiers,
the presence of a heater to cathode breakdown of the order of megohms can often give

18

rise to quite serious trouble. Heater to cathode insulation breakdown, either permanent
or variable, can also give rise to noise in valve amplifier circuits. If, on the other hand, the

value of cathode to heater circuit resistance is only of the order of a few hundred ohms,

as for instance where cathode biasing is used with high slope valves, then a cathode to
heater insulation breakdown of the order o f fractions o f a megohm need not give rise to

any serious trouble.

6. The next test normally to be made upon the valves is the measurement of some

or all o f its mutual characteristics. This may take the form of the complete plotting of
one or all of its characteristics, or the measurement of its mutual conductance, or the
comparative testing of the valve on the basis o f its mutual conductance. All these require

the manipulation of the main voltage and meter controls and, before such a test is under

taken and the Circuit Selector switch turned to position Test, one should be assured that
all the requisite controls are correctly set. This applies to the setting of the anode, screen

and grid voltage controls, the Meter Switch and the Electrode Selector switch. In pa r tic u la r,

where the p r o b a b l e anode current of the valve is unknown, the Meter Switch sho ul d b e se t

to 100 mA to a vo i d damag e to the mo v e m e n t if the current flo w i n g is such as to be co nsid er ab ly
higher than that cat e r e d f or by the lower meter range pos iti on s. It is always perfectly simple

and safe to set the Meter Switch at successively lower full scale current deflections to cater
for a valve, the anode current of which is less than that which can be appropriately read
on a higher range. If the reverse procedure is adopted, however, then it is quite possible
that a damaging current may have passed through the meter circuit before the latter is
set to a suitable high range. The procedure for taking the necessary valve measurements
is then almost self explanatory.

Where only a measurement of mutual conductance is required then the data for this

can be taken from the “AVO” Valve Data Manual or Handbook. The electrode voltage

settings should be made as indicated and consequent upon such settings an initial anode
current will be sh ow n on the meter which has been finally set to a suitable range. This

anode current reading should normally be compared with the anode current reading shown

in the tables, as it will give an initial indication of the valve’s “ goodness ”. Quite obviously
if a valve shows an anode current reading considerably below that which is appropriate
for the applied electrode voltages, then its emission is much lower than would normally
be expected and in normal circumstances the valve will not function at full efficiency. More
particularly does this apply in the case o f valves used either as oscillators or output valves,

for in both conditions the valve has to deliver an appreciable power which cannot
obviously be up to standard if the emission is low. At the same time care should be taken
not to jump to false conclusions on this basis when testing valves of very high slope and
short grid base, where it may be possible to double the valve anode current for a change
in bias of some .25V, and a very slight variation in the valve characteristics may give rise
to an erroneous impression of the valve’s “ goodness ” on the score of anode current.

7. After having observed the initial anode current reading and obtained therefrom
such information as is desirable, this anode current indication may now be backed off to
zero by the Backing Off controls and the Meter Switch Set to its 2· 5 position, any further
adjustment to zero being made by the Fine Backing off Control.

The Set mA/V Control should already have been set to the value given in the valve
data and it would be as well to explain here how the two scales on this control should be
employed. The outer scale marked I—L0 applies a potential to the grid such, that at
the slope indicated the rise in anode current is 1mA. Thus when the Set mA/V dial

19

indicates 1 mA per volt, the bias change is equivalent to IV, but when the control is set
at 10 mA per volt the bias change is only 1 /10th of a volt.

The inner scale marked 3—30 applies a potential to the grid such that at the slope
indicated the rise in anode current is 3mA. It therefore follows that for a slope of 10 mA/V
on this scale the voltage change at the grid will be 0-3V.

For general purpose use, the inner scale should be employed, but when checking (c)
a valve with slope less than 3 mA / V or (b) a vaive with a high slope/short grid base, the
outer scale should be used.

Whilst in (b) above, there is no necessity to use the outer scale, a more accurate result
should be obtained by its employment due to the smaller decremental voltage used to
produce the mutual conductance reading. To measure the comparative “ goodness ”
o f a valve in terms of mA/V, with the anode current backed off to zero as already explained,

any final zero adjustment having been made with the Meter Switch at its 2-5 mA position,
set Meter Switch to position mA/V. The comparative “ goodness ” of the valve will now

be given on the Replace/Good scale.

All valves coming within the green portion can be taken as satisfactory. Valves in

the red portion are suitable for rejection, whilst the small intermediate band between the
green and red portions denotes a valve which, whilst not entirely unsatisfactory, is not by
any means working at its full rated efficiency. Subsequent action on the valves whose
test figures come within this band will obviously have to be related to the particular
requirement of the moment.

Alternatively, where it is required to obtain a reading o f mutual conductance, and not

merely a gauge of the valve’s “ goodness ” factor on the basis of mutual conductance,
then after backing off to zero, the Meter Switch should be set to position mA/V and the
Set mA/V control rotated until the meter needle covers the calibration line at the centre
of the goo d scale (marked m A /V ). The mutual conductance of the valve may now be read

> from the Set mA/V Control.

Valves having a slope of less than ImA/V cannot be checked by the comparative
“ goodness ” method (using replace/good scale). In such instances, the set mA/V control
should be set to position 1, the standing anode current backed off, and the Meter Switch
set to position mA/V. The mutual conductance (slope) of the valve will now be directly
indicated oh the meter (using scale marked 0-1—1 mA/V).

Where more comprehensive tests of the valve are required, to assist in the solution
of development or more intricate test problems, the plotting o f one or a family of mutual
characteristics can often give a much more complete answer. This may readily be under
taken with the Valve Characteristic Meter and is performed with the Circuit Selector in
its position Test. The manipulation of the controls subsequent to the obtaining of the
initial anode current readings is not of course required, it being merely necessary to plot
the value of the appropriate electrode currents as read from the meter, against the settings
o f the associated electrodc voltage switches. Ia/Vg! curves will be taken at a pre-determined

setting of anode and/or screen volts, the reading of the anode current obtained being
plotted against the settings on the variable grid bias controls. Similarly l a/Va curves will
require a fixed setting of grid bias, anode current being plotted against the settings of the
anode voltage switch.

Where either mutual conductance characteristic curves are required for the screen

or g2 of the valve in question, then the Electrode S elector switch should be set to position

2 0

“ S ”, the meter current shown will be an indication of the screen (or g2) current and all
the above instructions can be related thereto.

Remarks in relation to the tests described above as applied to multiple or special types

o f v a l ve , will be fo und in subsequent test notes.

8. Where a valve is suspected of passing too much grid current, a measure of the
magnitude o f grid current at the desired conditions o f applied electrode voltage may be
made after having measured the mutual conductance o f the valve in question. With the

Meter Switch set to 100 mA on the la scale and the Circuit Selector turned to position
Gas the meter is now directly connected in the grid circuit of the valve under test and
gives a direct indication of grid current flowing. (To prevent damage to the instrument, a

limiting resistance is incorporated which affects the accuracy of readings at the upper end

of the scale). The scale is calibrated 0—100 μ Α .

9. The testing of rectifying valves should really be associated with the requirements

of the circuit in which these valves are to work, although in most cases, in the data for the
valve in question a figure is quoted denoting the standard emission to be expected for a
valve of the type under test.

The procedure for carrying out the test is again straightforward. All initial tests
should have been carried out as for amplifying valves, but before setting the Circuit
Selector to Test, the suggested l o a d current figure for the valve given in the Data

Manual should be set on the D/R scale of the Meter Switch. This l o a d current, it wil l be

realised, applies to one anode only. The setting of load current can either be determined
from the tabulated data as already mentioned, or alternatively can be related to the total
current that the valve is required to deliver. Thus in a piece of apparatus where the total

HT current drawn is say 50mA, then a rectifier load current setting of “ 60 ” will be an

adequate test for the valve emission (assuming half wave rectification). Alternatively,

if the valve is a new one, the maker’s rating foT maximum load current can be used as the
basis for the setting of the Meter Switch. It will be realised that since each hal f of a full
wave rectifier is t e st e d independently, then the setting of the range switch should indicate
half the total value of current that the valve would be expected to deliver in a full w av e

circuit. For instance a valve rated at a maximum current of 120 mA would be tested with
each an ode at the “ 60 ” positio n on the Meter Switch.

No further manipulation of the electrode voltage controls is required. The heater

voltage is already set whilst anode, grid and screen voltage controls are c ompletely dis 

as so c i ate d from the test circuit by the setting o f the Electrode Selector switch to Dl or D2,

all appropriate voltage and circuit connections also being automatically made. Having,
therefore, correctly set up the valve as explained, the indication of the meter needle on the
coloured scale will show the operative goodness of the valve in relation to the standard
load current chosen.

Similar remarks apply to the testing o f signal diode valves, with the exception that

these are always tested with the Meter Switch at “ 1 ” unless otherwise specified.

INST RUCTI ONS FOR TESTIN G SPECIFIC VALVE TYPES

The function of a valve, as distinct from its manufacturer’s type number is indicated

by a symbol in the form o f letters appearing at the extreme right of the test data; thus a

21

half wave rectifier would have the lette r “ R ” in the function colum n, whilst a full wave

rectifier would be designated by “ RR Similarly, diode valves will be shown by the
letter “ D ” the number of diode elements being indicated by the number of “ Ds ”, thus
“ DD D ” refer to a triple diode.

The testing of multiple diodes or rectifiers is carried out in the manner already explained,
the Electrode Selector switch being used to select the diode or rectifier element, the
comparative emission for which, being indicated on the meter. It will be realised that when

dealing with diodes or rectifiers Dl and D2 positions of the selector switch represent
diode or rectifier anodes 1 and 2 respectively and correspond to figures 8 and 9 in the set

up figure.

In the case of triple diodes since only two anode systems are normally catered for,

a special procedure is adopted in the set up figure. At the position in the se t up number
representing the third diode the symbol f is included, the first and second diodes being
indicated by 8 and 9 respectively in the normal way. The valve should now be tested
normally with the selector switch set to 0 where the f appears in the set up number. This
will give emission figures for diodes 1 and 2. Now rotate the Selector Switch rollers so
that the two rollers originally set at 8 and 9 are now set to 0 and set up the position | as 8
on the selector switch. A further test with the Electrode Selector switch at Dl will thus

give the emission o f the third diode, e.g., AAB1 will be indicated in the data as 0231|0980.
To test diodes 1 and 2 the set up on the roller switch will be 023100980 and diodes 1 and 2
will be tested in the normal manner. For obtaining the emission figure for the third diode
the Selector Switch will be altered to 023180000 and the Electrode Selector to position Dl.

Combined Diode and Amplifying Valves will be represented in the type columns by

“ DT ” and “ DD T ” for diode triodes and double diod e triodes, whil st “ DP ” and
“ DDP ” indicate diode pentodes and double diode pentodes. The testing of such valves
is automatic, the amplifying section being tested first with the Circuit Selector switch at

position Test and the Anode Selector at position “ Aj” whilst the rotation of the Meter
Switch to the appropriate load setting and the Electrode Selector t o “ Dl ” a n d/o r “ D2 ”
would cause the meter to indicate the comparative goodness of the valve. (Unless otherwise
stated the load setting will be position 1 on the D/R scale of the Meter Switch.)

Double Triodes, Double Pentodes or Double Tetrodes will be indicated by the letters

“ TT ” or “ PP ” in the type column and will be tested in the normal way for each half
of the valve, selection being made by the rotation of the Electrode Selector switch to A!

or A2 corresponding to set up figures 6 and 7. Note that screen current readings obtained

whilst checking double tetrodes or double pentodes will be a combined value for both

halves of the valve.

Frequency Changers of the Heptode, Hexode class employing the normal oscillator

section as a phantom cathode for the mixer section are not very satisfactorily tested in two
sections, as the nature of the valve construction is such that each section is dependent on
the other for its correct operation. For test purposes therefore, this valve is shown con
nected as a triode or pentode for which, where possible, anode current and/or mutual
conductance figures are given. Such valves are indicated by the letters “ H ” in the type
column.

Frequency changers of the Octode class designated by “ 0 ” in the type column are,
as will be seen from the data, tested as if they had two separate electrode assemblies,
separate data being given for each. In this case the oscillator section is tested with the

Electrode Selector at Ai and the mixer section at A2.

22

As a further test to ensure the probability of such a valve oscillating satisfactorily, an

indication of failing emission will possibly give the most useful results. It will be realised

that when a valve is up to standard its cathode will develop its full emission at the rated

heater voltage for the valve, and any slight change in the cathode temperature will not

result in a corresponding change in the emission. If, however, the cathode’s emission is

failing, then an increase or decrease in the cathode temperature will result in a noticeable
change in the emission for the valve. When a valve is oscillating it tends to run into the

positive grid region, and thus makes use of the full emission capabilities of the cathode.
Any failing emission will limit its utility in this respect. As a subsequent test, therefore,
on a valve designed to be used as an oscillator, it is helpful to note the anode current at
the rated test figures with the normal heater voltage applied and then decrease the heater

voltage by about 10 to 15% (the next tapping on the heater switch) for a short period.
In the case of a valve with failing emission this will result in a decrease in the anode current
considerably greater than the percentage decrease in heater volts. Such a result would
suggest that the valve will not oscillate very satisfactorily. A negligible or small percentage
decrease in anode current (or of the same order as the heater volts change) will show that
the valve is developing its full emission at the rated heater voltage, and provided that the
circuit conditions are correct it should oscillate normally.

Frequency Changers employing separate electrodes assemblies for oscillator and mixer

functions are designated by “ TH ” (Triode Hexode) “ TF ” (Triode Pentode). The separate
sections o f this type o f valve are not interdependent, as in the case of the phantom cathode
types, and they can thus be tested in two separate sections as triode and pentode respectively.

This arrangement is catered for in the set up figures given, 6 corresponding to the triode

section and tested with the Electrode Selector at AI whilst 7 in the set up figure corresponds
to the mixer section which is tested with the Electrode Selector at A2. The figures to be

expected from both halves of the valve are given in the tables where available, but it is
often informative to apply a test for failing cathode emission to the triode or oscillator

section in the manner already described.

In the case o f normal triodes and pentodes (including beam tetrodes) the test procedure

for which has already been fully outlined, the type column will show the symbol “ T ”

and “ P ” respectively.

THE USE OF THE LINKS O N THE VALVE PANEL OF THE INST R U M E NT

These links enable a load t o be inserted into either anode circuit of the valve under

test when an anode current or mutual conductance test is being undertaken on the electrode
circuit in question. They therefore enable dynamic figures for the valve or electrode
system concerned to be obtained, the procedure being to disconnect the shorting link and
to connect across the terminals a resistance or other load which it is desired to include in

circuit.

Tuning indicators (Magic Eyes) are tested with the controls set according to the figures
given in the separate data table, using the screen switch for obtaining target voltage and
inserting the anode load, shown in column marked “ Ra ” by means of the link on the

valve panel of the instrument. At the approximate bias given in the table the triode section
should be at cut-off and the “ eye ” fully closed. On varying the grid bias to zero the
“ eye ” should open fully and the value of anode current should be approximately that
appearing in the table. In the case of double sensitivity indicators giving multiple images
responding to different sensitivities, two sets of data (where possible) are given, the first

set referring to the more sensitive indication.

23

Gaseous Rectifiers

These also necessitate the use o f the link, as such valves would normally pass a
damaging current i f tested without suitable limiting load in the anode circuit. They are
tested with the Circuit Selector switch turned to Test, anode voltage and representative

anode current figures being given in the Valve Data columns. The value of load resistance

(of suitable wattage) which must be included across the link, before the valve is tested, is

shown in K Ω in the “ mA/V ” column (which would not normally apply to a rectifier

valve).

Full wave examples of this class of valve are o f course tested at Electrode Selector

switch positions Ai and A2 and the appropriate load connected across each link on the

top panel of the instrument.

Cold Cathode Rectifiers designated by the symbol “ CCR ” can be tested in a similar

manner to Gaseous Rectifiers, the anode voltage, approximate anode current, and load

resistance being given in the data columns.

Thyratrons can be checked by comparison i f set up as a normal triode, with a limiting

resistance included in the link, the control ratio being indicated by a comparison between

the peak value of the applied anode voltage, and the setting o f the grid bias control which
will prevent the valve striking and passing anode current. It must be emphasised, however,

that the main value o f such a test is in comparison only, as the hold off grid bias value
shown on the grid bias control is only approximately half that of the bias which would

normally be required to hold oif the anode current of the valve at the peak anode voltage

in question.

In the data columns where information is given on common thyratrons, it will be
seen that this comprises a Roller Selector Switch No., Heater Voltage, Anode Voltage,
expected Anode Current, and the value o f the limiting resistor required. The resistor
should be o f suitable wattage and connected across the link terminals before the valve is
inserted in its holder. Grid volts should be at their maximum setting. With the Meter

Switch set to “ 1 00 ” on the l a scale, the Electrode Selector at “ Al ”, and the Circuit
Selector at Test, the bias on the valve should be reduced until the valve strikes and

anode current flows. A good valve will pass approximately the anode current given in
the Data. (If necessary, reduce setting o f Meter Switch.) This test is suitable as an
emission check on thyratrons used in television and commercial radio equipment.

Neon Indicators may be tested for striking, by setting up the roller switch so that

anode and cathode pins of the tube are set to 6 and 1 respectively, all other rollers being
connected to 0. A suitable load resistance (normally between 5,000 and 15,000 ohms)
should be included in the anode circuit link and the anode voltage switch should be set to a
peak value as near as possible to (and in no cases lower than) the striking voltage of the
neon in question. The striking of the neon will, of course, be indicated by a passage of
anode current shown on the Meter Switch being set to “ 100 ” on the la scale. It should
be noted that where the anode voltage refers to the peak applied voltage, as in the case
of thyratrons and neons, the actual peak voltage applied to the valve is higher than the
indication on the anode voltage switch. To obtain the peak voltage equivalent to a given
setting o f the anode voltage switch the figure shown on the switch should be multiplied
by approximately 1-5; thus with the anode voltage switch set to represent a DC voltage
of 100V the peak applied voltage is approximately 150V.

24

GENERAL PRECA U T IONS TO BE OBSERVED WHEN

USING THE VALVE CHARACTERISTIC METER

It will be realised that when dealing with an instrument such as the Valve Characteristic
Meter with such flexibility of control, it is almost impossible to protect the instrument
to such an extent that the operator cannot cause damage to either the valve or the instrument
by some combination or wrong setting of the controls or incorrect use of the meter. It is,
therefore, important that the correct procedure, as previously outlined should be used in
the sequence o f the tests applied. Valves should be tested for insulation or breakdown
before full voltages are applied for characteristic tests. Where any doubt whatever exists
as to the probable electrode current likely to be passed, the Meter Switch should always
be turned to its highest current range and then gradually reduced in order to facilitate
reading of the electrode current.

In experimental work where a variable voltage is required to be supplied to the anode
or screen electrodes of the valve, always start with the lower voltage tappings and increase
only after correct adjustments have been made to the Meter Switch to ensure that the meter
circuit is not overloaded by an unknown current. Always make sure that the selector
voltage switches have been correctly set for the valve before the instrument is switched on.
In this respect it is a good practice to return the selector voltage switches to zero (particularly
Heater Voltage switches) after a test has been applied and before a new valve is inserted.

Take care in setting the Roller Selector Switch to avoid wrongly connecting the

electrodes of the-valve under test. In this respect the automatic cut-out is advantageous
in that it will usually save a valve if high tension voltage is inadvertently applied to the

heater by incorrect setting of the switch, but it must be pointed out that after the switch

is correctly set nothing can save the he ate r fr om being burnt out if an o ve r lo a d h ea te r voltage

is a p p l ie d b y wrong se tt in g of the he ate r vol tage switches .

D o not apply test voltages t o the valve without ensuring that where necessary top cap

connections have been correctly made, as a valve can often be irreparably damaged by
running it with its grid or its anode wrongly connected.

Where a valve appears to be performing abnormally, as indicated for instance by a

continuously rising or falling anode current which does not attain a condition of stability,
do not leave the valve “ cooking ” for a long period to see what will ultimately happen,
as this will in all probability result in the damaging of the valve due to excessive currents
in the anod e or screen circuits. In general, it is not necessary or helpful to leave a valve

on test for a considerably longer period than is necessary to complete the test in question.

Finally, it must be stressed that whilst every care has been taken in the compilation of
this publication, the “ AVO ” Valve Data Manual and the “ AVO ” Valve Da ta Handbook
to ensure that all data given is correct as far as is known at the time of going to press, it

is no t impossible that with the many thousands of figures involved, errors will have crept
in. T h e manufacturers cannot hold themselves responsible for any damage that might

occur to a valve or to the instrument from such a cause.

ABBREVIATED WORKING INSTRU C T I O N S FOR TH E “ AVO ” VALVE

CHARACTERISTIC METER MARK Ι Π

Before switching “ ON ” the full instruction Book should be read and always used for

reference when testing unusual types o f valves.

1. Check mains adjustment tap and connect mains lead to the supply, red and black
leads are line and neutral, green or yellow being the earth connection.

2 5

2. Set “ Circuit Selector ” to “ Check C ” and “ Electrode Selector ” to “ A

3. Set “ Meter Switch ” to 1 00 on the la scale.

4. Turn “ Backing Off ” controls fully anti-clockwise.

5. Set “ Heater Volts ” switches to value indicated in Valve Data. (Heater volts in
parenthesis should be ignored.)

6. Set “ Anode V olts”, “ Screen Vol ts” and “ Grid Volt s ” to values indicated in
Valve Data.

7. Rotate the “ Set Ma/V ” control to figure given in Valve Data using (where possible)

the inner scale and appropriate setting of associated switch.

8. Set “ R oll er Selector ” switch as indicated in Valve Data and ensure that Aj and A2

links are tight. (For “ * ” in data read “ 0 ” .)

9. With leakage switch at “ ~ ” switch on, and allow instrument to warm up, Adjust
pointer to position “ ~ ” by means of “ Set ~ ” switch.

ALL VALVES

1. Insert valve, and make any top cap connections if required.

2. Fully rotate “ Leakage ” switch. Check heater continuity at “ H ” and insulation

on all other positions.

3. Set “ Circuit Selector ” to “ Check H ” to measure leakage from Heater/Cathode to

all other electrodes strapped together with valve hot.

4. Turn “ Circuit Selector ” to “ C/H ” to measure leakage between heater and cathode
with valve hot (if valve is indirectly heated.).

TRIODES, DOUBLE TRIODES, DIODE TRIODES PENTODES, DOUBLE PEN

TODES, DIODE PENTODES AND TETRODES IN SIMILAR COMBINATION.
ANODE CURRENT. With “ Electrode Selec tor” at “A,” set “ Circuit Selector” to

“ Test ”. Meter should then indicate anode current. Reduce Meter Switch setting if

required. If protective relay operates, switch o f f and check for incorrect setting o f “ Roller

Selector ” switch or panel controls. If all controls are correct and relay continues to operate

when instrument is switc he d on again, the valve is prob ab ly soft an d the test should be dis

continued.

MUTUAL CONDUCTANCE. Reduce meter reading to zero by means of “ Backing Off ”

controls. Set “ MeteT Switch” to “ 2-5 ” position and re-adjust zero if necessary. Turn
“ Meter Switch " to “ mA/V ” position, when a good valve will give an indic ation in the
green band on the meter scale. To obtain actual mA/V reading, adjust “ Set mA/V ”
control until needle reads on calibration point 1 mA/V, in centre of green band. The “ Set

mA/V ” control will now indicate the mutual conductance of the valve under test. To

26

obtain a reading for valves with mutual conductance below 3 mA/V, use outer scale setting
on “ Set mA /V ” control and fol low the procedure outlined above.

For double valves, check data for difference in electrode voltages and repeat above

operations with the “ Electrode Selector ” set to “ A 2

GAS TE ST. To measure grid current, set “ Circuit Selector ” to posit io n “ Gas ” and the

“ Meter Switch ” to its 100 mA position. Meter will now indicate gas current, full-scale
indication being Ι Ο Ο μ Α .

DIO DES. To check diodes turn “ Electrode Selector ” to “ Dj ” and “ Meter Switch ” to
“ 1 mA ” on D/R scale (unless otherwise indicated in Valve Data). Turn “ Circuit Selector ”
to “ Test”. The condition o f the valve will now be given on the “ Replace/Goo d” scale,
Cheek double diodes at D[ and D2 position of the “ Electrode Selector

RECTIFIERS. To chcck rectifiers, set “ Electrode Selector ” to “ D f’ and set anode
loading given in Valve Data, on D/ R scale o f “ Meter Switch ”. Turn “ Circuit Selector ”
to “ Test The condi tio n of the valve will now be indicated on “ R e p l a c e /Good ” scale.
Load reading is per anode. Check full-wave rectifiers at position “ Dj” and “ D2 ” of
“ Electrode Selector ” switch.

On completion of tests return controls to their fully anti-clockwise position.

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