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 ^
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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.
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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
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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
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