AVO VALVE CHARACTERISTIC METER Service manual
THE “AVO”
VALVE CHARACTERISTIC METER
WORKING INSTRUCTIONS
THIRD EDITION
PUBLISHED B Y
THE AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO. LTD.
WINDER HOUSE, DOUGLAS STREE T, LONDON, S.W.l
Telephone: Victoria 3404-9
THE AVO VALVE CHARACTERISTIC METER Mk II
F O R EWO R D
o r more than a quar ter of a century w e have be en engaged in the design and
manufacture of “AV O ” Electrical Mea surin g In struments. Throughout t hat tim e we
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have consistently pio neer ed the design of modern multi-range instruments and have
kept abreast of and catered for the requir em ents of the e poch-makin g developments in
the fields of radio and electronics.
The success of our steadfast p olicy of main taining high st an dards of performanc e in
instruments of un excell ed accu rac y, and making such instruments avail able at rea sonable
cost, is reflected in the great respect and genu ine goodwill which “AVO ” pr od ucts enjoy
in every part of the World.
I t has been gra tifying t o note the very large number of instances where the satisfaction
obtaine d from t he perfor mance of one of our instruments has led to the aut omatic cho ice
of other instruments fro m the “AVO ” range. This process, ha ving continued ov er a long
period of years, has resulted in vir tual standardi sation on our products by numerous
Public Bodies, The Services, Railway Systems, and Pos t Office and Telegraph Undertaki ng s
throughout the worl d.
O ur design ers have thereby been encouraged to en sure that new instruments or
accessories for inclusion in the “AVO ” r ange fit in with existing “AVO ” apparatus and
serve to extend the usefulness of instruments already in use. Thus, the user who sta nda rdises
o n “AVO ” products will se ldom find h imself shor t of essential measuring equipment, for,
by means of s uitabl e accessories, his existing eq uipment ca n often be adapted to meet
unu sual demands .
It is with pleasure that we acknowledge that the unique po sition attained by “AVO ”
is due i n no small measure to the co-oper ation of so many users who stimula te our Research
and De velopment staffs from time to time wit h suggestions, criticisms, and even requests
fo r the pr od uction of ent irely new instrum ents or accessories. It is our desire to encou rage
and preserve this relationship between those who use “AVO ” I nstruments and those who
are respo nsible for th eir design a nd manufac ture, and correspondenc e is t herefore
welcomed, w hilst suggestions will receive prompt and sympathetic consideration.
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INDEX
For eword .. .. ..
Int rodu ction .............................................................
The Basic Method of characte ristic c h e c k in g
The Basic Method of checking diodes and re ctifie rs
Insulation Testing .. .. .. .. .: .. .. .. .. 8
T he Safety C ut-out.. ...................................................................................... 9
The Valve Panel and Selector S w itc h .......................................................................... 9
Pro cedure for setting up valve base connections .. ,, .. .. 10
Provision for new valve bases .......................................................................... 12
The prevent ion of Self oscillation of valves under test
Dia gram of Standard base pin conne ctions
Special pro cedure for valves ha ving internally connected pins .. . . . 14
The controls on the front panel, their functions and op erati ons
The Set 'v Control ...................................................................................... 16
The Electrod e Le aka ge S w itc h .......................................................................... 16
The Circuit Selector Switch .......................................................................... 16
T he Anode and Screen Voltage Switches
The Heater Voltage Switches .. .............................................................. 17
The N egative Grid Voltage Control .............................................................. 17
The Press Buttons ..
The Set Zero Control ...................................................................................... 17
The Meter Selector Switch...................................................................................... 18
The Set mA/V C o n tr o l ...................................................................................... 18
The Anode Selector S wit ch .......................................................................... 18
T he Sp ecial Ad ju stment Panel at the rea r o f the instrum ent .. .. . . .. 19
G eneral Procedure for testing a v a l v e .......................................................................... 19
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 test ing specific val ve ty pes .................................................. .. 22
Multiple diodes and rectifiers—double triodes and double pentodes—combined diode
and amplifying valves—frequency changers of heptode and hexode types—frequency
changers employing separate electrode assemblies.
The Use of the Link on th e Back P anel of the I nstr ument ..
Tuning Indica tors .. .. .. ...................................... .. . . 24
Gaseous Rectifiers .. .............................................................. .. .. 24
Cold Ca thode R ectifiers ...................................................................................... 24
Thyratrons .. .. .............................................................. .. ., 24
Neo n Indicators .. .. .. .. .. .. .. .. .. 25
General Precaution s to be obs erved when using the Valve Characterist ic Meter ., 25
N otes upon simple maintenance of instrument .. .. .. . . .. 26
Ci rcuit diagra m of Valve Ch ar acteristic M ete r
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...................................... 12
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The “ AVO Valve Data Manua l
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 has 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 further announcements.
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Introduction
to
THE “AV O” VALV E CHARACTERISTIC METER
'T’he proble m of de signing a Valye Testing Instrument capable of giving a true and
A comprehensive pic ture of the state of any valve, has always been one of co nsiderable
magn itude, increasing in complexity as new valve types are brought into gen eral use.
For a quick general purpose test necessitating a minimum of time and technic al effort,
a mut ual conductan ce figure w ill give an adequ ate i dea of a valve’s usefulness, and the
original “AVO ” Valve Tester was design ed to tes t the efficiency of valves on t his basis.
Whilst a Valv e T ester must, of necessity, be accompani ed by a data book correlating
the results of the Tester wi th the cond ition of the valve in question, a purely empirical
figure, if u sed as a standard, will always give ri se to doubts in the mi nd of the operator.
Th e instrumen t should therefore , produce a figure which can be compared with so me
st andard quoted b y the valve ma nufactur er, if the opera tor is to use his in strument with
confidence. For this reas on the “A VO ” Valve Tester used the static ze ro bias mutual
conductanc e figure as a ba sis of comparison, this figu re being at that time almost universally
quoted by the valve manufacturer .
In order to reproduce this stan dard correctly, it was also nec essary to reproduce the
stated values of DC anode and screen voltage, a matter of some considerable difficulty
when it is realised that for an y stated condition of anode and/ or screen volts the correspond
ing electrod e currents can vary over very wide limits, and in the case of valves of low
initial a no de current a nd high slope, the actuation of the control w hich pro duc es the
milliamp-per-vol t reading might eas ily double the anode cu rrent flowing. With D.C.
methods of testing t he inherent internal resistanc e of the rectifying circu its use d could be
such as to give regulation errors wh ich c ould 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 not only re nder the Tester of
prohibitive price and size, but would considerabl y increase the complication of ope ration
for the non-technic al user .
The problem was overcome by th e intr oduction of the AC method of ope ration (Patent
No. 480752) by which m ean s the nec essary DC test conditions were correctly simul ated
and a true mutual co nductan ce figu re produced b y the application of AC voltages of
suitable amplitude t o all electrodes. This e normously simplified the power supply problem,
rendered regulation er ror s n egligible , and obvia ted the necessity forvoltagecircuitmonitoring.
The “AVO ” Valve Tester thus fulfilled normal testing needs for a long pe riod. Du ring
recent years, however , electronic tec hniqu es have bec ome much more precise and the
natu re and mul tiplicity of valve types have contin uo usly incre ased. T he zero bias mutual
co nduct ance figure is seldo m quoted by the valve manufacturers, who, usually, publis h t he
optimum wo rking point mutual conductance and voltage figures, and in a large number
of cases give full fa milies of curv es, from which, precise operatio n, under a variety o f
working conditions, can be judge d. To cater for present day requireme nts therefore, a
valve testin g d evice should n ot only be capable o f producing a working point mutu al
conductance figure at any reasonable val ue of anode, screen or grid voltage r ecomme nded
by the manufacturers, but should also be cap able, if necessary, of reprodu cing any one
of the mutual characteristics associ ated with the valve i n qu estion. The instrument thus
has to simulate the performance of a compr ehensive valve mea surin g s et-up of laboratory
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type and yet, at the sam e tim e, be sufficiently cheap and s imp le to ca ter for the needs of
the comparatively inexp erienced rad io test assistant. It is obvious that the very much
wid er application of an instrument of this class, would render the regulation difficulties,
already r eferred to, much mo re critical.
Investiga tions were, therefore, put in hand t o see whether th e AC test method would
reproduce DC conditi ons n ot only in respect of the mutual conductance figure taken at
a singl e disc rete poi nt, but at all points o n ail characteristi cs fr om zero bias to c ut off.
In other words, it was necessary t o de termine whether the general funct ion for a DC stati c
valve characteristic
(Va + ^ V gl + μ 2ν ?2)
la = f ---------------------------------
Ra.
would hol d when la was measured in terms of DC current, but wh en Va, Vg2 an d, if
necessary, Vgl, were replaced by 50 cycle AC voltage s of suitable magnit ud e. It was
eventu ally fou nd that a complete co-relation between the se two sets of conditions was h eld
when the grid voltage took the form of a sin usoidal wave form with the positive half cycle
suppressed (in other words , rectified but completely unsmoothed AC), and th e following
relationships were maintai ned :—
Va RMS — 1*1 Va in dicated DC
Vg.2 RMS = 1 ·! Vg3 ind icated DC
Vgx ( mean un smoothed) = 0*52 Vgj indicated DC
la (mean DC) = 0*5 ind icated la
From the above c onditions, therefore, the re quired relationship s were obtaine d which
fo rmed the basis of oper ation of th e Valve Characteristic Met er (Pat ent No. 606707).
Such an in strument, whilst retaining the advantages of simplicity, size and reas onable
price, res ultan t upon the elimination of complicated regulated DC supply systems and
univ ersal mon itoring, woul d have the inherent regu latio n easily ob tained from a well-
designed AC transformer. It would enable a valve to be c heck ed at any point on any one
of i ts many mutual characteristics and if necessary would allow a full family of character
istics to be drawn.
The basic method of characteristic checking
The fundamental circu it of operation of the instrument is shown in Figure 1 , the nature
of the wave forms present in the vario us parts of the circuit being indicate d thereon. As
in the origina l Valve Tester, the process of obtaining a direct readi ng mutual conductance
figure is simplified by the production of a backin g off circuit, whic h balances out the deflection
due to the standing anode current at the desired test condi tions prior t o the operation of
the mutual cond uctance button. Only the desire d figu re appears on the meter scale,
thus ena bling the meter to be set at a sufficiently sensitive range for preci se determ inati on
of mutual conductance. It will be noticed that the cu rrent flowing in this backing off
circuit is similar in wave form, but precisely opposite in direction to the an ode current,
thus eli minating any u nd esira ble rip ple that could otherwise bec ome appar ent when the
meter, after b acking off, was set to a sen sitive range.
The basic method of checking diodes and rectifiers
Any simp le emission te st at lo w applied voltage must necessarily give rise to a purely
em pirical figure for the valve in question which cannot necessar ily be co-r eiated wi th any
one of the maker’s characte ristics and w hich, owing to th e f act that it relates to the lower
bend portion of the rectifier characteristic may vary very w idely for any given type of valve.
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The important function of a rectifying valve is that it will, under suitable reservoir load
conditions, produce sufficient curr ent to operate the apparatus which it is intended to supply.
This fundamental requirement,' therefore, is the basis of 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 DC current conditions, i.e. 5mA, 15mA, 30mA, 60mA and 120mA. The actual
current flowing in the load circuit is then indicated on a met er shunted to correspond with
the DC load required. The meter read ing will then indicate as a percentage, the comparative
efficiency of the valve on the basis of this required DC load. Each half of a full wave
rectifying valve is tested sepa rately thus enabling matching of two halves to be checked
and any tendency to pr oduce hum by part ial h alf waving to be indicated.
The pre-det ermined lo ad figures are chosen so that they not only give a sufficiently
wide range o f c urrents to cater fox the no rmal requirements of electronic appara tus, bu t
also correspond to the DC maximum emission figures usually quoted by manufacture rs
in th eir rectifyin g valve data. Signal diod e valves are similarly tested, but a lower AC
voltage is applied and comp arison is mad e with a single DC load figure of 1mA, this figure
being normally more tha n sufficient to cover the rectified signal current that would be
obtained. The basic operating circuit of the diode and rectifier system is shown in Figure 2.
Insula tion Testing
To cover all eventualities, three distinct forms of insulation measurement are catered
for in the Valve Characteristic Meter. Measurements are taken with DC applie d voltages,
and direct indication of the insulation value in megohms is shown on the meter scale.
As an initial test, prior t o the application of operating voltages to the valve, th e rotation of
a switch enables the insulation figure to be shown, which occurs between ea ch of the valve
electrodes taken i n order and all the others strapped together. The denominat ion of the
Fi gure 2
elect rodes between which any bre ak down exists will thus be autom atically indicated and
fu rther , the continuity of the heater circuit is shown as a zero resistance at th e heater (H)
positio n of the switch.
Wi th d irectly heated valves it is not un common for electro de sagging to occur on the
appl ication of heater voltage, with the result that a breakdown occurs between heater and
an adjacent electrode. To show up thi s condition a test circui t is provided indi cating the
insulation resistance betw een t he heate r and cathode of a valve and all oth er e lectro des
strapped when heate r v olta ge has bee n applied.
Fin ally the very important factor of heater to cathode insulation when the heater is
hot c an be tested , the insulation again being shown directly in megohms, the usual cat hode
to heater connection b eing ope ned for this purpose an d the appl ied voltage being in such
a direction as to make the cat hode negative with respect to the heater, thus avoiding false
indic ations of i ns ulation r esi stance due to el ect rode emission.
Safety Cut -O ut
To preven t dam ag e to in ternal components of the Valve Charac teristic Meter, due to
inadvert en t o r delib erate shorting of the sup ply voltages, a safety cut-o ut is incorp orated,
op erative when da maging overlo ads of AC current are taken fro m either t he anode or screen
voltage sources. The cut-out takes th e form of a two circuit polarised elect ro-m agne tic
relay which has two windings incorporated in its ele ctro-m agn etic system, one as sociated
with the screen vol tage supply and one with the anode voltage supply . It will be appr eciated
that with the valve electrodes taking normal curre nt, half wave DC pulses only will flow
through these windings and the dir ection and magnitude of the windings are such that with
anode cu rrent only flowing, or alternatively, with a c onsiderab ly larger anode curr ent than
screen current flowing, the cut-o ut will be held in contact and the inst rument will work
normally. It is obvious, however, tha t if an internal valve sho rt occurs on any one of its
high voltage electrodes, or alternati vel y, if such a short is a pplied externally via the valve
holder sockets, or other part of the circuit, or further if any int ernal short occurs as sociated
with t he ano de or screen supply circuits, th en the current flowing i n these circuits will not
ta ke the form of un i-dir ectional pulses, but will be ordinary AC curren t.
In such circumsta nces, the effect of the first ha lf cycle of AC current in the reverse
direc tion fro m no rma l w ill be such as throw out t he c ut-ou t and thus break both anode
and screen sup ply circu its. The overload is, th erefore, removed from the supply system
and bur n out of transformers and associate parts is obviated. Note t hat this protection
does n ot apply in the ca se of a sho rt applie d to the heater voltage windings as these norm ally
pass sinusoidal AC current . Further, if for any reason w hen te sting a pentode the anode
circ uit should become disconnected (this can occur when the rolle r sw itch is wrongly
set up) then the nor mal result would be for a dam agin gly heavy rectified cur rent t o flow
In t he screen circuit; the rel ative direction and magnitude of the two windings on the
cut-out is then such that w hen the current in the screen circu it seriously exceeds the cur rent
in the anode circuit the cut-out is t hr own and damage both to va lve and circuit is obviated.
It must be stressed that this cut-out will not operate upon the passage of normal heavy
currents o f a DC nature occurring in the valve anode circuit, and it will not protect the
movement if the latter is wrongly set on a range not corresponding to the current passing.
This problem is dealt with by ensuring that the movement is always set to its maximum
current range when the probable magnitude of the current is unknown.
THE VALVE PAN EL AN D SELEC TOR S WITCH
The Valve Pane l com prises 18 valve holders of the fo llow ing types :—English—4/5
pin, 7 and 9 pin, 8 pin side contact, B7G. B8 A, B8B ( American Loc tal), B9G, Englis h Octal,
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