AVO MK III, VALVE CHARACTERISTIC METER Service manual

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TH E “A V O ”

VALVE CHARACTERISTIC M ETER

Mk. III.

PUBLISHED B Y

AVO L I M I T E D ,

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

Telephone: Victoria 3404 (12 lines)

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FOREWORD

OR more than a quarter of a century we have been engaged in the design and Fmanufacture of “ AVO ” Electrical Measuring Instruments. Throughout that time we have consistently pioneered the design of modern multi-range instruments and have kept abreast of and catered for the requirements of the epoch-making developments in

the fields of 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 to the automatic choice of other instruments from the “ AVO ” range. This process, having continued over a long period of 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 position attained by “ AVO ” is due in no small measure to the co-operation of so many users who stimulate our Research and Development staffs from time to time with suggestions, criticisms, and even requests for the production of entirely new instruments or accessories. It is our desire to encourage and preserve this relationship between those 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.

INDEX

F o rew o rd .........................................................................................................................									3
Introduction ..............................................................					......................................				5
The Basic Method of characteristic checking ...					...	...	...	...	7
The Basic Method of checking diodes and rectifiers					......................................				7
Insulation Testing ...	...	...	...	...	...	...	...	...	8
The Protective Relay	.........................................................................							...	9
The Valve Panel and Selector Switch ..........................................................................									9
Procedure for setting up valve base connections					......................................				10
Provision for new valve bases		..........................................................................							12
The prevention of Self oscillation of valves under test......................................									12
Diagram of Standard base pin connections				..................................................					13
Procedure for valves having internally connected pins......................................									14
The controls on the front panel, their functions and operations						..........................			14
The Set ~ Control	.....................................................................................								15
The Leakage Switch ..............			,.............................................................						15
The Circuit Selector Switch .........................................................................									15
The Anode and Screen Voltage Switches				..................................................					16
The Heater Voltage Switches ......................................						...	...	...	16
The Negative Grid Voltage Controls			..............................................................						16
The Backing Off Controls	.........................................................................								16
The Meter Switch ..................................................................................................									16
The Set mA/V Control ................................................			....................................						16
The Electrode Selector Switch .........................................................................									17
The Mains Adjustment Panel at the rear of the Instrument......................................									17
General Procedure for testing a v a l v e .........................................................................									17
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 ..........................									23
Tuning Indicators (Magic E yes).........................................................................									23
Gaseous Rectifiers.................................................................................................									24
Cold Cathode R ectifiers.....................................................................................									24
Thyratrons.............................................................................................................									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

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.

Introduction

THE “ AVO ” VALVE CHARACTERISTIC METER Mk. Ill

'The problem of designing a Valve Testing Instrument capable of 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 of 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 of 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 of 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 of 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 not only render the Tester of prohibitive price and size, but would considerably increase the complication of 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 of 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 of 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 of cases give full families of curves, from which, precise operation, under a variety of working conditions, can be judged. To cater for present day requirements therefore, a valve testing device should not only be capable of 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

to simulate the performance of a comprehensive valve measuring set-up of laboratory type and yet, at the same time, be sufficiently cheap and simple to 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 ^ + μ ι λ ^ - Ι ^	Vg2 )
l a	-------	-

Ra.

would hold when la was measured in terms of DC current, but when Va, Vg2 and, if necessary, Vgi, 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 following relationships were maintained:—

Va RMS = 1.1 Va indicated DC Vg2 RMS = 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 of operation of the Valve Characteristic Meter (Patent No. 606707).

Such an instrument, whilst retaining the advantages of 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 welldesigned 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 characteristic checking

The fundamental circuit of operation of the instrument is shown in Figure 1. As in the original Valve Tester, the process of obtaining a direct reading mutual conductance figure is simplified by the introduction of a backing off 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	of 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 of 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.

The important function of 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 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. 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 of the required DC load. Each half 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 shown 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 of 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 “ hold-off ” 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 protect the valve when incorrect heater voltages are applied.

It must also be stressed that the relay will not operate on	the passage o f normal
heavy current o f a D C nature occurring in a valve anode	circuit, and it will not

protect the movement if the latter is wrongly set on a range too low to accommodate the current passing. This problem can only be dealt with by ensuring that the move ment is always set to its maximum current range when the magnitude o f the expected 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 of 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 of 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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