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Practical Data on the 1934-1935 Valve Types

Wireless World, November 30, 1934.
    
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It cannot be expected that a wireless receiver will function correctly unless suitable types of valve are chosen for use in it. The multiplicity of types now available makes this task one of considerable difficulty if the necessary information concerning the specimens is not conveniently available. As in previous years, therefore, The Wireless World Valve Data Supplement provides this information in a compact form for all the chief receiving valves. Owing to the vast number now listed by the makers it has been found necessary to delete those types which must be considered as obsolete. The guiding principle here has been one which it is believed will commend itself to all users of the supplement 4 Volts and 6 Volts battery valves have not been listed, nor have other types where later specimens are available with characteristics so similar that they can be used as replacements.

Frequency-changers

The widespread use of the superheterodyne has led to the development of numbers of special valves designed to operate as frequency-changers, so these now occupy a section to themselves. Without exception these valves perform two functions and even if they are not always two valves built into one bulb, they have the equivalent number of electrodes. Each valve, therefore, occupies two lines in the tables - one for that section functioning as the first detector or mixer and the other for the oscillator portion.

The triode-pentode is a multiple valve and consists of an HF pentode and a triode mounted in a single glass envelope. The two valves are quite separate save for a common cathode and external oscillator coupling must be provided. The valve exactly replaces the now obsolete two-valve frequency-changer.

The heptode, on the other hand, is a multi-electrode valve which functions differently, for although it is provided with oscillator. electrodes and acts as a mixer, it cannot be described as a double valve. The various electrodes are arranged concentrically and the nearest description in terms of ordinary valves would be a triode and a screen-grid valve connected in series. The tetrode section has variable-mu characteristics and the oscillator coupling is internal and electronic. The octode is a modification of the heptode, and if it be allowable to describe the latter as a combination of a triode and tetrode, the octode must be termed a composite triode and pentode - in other words, the octode is to the heptode as the HF pentode is to the screen-grid valve.

A recent addition to frequency-changers is one which is-widely used on the Continent - the triode-hexode. This consists of two separate electrode assemblies a triode and a hexode, with a common cathode. The triode functions as an oscillator and its grid is internally joined to one of the hexode grids to provide internal coupling. Mixing takes place in the hexode and is entirely electronic. The valve is claimed to be particularly good for short-wave receivers.

Screen-Grid Valves

The screen-grid valve is now rarely used as an HF amplifier, having been largely displaced by the variable-mu type, and it is rapidly being superseded even in the frequency-changer of a superheterodyne. As an amplifier, the grid bias applied is usually about 1.5 Volts in mains sets in order to avoid a flow of grid current which would damp the tuned circuits, but no bias is necessary in a battery model.

When the valve is used as an amplifier With a 1:1 ratio intervalve coupling, the amplification is readily calculable provided that the dynamic resistance of the tuned circuit is low compared with the internal AC resistance of the valve, for it is equal to the product of the mutual conductance and the dynamic resistance divided by 1,000. In cases where a step-up ratio is used, the amplification is calculated as above, but is divided also by the ratio.

When the valve is used as a first detector in a superheterodyne, it is usually biased more highly so that its AC resistance is increased. The conditions are not easily calculable, but it is convenient to remember that the effective amplification obtainable is usually about one-third of that given by the same valve acting as an amplifier.

As a detector-oscillator the valve must be carefully selected, and it is usually wise to choose a screened HF pentode with a high mutual conductance. The HF pentode has the advantage over the screen-grid tetrode in not possessing a negative resistance kink in its characteristic, and it is consequently capable of giving a larger undistorted output.

Variable-mu Valves

For amplification purposes variable-mu valves are now almost universal. The stage gain obtainable at minimum and maximum bias is calculated in exactly the same manner as for screen-grid valves, and when operating at minimum bias they function in an identical manner.

The amplification, however, can be varied within wide limits, without introducing distortion or upsetting the tuning, by the simple expedient of varying the grid bias. In a mains set with manual control the variation of bias is usually obtained with the aid of a potentiometer in the HT circuit, to the slider of which the valve cathode is connected. In a battery set, however, a potentiometer must be connected across the bias battery and the grid return lead taken to the slider. Automatic volume control is now rapidly becoming a standard fitting, however, and no manual control of bias is then needed, so that the grid return leads of the variable-mu valves are taken through suitable filters to the AVC bias source, which is usually the detector.

The maximum bias required to effect a given reduction in signal strength becomes a matter of some importance. The smaller the bias required to effect a given change in mutual conductance the better will be the action of AVC, or in the case of a battery set with manual volume control, the smaller will be the necessary bias battery. It is unwise to proceed too far in this direction, however, in cases where the set must be used near a local station, for the input handling capacity of low bias valves is usually much less than that of types requiring a large bias, and serious distortion may occur on a strong signal. In this connection it should be pointed out that variable-mu screened HF pentodes have some advantage over the ordinary types in that a larger output can be obtained.

Diodes

Recent months have seen the introduction of diode valves specially designed for detection and AVC purposes. The valves usually contain two diodes with a common cathode, but while the connections are both often brought out to pins in the base, in some cases only one of the anodes is taken to the base and the other is joined to a top cap. These valves are rated to operate with a much larger signal input than the diodes usually fitted to the multiple valves, and can safely pass a heavier current. Westectors will be found in this section, since they fulfill the same functions as diodes of the thermionic type.

Multiple Diode Types

The uses of these valves are many and varied, but they find their chief application in the provision of automatic volume control. A duo-diode-triode can provide detection, delayed AVC and first stage LF amplification, While a triple-diode-triode can give, in addition to detection and LF amplification, quiet delayed amplified AVC. Two types of duo-diode-pentode are to be found. One embodies a pentode of the variable-mu type, and this is intended to act as an LF amplifier and be controlled for AVC purposes, In the other type the pentode has the characteristics of an output valve, and no additional LF amplification is needed.

Triodes

Triodes with internal resistances greater than 7,000Ω find their chief application as grid or anode-bend detectors, first-stage LF amplifiers and oscillators for superheterodyne frequency-changing purposes. For a grid or power grid detector it is usually best to choose a valve with an internal resistance of some 10,000Ω, and with a moderately high amplification factor.

In the LF amplifier, a similar type of valve is usually best, and with normal designs a resistance of about 10,000Ω leads to the most even frequency response. In cases where quality is the first consideration, the trend should be towards a valve in the 7,000 - 10,000Ω range, while small sets where amplification is at least as important as quality will be best served by the choice of a valve with a resistance between 10,000 ohms and 20,000 ohms. The basis of choice, of course, assumes that the valves have similar values of mutual conductance.

The selection of an oscillator valve is in no way difficult, for almost any valve will function. In general, however, one with an internal resistance of some 10,000Ω and a mutual conductance of some 1.5 - 2.5 mA/V is the best.

Output Triodes

Those valves with resistances less than 7,000Ω are chiefly of the output type. The few specimens which come into this section and which have resistances above about 4,000Ω are usually more suitable for acting as low-gain LF amplifiers in ultra-high quality receivers.

The most important characteristic of an output valve is the power output, for unless this be sufficient it is impossible to obtain good quality reproduction at reasonable volume. For ordinary room strength at good quality some 2 Watts is necessary, but where the very best quality is desired, and particularly when the high and low frequency responses are unusually well maintained, some 4 Watts should be allowed. So much depends upon the efficiency of the loud speaker, however, and even upon the characteristics of the room in which it is used, that experience is the best guide for any particular conditions.

The output is calculated upon a basis of 5% second harmonic distortion, and this involves the statement of the optimum value of load impedance. Since it is unlikely that the speaker will have the correct impedance, a transformer must normally be used for matching, and its ratio can be calculated by dividing the required load impedance by the speaker impedance and taking the square root of the result. In all cases where. the speaker impedance is less than the valve load, the transformer will be of the step-down type.

Output Pentodes

The chief advantages of the pentode over the triode are its increased efficiency and sensitivity - a greater output is obtainable for a given expenditure of energy from the HT supply, and a smaller signal input is required. To counterbalance this, however, the valve is much more critical in the matching to the loud speaker, and if undue accentuation of the high notes is to be avoided a tone-correction circuit is necessary. Although high-quality reproduction can be obtained with a pentode, it is more difficult than with a triode, and the latter is usually selected where quality is of prime importance. The pentode, however, is invaluable in the smaller class of receiver where sensitivity and cost are at least as important as quality.

In the case of receivers of the Universal type and those intended for operation from DC mains, the fact that a pentode requires a much smaller grid bias than a triode is of great importance, and there is really no alternative to the pentode for the output stage.

It will be noted that columns are included for the self-bias resistance in both triode and pentode output valve sections. The Watts rating of the resistance can be found by multiplying the resistance by the square of the sum of the screen and anode currents, and dividing the result by 1,000,000.

Quiescent Output Valves

Recent development in battery valves has resulted in the production of a new range of output types combining large output with extreme economy of power drawn from the HT battery. The Class B valve is really two triodes mounted in the same bulb and operated in push-pull. It is worked with zero or only a small negative bias, and grid current flows during a large portion of the cycle of input voltage.

As regards the output circuit, matching to the loud speaker is obtained with a transformer, the ratio of which must be calculated in the same manner as for other output valves. It is, however, important that the component used should be of suitable type with a low DC resistance and a small value of leakage inductance. Since the characteristics of Class B valves are similar to those of pentodes, it will usually be found that a resistance-capacity tone-correction circuit across the primary of the output transformer is needed to avoid undue stressing of the upper register.

Because of the grid current flow, the Class B valve has a low input impedance, and it is usually. necessary to feed it from the preceding LF, or driver, valve through a step-down transformer. The ratio can be calculated by dividing the load required by the driver valve by the grid-to-grid input impedance of the Class B valve and taking the square root of the result.

In a few cases it will be found that this leads to a step-up ratio instead of the usual step-down, and it should be noted that the figures for transformer ratio in the columns are arranged to show this automatically. In every case the first figure refers to the primary, thus, 2:1 ratio means that the primary has twice as many turns as the secondary and that the transformer is of the step-down type. A ratio of 1:2, however, means that the primary has one-half the secondary turns and the transformer is of the step-up type.

Class B working is primarily intended for the battery user, and it does not go happily with mains operation as far as the smaller class of valve is concerned. One or two types for mains operation are making their appearance, however, and they are intended chiefly for cases where a very large output is required, such as public-address equipment.

Class B amplification is now faced with a serious rival in the QPP valve, this consists of two pentodes built into the same bulb and operated in push-pull with a large grid bias so that the quiescent anode current is small. The requirements of the output circuit are not dissimilar to those of a Class B stage, but as grid current is not permitted, the input circuit can be of any standard push-pull type and it is customary to employ a transformer of high step-up ratio.

Rectifying Valves

Little need be said about the rectifier for the supply in an AC receiver, since the questions arising are well known. It may be remarked, however, that indirectly heated cathode types are now common among the specimens rated for the higher voltages and that they are well worth consideration. By their use it is possible to avoid the necessity for including a thermal delay switch in order to ease the strain on the smoothing capacitors. The figures given for output assume in all cases the use of a 4 μF. reservoir capacitor and this should in general be rated for working at not less than 1.4 times the RMS AC input voltage. Thus, the capacitor used with a 500 Volt rectifier should be rated for at least 700 Volt working.

Many new types of rectifier with high heater voltages are to be found and these are intended for use in Universal type sets in which the valve heaters are series connected and no mains transformer is employed. Many of them are of the half-wave type, but some contain two anodes and two separate cathodes. These valves can be used as two separate half-wave rectifiers, or as a full-wave rectifier or a voltage doubler on AC supplies only.

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