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New Valves for Receivers

F E Henderson Wireless World, November 30, 1934.
    
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One of the problems of the listener today is to know how to keep up to date. It is easy to say relegate to the scrap-heap any set when it begins to get obsolete, but considerations of economy often decide otherwise, and it then becomes necessary to see what can be done, even if we may have to compromise a little, to effect improvements. The purpose of this article is to guide the listener and discuss valve changes which are permissible whilst giving warning of changes which might result in trouble.

There are a vast number of listeners unwilling to scrap the receiver which they know how to handle, gives them satisfactory service, and adequately meets their particular requirements. On these occasions the user feels that the introduction of one or two more modern types of valves should give the set an increased efficiency either by a saving in running costs or better performance. Perhaps also such a receiver has been prone to microphonic or ringing noises associated with the design and types of valves available at the time the set was produced, and some improvement is called for in this respect.

Illustrating (a) simple construction of original DER triode. The low efficiency and heavy filament enabled simple design to be used, without danger of introduction of microphony. (b) Early design of hairpin filament which led to microphonic troubles as the efficiency increased. (c) Modern design of battery valve filament, employing anchored and sprung supports to shorten length of free filament and maintain constant tension.

It is seldom practicable or possible to introduce new features in the way of components in bringing an set up to date, with the exception of the valves. Batteries, in the case of a non-mains driven receiver, are, of course, calling for constant renewal if the receiver is to be kept up to a certain standard of efficiency, but how often is the other replaceable component, namely, the valve, allowed to deteriorate before serious attention is paid to the renewal of this essential part.

Most valves give an exceedingly long working life if treated kindly, but, even so, are not everlasting, and sooner or later it is false economy to keep them in service, and the question of new valves calls for urgent attention. The problem then becomes whether to fit a valve of identical type and make to the original, or whether to attempt to use the higher working efficiencies of more modern types.

In the first place there is usually a greater flexibility in the choice of valves for a home-constructed receiver or kit set than for a manufactured receiver. This is because the set manufacturer usually designs rigidly around a given combination of valves both on account of physical size and electrical characteristics, and often for either of these reasons a departure from the types specified is dangerous or impossible. In any case it is usually good policy to refer the matter to the actual manufacturer concerned, or to the valve manufacturers, who can usually recommend a type suitable for the particular receiver in question.

In the case of the home constructed outfit or kit set it is certainly worth some consideration before re-valving, and to assist in this it may be helpful to analyse the various changes which have taken place in the design or characteristics of receiving valves during the past eight or nine years.

The principal features which affect the replacement of one type of valve by another are:

Size, type of fitting (base, pins, etc.), filament voltage, filament current, mutual conductance, grid bias, anode feed current, grid current characteristic, inter-electrode capacity, nature of characteristic - triode, tetrode, pentode, etc.

Apart from the first two considerations, which are purely physical and obvious ones, any or all of the remainder may take part in determining whether a replacement valve of another type may improve a set or render it unworkable.

Let us consider how each of these points in turn will affect performance and review the major changes in each instance over the past decade of valve manufacture.

Filament Voltage

Although now probably representing only a small minority, there are still users of battery sets who employ a 6 Volt accumulator, this being a relic of the days when 6 Volt valves were used in order to get a performance not then possible with valves of lower voltage. By the improvement of filament technique enabling a greater electron emission to be obtained from a modern 0.2 Watt filament than was possible for a 1.5 Watt filament of ten years ago, manufacturers, by common consent, have ceased the production of broadcast battery valves with filament voltages exceeding 2.0. It is, therefore, false economy to continue the use of 6 Volt battery valves with consequent heavy bulk and heavy charging costs of the accumulator, and a change to 2 Volt valves would be a good one.

Owing to the improvement in filament technique, actually a better performance can be obtained from a set of modern 2 Volt valves providing the correct types are carefully chosen.

A striking example of this improvement is given in the table which compares a popular line of 6 Volt battery valves in 1925 with 2 Volt types of modern design which will replace them.

A similar argument applies to 4 Volt battery valves which can be replaced by modern 2 Volt types with a reduction in the size and charging costs of the accumulator.

In the case of mains-driven receivers the filament, or, in this case, the heater voltage is, of course, fixed by the transformer incorporated, which normally would be wound for 4 Volts, being the figure standardised in this country for AC valves.

Filament Current

Since the introduction of the oxide-coated or barium technique to replace the thoriated filament, which in its turn replaced the bright emitting filament, there has not been any marked change in the filament current taken by any particular class of battery valve. Very few sets of to-day will be employing bright emitter types of valves, but there still may be users who have valves fitted with the thoriated filament. Such valves were often exceedingly long-lived, and may even yet be giving good service. It would, however, be an economical move to replace these with modern valves, but, here again, very great care must be taken in the choice of valve and characteristics owing to the greater efficiency of these over the thoriated types. To minimise the chances of instability a modern valve of moderately low impedance and low amplification factor should be chosen and worked at a fairly low HT voltage (except in the output stage), or disappointment may result.

A striking instance of the improvement in filament current efficiency resulting from changes in filament technique is afforded by a comparison of the types shown below:

In certain sets the volume control is effected by means of a variable resistance in the filament lead. Should a valve of lower filament current replace an older type the voltage drop across this resistance will be less and the control of volume may be adversely affected. The remedy is, of course, to utilise a higher value of variable resistance.

In the case of DC mains-operated sets where the valve heaters are wired in series at a constant current it is not practicable to introduce any change in this direction owing to the constants of the heater circuit.

A modern type of double helical spiral heater for mains valves, to minimise hum.

With AC valves the heater current is more or less standardised and no economy is worth considering in this direction.

The form of bulb shaping and mica electrode support anchored to bulb shown in this sketch is adopted in many modern valves to ensure rigidity and characteristic consistency.

Perhaps the most striking change in valve characteristic over the past decade is the rapid climb in mutual conductance which applies to every class of valve and is directly the result of improved emission efficiencies. This, while affording a means to greatly improve efficiency in performance when treated with discrimination, represents also the biggest difficulty in the way of introduction of modern improved valves to replace valves of older types. The reason is, of course, that the improvement in mutual conductance will probably result in an increased overall gain per stage and unless the receiver is designed with adequate screening and decoupling of the various stages this increased gain is reflected in feedback, giving rise to uncontrollable oscillation either of radio or audio frequency.

Great care must, therefore, be observed in the choice of modern valves, and in the case of most earlier set designs it is not practicable to take full advantage of the increased amplification that they offer.

Most valve makers, however, have a range of valves having modern characteristics in every other respect but so designed as to give the advantages of present-day technique in an older type set without introducing instability. This question is best considered under two heads:

  1. Valves for radio frequency, ie, HF and detector stages
  2. Valves for audio frequency, ie, LF and power stages.

On the radio-frequency side the effect of increased mutual conductance must be considered in conjunction with the value of anode-grid capacity. To take as an example a triode used as an HF amplifier (common practice until recent years) any attempt to introduce a valve in which the product of factors representing mutual conductance and capacity is increased will undoubtedly lead to instability. In a triode valve this state of affairs cannot be avoided, and hence with HF amplifiers using triodes (either aperiodic or neutralised tuned circuits) any improvement in the mutual conductance of the valve types hitherto used is impracticable.

In the case of the detector stage, the effect of any increase in the mutual conductance &mult; capacity factor will have its effect on the reaction circuit - usually an important section of older type sets, and if ganged circuits are used some retrimming of the condensers will probably be necessary.

Illustrating a modern development in battery valve design the HL2/K triode, primarily intended to reduce microphony, increase uniformity, and economise in space. The type is suitable as detector in practically all battery sets using triodes.

Renewal of the detector valve is, however, a move with strong recommendation and very often results in markedly improved range-getting properties. A valve of medium impedance is generally preferable, and the increase in sensitivity often allows a reduction in HT voltage, with resulting saving in HT current.

With screen grid valves, the problem of attempting to improve results with a modern valve is more complicated as so much depends on the lay-out and degree of screening provided in the set. It is usually impracticable to attempt the introduction of a valve having more than 1.5 times the mutual conductance of the original, unless the screening is very complete, but modern design screen grid valves with a restricted gain can be used, such valves often showing a reduction in HT current at the same time.

The following summary of advice may be helpful

Detector

 Use medium impedance valve.

 Reduce HT volts if reaction too fierce.

 Reduce value of grid leak if reaction "Ploppy"

HF Amplifier Triode

 Use valve of similar mutual conductance as in type originally specified.

HF Amplifier Screen Grid. One stage

A higher mutual conductance may be beneficial. Decrease screen volts if instability experienced.

HF Amplifier Screen Grid. Two or more stages

 Use valve of similar mutual conductance unless otherwise recommended by makers.

A material improvement can often be effected in a single-stage HF amplifier, but the advice of the manufacturers should be obtained before re-valving a multi-stage HF amplifier.

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