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Choosing a Valve

J B Dance MSc, Radio Constructor, August, 1960.
    
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The number of different types of valve from which a constructor can choose has now become so great that it is only possible to discuss some of the more important points in this article. A valve has to be chosen not only for its electrical performance but also for the ease with which it fits into the apparatus in which it is to be used. Obsolete and replacement types should be avoided in new equipment as difficulty may be experienced in obtaining replacements at some future date.

Forty years earlier the choice was simple - use an R Type

General Remarks

One of the most important characteristics of a valve is its heater voltage, or, in the case of AC/DC. equipment, its heater current. In equipment using a mains transformer, all of the valves are connected with their heaters in parallel and therefore the heater voltage of each valve must be the same; the most common value is 6.3 Volts. When this type of heater connection is used the current taken by each valve may be different. Power valves, for example, require more heater power because they have a larger cathode which must be kept hot; this extra power is obtained by taking more current than the other valves. Directly heated valves with 1.4 Volt heaters are made for operation from dry batteries; the heater power required is usually very small. In equipment for AC/DC. operation a mains transformer is not used, the valve heaters being connected in series. In this case the heater current for each valve must be the same (often 0.3 or 0.15 Amp). Valves which require more power than the others have a heater with a higher working resistance and therefore a higher voltage is developed across the heaters of these valves. Thus the PL82 has a 16.5 Volt, 0.3 Amp heater. The heater voltage supply to any valve should be correct to within 5%.

Care must be taken that the maximum heater-cathode voltage rating of a valve, as indicated in the appropriate data, is not exceeded. Many rectifiers have high heater-cathode voltage ratings. Twin triodes which have a high heater-cathode voltage rating are useful in certain phase splitters.

Miniature valves may have advantages over the larger types at high frequencies because of the shorter lead lengths to the electrodes. Physical size, availability and cost are other considerations when deciding between miniature and other valves. Miniature valves are now used by almost all commercial manufacturers except possibly for power valves and rectifiers. The fairly new sub-miniature valves (having lengths of 28 mm and diameters of 5.4 mm) may become very popular in the future, but at present they are mainly used in large commercial equipment.

The choice of valves for the various stages of radio receiver will now be considered.

RF Amplifiers

If it is likely that an RF amplifier will be required to handle a large signal at any time, a variable-mu valve with appropriate automatic (AGC) or manual biasing should be used in order to avoid overloading. On the other hand, if it is desired that an RF pentode amplifier should generate the minimum possible amount of noise, a sharp cut-off pentode should be used. Sharp cut-off pentodes can normally be made with a higher mutual conductance than variable-mu valves and therefore tend to generate less noise and to give a higher gain. Even less noise is obtained from grounded grid triodes, but the amplification is much smaller. Other special low noise circuits such as cascode amplifiers are also available. Pentodes such as the 6AK5 (EF95) and the 6AG5 have been designed for use in low noise RF stages operating at high frequencies. They have a fairly high mutual conductance and a very high input impedance - the two most important qualities for a very high frequency amplifier. In the two types mentioned, two separate cathode pins are used so that the input and output circuits can be kept entirely separate.

Converters (Mixers)

The main considerations concerning the design of a converter stage are (a) the amount of noise generated by the valve, (b) the means to be employed in avoiding pulling of the oscillator frequency and, possibly, (c) what gain will be obtained. The gain is determined by the conversion conductance of the valve. The multigrid mixers such as the 6K8 triode-hexode, the 6BE6 and 6SA7 heptodes are usually used as self-oscillating converters in broadcast receivers and generate a relatively large amount of noise. Very little pulling of the oscillator frequency occurs with a well designed multi-grid valve; this type of mixer is also the most economical with respect to components used in the circuit.

Multi-grid valve converters have a conversion conductance of about 0.3 to 0.4 mA per Volt. A conversion conductance of about 2.5 mA per Volt can be obtained by feeding the oscillator voltage to the cathode of a pentode mixer, or alternatively, to the same grid of the mixer that the signal is fed. This method gives much higher gain and much lower noise than when multi-grid converters are used, but an extra buffer amplifier should be used between the oscillator and mixer or pulling will almost certainly occur. Three separate valves (oscillator, buffer amplifier and mixer) are therefore needed in this type of converter; this may be well worthwhile in a good communication receiver. (In television and FM tuners a buffer amplifier is not, however, employed.Ed.)

IF Amplifiers

Variable-mu pentodes are almost always used in the IF stages of a radio receiver because the receiver gain as a whole is controlled almost entirely by the IF gain. If the IF valves have a high value of anode resistance, the tuned circuits will be only slightly damped and the selectivity may be made as great as possible. The higher the mutual conductance of an IF valve, the higher the gain of the stage. If a valve is chosen so that its screen grid is designed to operate at the same voltage as its anode, fewer decoupling components will be required. Sharp cut off pentodes (i.e. not variable-mu), are often used in broad band IF amplifiers for use in radar, television and FM receivers.

Detectors

Most small diodes give very similar results as detectors. Any small triodes, or triode connected pentodes, are suitable for use as cathode follower detectors. Diode detectors load the previous tuned circuit and thus reduce its selectivity, but the load imposed by cathode follower detectors is very small (hence the name 'infinite impedance detector'). Cathode follower detectors should be operated at a fairly high signal level. The choice of valves for these two types of detector may be made solely on physical size, preferred base, etc. The leaky grid detector virtually consists of a diode combined with an AF amplifier although no separate diode is employed. Leaky grid detectors are very sensitive to small signals, but the quality of reproduction obtained is poor and considerable loading is applied to the previous tuned circuit. A triode or pentode may be used.

Radio Audio Amplifiers

Valves for use as very low level audio amplifiers should generate the smallest possible amount of noise, as any noise in the low level stages is amplified enormously. It is also desirable that low level audio amplifiers should have a bifilar heater construction to minimise hum. The valve holder should be made of a good electrical insulator such as PTFE or hum will be introduced by capacitative coupling through it. It is most important that microphonic valves are avoided in low level stages and good screening is essential. The 6BR7 and the EF86 have been specially designed for use in low level audio amplifiers. For the very best results it is advisable to pick out one valve from several of the same type. A valve with a very small reverse grid current should be chosen.

Audio amplifiers for use in stages at higher signal levels should have very linear characteristics or distortion will occur. Triodes tend to give much less distortion than pentodes at the higher voltage levels; negative feedback is essential for reasonably good reproduction when pentodes are used. If a high gain is required, an audio amplifier should have a large amplification factor. Pentodes give much higher gain than triodes.

Audio Power Amplifiers

Valves required to give an appreciable power output have a lower anode resistance, a much greater maximum value of anode dissipation, and a greater anode current, than valves intended for use as voltage amplifiers. The valve chosen for the power amplifier must be capable of giving the amount of power required at a certain permissible distortion level. Small amounts of power may be obtained by using valves which are normally only used as voltage amplifiers. The impedance of the device (speaker transformer, etc.), into which the power is fed must match the optimum load impedance of the valve fairly closely. When deciding on the power output required from a high quality audio amplifier, it should not be forgotten that the peak power is much greater than the average power. The amplifier must be able to handle the peak transient power or distortion will occur. A valve with a high slope (mutual conductance), will give a large voltage amplification; that is, the voltage input to the amplifier for a certain power output will be smaller than if a low slope valve were used.

Beam tetrodes and pentodes can usually be made more efficient in operation than triodes, but triodes give less distortion for the same amount of negative feedback. At high power levels triodes need a higher HT voltage than tetrodes. Tetrodes may, of course, be triode connected if desired. The method of operation known as 'ultra-linear' is popular. In this method, the valves are effectively operated somewhere between triode and tetrodes by connecting the screen of the valve to a tapping on the primary of the output transformer. Ultra-linear amplifiers are almost always of high fidelity standards and are therefore push-pull.

A power amplifier often takes as much current from the HT line as all of the other valves put together. When the amount of HT is limited (as when battery supplies are used), it is important to choose a power valve which is economical on HT current and which does not give much more power than is likely to be required.

HT Rectifiers

A heater supply of the correct voltage must be available for the rectifier (except in AC/DC equipment); the heater power required by rectifiers is usually comparatively large. The 6X5 (octal), and 6X4 (miniature B7G) valves are sometimes useful because the 6.3 Volt supply for their heaters can normally be taken from the commonly used 6.3 Volt heater line feeding the other equipment. These two types of valve are limited to a maximum DC output of 70 mA, however, In most other types of rectifiers the cathode is connected to the heater and a separate winding on the mains transformer is necessary for the rectifier heater.

It is usually advisable to choose an indirectly heated rectifier where possible so that, after switching on, the HT voltage is not applied to other valves until they have warmed up. This also prevents many of the capacitors in the equipment from having to withstand a higher voltage than normal just after the apparatus has been switched on.

The rectifier must have a peak inverse voltage rating which is larger than any inverse voltage which will appear across the valve. In the usual type of full wave rectifier circuit, the peak inverse voltage is 1.41 times the RMS (root mean square) voltage appearing across the whole of the secondary HT winding of the transformer providing that capacitor input is used. This is equal to double the HT output voltage at no load.

In capacitor input filters there is a limit to the maximum size of reservoir capacitor which can be used. In choke input filters there is a limit to the minimum size of choke which can be used. These values are normally given in the valve data for the rectifier concerned. If they are not observed the peak cathode current rating of the rectifier may be momentarily exceeded during each cycle.

Combined Valves

It is often possible to reduce the size of equipment by employing two or more valves in the same envelope. Triode-hexodes are often used as oscillator-mixers and double diode triodes as combined audio detectors, AGC detectors and audio amplifiers in broadcast receivers. Choice of these valves depends on the electrical characteristics of each separate part of the valve. Care should be taken to avoid undesired feedback from one section of the valve to another. Double triodes are particularly useful for a wide variety of applications, but the uses of some double triodes (e.g. 6J6 and 6SC7), are limited because there is only one common cathode for the two triodes.

Voltage Stabilisers

A simple gas filled voltage stabiliser tube must be chosen for adequate stability, the current ratings being suitable for the circuit concerned. Stabilisers should not be connected in parallel in order to increase the maximum permissible current for the reason that one of the tubes would probably then take all of the current. Stabilisers may be connected in series, however, in order to increase the voltage of the stabilised supply. The VS110 and VR105/30 are quite suitable for stabilising the HT supply to an oscillator

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