The Choice of Power Valves

Radio Record, Volume III, Issue 10, 20 September 1929, Page 10

The Choice of Power Valves

Meaning of Undistor ted Power |

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HE advent of the all-elee tric set has to a very great extent solved the problem of power valves. _ Prior ‘to this, the last valve in a multi-vulve

set had to be more or less economical in both "A" and "B" current, with the result that certain compromises had to be made.’ This usually amounted to a reduction of the "A" current and "B" voltuge; but the all-electric set has rendered economy in these directions unnecessary. The primary function of a power amolifier is to supply undistorted power tv a loudspeaker, in amounts suliicient s that the speaker can give undistort- «. reproduction with the amount of volume desired by the listener. Power valves and power amplifiers are essential for satisfactory reproductiou because of the low efficiency of toudspeakers. This lack of efficiency necessitates that the speaker be supplied with perhaps fifty times as much power as it will finally radiate as sound. If the power valve has not sufficient capacity to supply the required. amount of

undistorted power, ethe speaker has either to be operated at a volume level lower than desired (so that less power is drawn from the valve), or the power valve hag to be overloaded. Under these conditions serious distortion is produced. . Distortion makes itself evident by a very large number of symptoms, and all but the most untrained ear can pereeive this condition, With the magnetic cones and horns a rattle indicates that the speaker is being. overlouded, but with the dynamic cone spenker, overloading is almost impossible under normal household conditiohs. Overloading of the power valve. shows itself generally in harshness. lack of clearness, confusion of both lower and upper notes, and a tendency to blast. Types of Power Valves. HERE are now available the following types of power valves: 1124, 171A, the 245, 210, and the 250, All of these can be used singly or in pushpull, and there are consequently ten

possible combinations. The. problem arises in determining which combination will be chosen by constructor or purchaser. It will be noticed that seyeral types of valves have been given. These names apply to Radiotron valves. but almost every manufacturer makes the equivalent, and the onus is on each te gg fg ew

«MRR URE CRERES SEER ESATA SR ER EERE SRE ERS RSE SERRE RBA consiructor to find out the designation of these valves in the make he prefers. Table 3 shows a few of the betterknown examples. _ In deciding which power valve or combination of power valves to use, the first point to be determined is how they compare with regard to power output. mens

In Table I has. been listed these types of valves and their combinations indieating the power output at various voltages. The table has been arranged in order. of power output. In all cases it has been assumed that the power output to be obtained from two valves in push-pull is equal to three times that obtainable from a single valve operated at the same plate ‘voltage. -_ To bear this out we see-that the out-

PEGRSESTSHSSS ESET ESS SSA Se Rees enero put of a single 171A operated at 135 volts is .88 watts, while two 1714’s in push-pull ‘give an output of 1 watt. The wattage is computed by multiplying the voltage applied to the valve by. the milliamps. emission,.and then dividing the equation by 1000. An example will serve to make: per- _

power amplifler capable of supplying 3 watts of power. Since the amplifier must supply at least this, ‘all artangements up to No, 15 in Table 1 may be disregarded. ‘This latter will supply 8.2 watts, and will be sufficient for our purpose. We sée, however, that it requires 400 volts on the plate, and this necessitates the construction of a big transformer, Glancing down the column of plate voltages, we come to 250 volts as-being required by two 245 type valves operating in ‘push-pull. This gives us an output of 4.8 watts, which is ample for. our F PUTDORE, Be-

cause of the comparatively low plate voltage, it is passible to effect a considerable saving in the construction cost, In addition, there is much less danger of filter condensers breaking down, and 250 volts ig much less dangerous in the eage of accidental shocks, than is 400. . We would, then, feel more disposed to construct an amplifier using two 2435's 1n push-pull rather than either 250 singly, or the 210s in push-pull. ~ Output Expressed as Volume. For some time reference has been made to the various power outputs, and it will be worth while to translate "power output" in the terms of volume of sound. The following is. about the average: Power output up to .3 ot .6 watts will give fairly low reoni volume; about 15 -watts. gives very good room volume; and 5 to 10 watts gives volume sufficient for a hall, It ig unwise to build an amplifier to give an output of less than,.3 watts, so in designing a power. amplifier we must consider either the outputs .3 to .5, 5 to 1.5, or 1.5 to 3 watts. For usual home use for ‘Wireless set or gramophone such as in the amplifier described in another section of this issue, two 17iA’s (or their equivalent) in pushpull can be used successfully, These valves;were used in the original model, and were found to give ample volume from ‘either gramophone or crystal set on the local station. A second possibility for this range is the use, of a single 245 type with 200 volts on the plate, but the advantage of the two 1T1A’s with 135 volts is quite evident, For a greater outpyt, 1.5 watts, a single 245 valve may be used with 240 volts on the plate. By increasing the voltage on the 171A’s to 180, two watts may be déliyered comfortably. From 2 to.3 watts of undistorted power may be supplied by two 245’s with 250 on the plates. For higher outputs, use twe 250's in push-pull. -This position has been summarised in Table 2°: From the following discussion, it: has become evident that the 210 type yalve does not appear.’ This valve did not enjoy great popularity in New Zealand. It was used in America when

" AC. amplifiers first became popular, This valve is capable of supplying ap-. proximately 1.5 watts at a platé voltage of 425, whereas a single 245 type can supply 12.6 watts with only 250 volts, It then seems hardly feasible. to use the 210 with all the difficulties, and expense of constructing a sufficiently powerful amplifier, Another interesting fact is that the 245 valve can be used to supply up to 5 watts of power, and this is sufficient for a small hall, Another interesting point is that the power valves have been limited to the 171A for general

house. purposes, the 245 tor slightly greater power and hall. use, and the 250 for still greater power, The 112 valve is also absent, This is a medium-power ‘Yalve that is hardly used because. whilst ample power. is- available, one might as well use the 171A, and obtain the greater output.

Push-pull y. Single Amplifier. OME years, ago, push-pull was very fashionable. With the coming of the super-power valves it then lapsed, but now, with the popularity of A.O, ‘operation, it is coming back into greater use, Among the reasons for its popularity are increased power output, Jess. hum with A,C.-operation, and less distortion. Distortion in’ a power amplifier fre--quently takes the form of the introduction. of new frequencies-harmonics, they are called. From experiment, it has been found that the maximum permissible harmonic output to be 5 per cent. The maximum that a single a in these tests could handle was .6 or . watts. At the same output, the valves in push-pull had only 7-10th of one per cent. harmonic, Therefore. if the single valve is always operated below .7 watts, we can get essentially distortionless output from the valve, but ‘with push-pull the-output may be raised considerably. When single valves are used in an amplifier it is essential that they have a power rating sufficiently high so as .to ‘prevent any possibility of overload-: ing. When push-pull is used the normal rating of the valves may more nearly coincide with the value required for good reproduction, because good push-pull valves do not produce serious distortion even when called upon ‘to

nee supply more power than their rating. Those who have been following the design of manufactured receivers will note this decided tendency towards this use of valves in push-pull. Because the home experimenter is always anxious to .construect a power amplifier giving the best possible reproduction, it generally is advisable to use this type of amplification, For this reason, the amplifier we have selected for this issue has been one embodying this popular and distortionless circuit, It may be used with gramo‘phone pick-up, ¢erystal set, radio and

detector stages from A.C, or a D.C. set, short wave adapter, or phones and radio umplifier- Believing that a separate amplifier is-of more service than the audio stages of a set; they have been separately constructed, and the A.C. Browning Drake-has been!

designed as a unit merely to feed this amplifier. In the writer’s opinion, it is not worth while constructing a straight-out all-electric set. Sooner or later, not only will a gramophone be introduced, but a short-wave adaptor will find its way into the home of the wireless enthusiast, and he will want a good amplifier apart from his set. The choice of valves for this set has now been made reasonably clear. It is merely a matter now of decision as to the amount of power required. Most of the power transformers supply up to 200 volts, and this renders them ad-

mirably suited to the 171A type of valve. This is all that can be desired for the ordinary home use, and unless unnecessary power is required, the writer Would not recommend the use of any other valve. Generally speaking, the following are the best known examples of the 171A. type of valve on the New Zealand market: Mullard AC4 and ' PM256; Philips C608, C605; Ce-Co J71A; Corror 610P; Osram P625A3 Six-sixty 620S8P. Some of these :re-" quires only 150 on the plate. The 245 Valve. ' JT various points in this article, we have: referred to the type 245 valve. Because this valve is not read--ily known, it is worth while to -give some data referring to it. The details of the original 245 are as follows: Filament voltage, 2.5 A.C.3 fil. current, 1.5 amps. A,C,; plate volt age, 180 to 250; plate current, 26 to 82 mills.; neg, G.B., 33 to 50 volts; amplific. factor, 3.5; impedance, 2000 ohms; power output, .75 to 1.6 watts, Other makers are producing this type of valve. We refer in Laboratory Jottings to the new Osram P625A. In summarising the characteristics. of the new valve, we can say that it is designed to supply fairly large amounts of power at medium plate voltages. We ure thus able to obtain comparatively large amounts of power from a. power supply of reasonabie cost. Regarding the circuits for the 245, we show in the diagram a suitable cite cuit, though it may be used in pushpull in the amplifier previously referred to. The grid bias resistances have alBo been given in this article. Note that in using a single 245, none of the circuit constants need be changed except the bias resistance. The question might be asked, "Can the power for this valve be taken from the 2.5 winding for the 227 valves?" It can, if:the wire comprising the sec-

ondary of the transformer for this particular purpose is sufficiently heavy. For two valves in push-pull, 3 amps. Will be needed, and each 227 valve requires 1.75 amps. If an amplifier alone is to be constructed, the total power to be supplied by this winding is a total of 4.75 amps., and to supply this, No. 14 gauge wire must be used. Where yadio and detector valves are to receive their power from this winding, it must be at least No, 12 gauge. There are mechanical difficulties in winding a transformer with heavy gauge wire, so "an extra winding: would: be preferable.

Arrange- Power Type Plate ment Output Of Single` or Voltage No. in Watts: Valve. Push-Pull: Required; .120 1124 Single 135 2 .195 1124 Single 157 3 .330 1714 Single 135 360 112 Push-Pull 135 0 .60() 1124 Push-Pull 157 .800 210 Single' 300 G :700) 1714 Single 180 750 245 Single 180 8 1.0 1714 Push-Pull 135 10 15 210 Single 425 11 1.5 250 Single 300 12 1.6 245 Single 250- 13 2.1 1714 Push-Pull 180 14 2.25 245 Push-Pull 180 15 3.2 2,70) Single 400 16 45 210 Pusb-Pull: 425 17 4.5 250 Push-Pull 30 18 4.6 250 Single 450 19 4.8 245 Push-Pull 250 20 40 9.6 250 Push-Pull 400 21 13.8 250 Push-Pull 450

Table No. 1.

> ‘SV atta Rest arrangement output, 6 une, 0.5 oy less .... One 171A at, 180 volts 0.5 to A penee U71A’s) in | push-pull ‘ . With 185 volts; or a og single 2435 with about 200 volts on ° the plate 1 to 15 11... Single 245-at 250 volts 15 to.2 «e000 ITLA’s in push-pull at « 180 volts : 2 to SB w.cseee « 245 valyes in push-pull Higher powers . 250 valyes in push-pull

Table 1.