The VALVE—What it Is—and Does

Radio Record, Volume V, Issue 2, 24 July 1931, Page 6

The VALVE—What it Is—and Does

Characteristic Terms Explained

sew INCH valves are very different 4:from one another, we must "ave some means of illustrat‘ing the points of difference. so with each valve is a set of'characteristics, A typical one accompanies this article. Letius go through the items and see what:they all mean, The most important undoubtedly is the filament. or heater yolts. If we do not know this we can ruin our valve in a fraetion of a second. In the "filameut"’ in the case of battery valves and the "heater" in the ease of uc, valves is the first of the three elements in the Average valve. It is a piece of fine wire which, when hot, emits electrons--they are the little things that do. all the work in the valves. In battery valves the electron emitting substance is heated directly with the current. With the a.c. valves the substance is close by the wire that carrics the heat. ing current. This body is known as the "cathode," whereas the wire carrying the current is known as the heater. It is the same as the filament of the ordinary valve. The filament in the ordinary valve, remember, incorporates both: cathode and heater. This filament or heater has a certain resistance and if a higher voltage is used than the- manufacturers recommend, more current will flow through than is supposed to, but it will only carry a certain current with the result that ii boys out and the valve is useless. Plate Voltage. pam plate (anode) voltage is that which is applied to the third electrode, the plate or anode. As the direct current has to pass from the plate to the,filament, across the vacuum, while not. aS critical as plate voltage, the maker’s recommendation should not be arently exceeded. Grid Volts. pits relates to the number of negative volts (usuaily small in comparison with the plate voltage) that are applied to the third element or grid of the valve in order to keep it charg éad" tegatively. By so doing we ca» control the electron flow. This is im-

portant if quality and economy are desired. . Sereen Grid HIS relates to the voltage to be use’ on the extra grid in the speciai sereen grid, variable mu or pentod. valves. It is sometimes referred t* as the auxiliary grid. It, like the plate, completes the circuit through the vacuum of the yalve. Pilate Resistance. HIS refers to the resistance or impedance the valve offers to current passing between the plate and the filament. The impedance of a valve can always be found by dividing the change in the plate voltage with the change in plate current, Plate Current. TPHIS indicates the amount of curreft that the valve will pass from the

plate to the filament when the recommended plate voltuge is used. The same applies to current," but referring, of course, to the filament. Mutual Conductance. AS, the grid of the valve is made ore negative with respect to the the plate current decreases, and vice versa, and it is the ratio of changing grid voltage to plate milliamps which denotes the mutual conductance. Let us refer to the characteristic curve reproduced on the next page. The vertical lines represent current flowing in the plate circuit and the horizontal "ines grid bias volts. Thus with this valve when there are 8 volts bias five wils will pass if there are 150 volts on the plate, and one mil if there are only 100 volts on the plate. From the {50-volt curve we see that by applying 4 volts bias we reduce the plate current from 16 (at 0 bias) to 10 (at 4). Thus 4 volts bias reduces the current by 6 mils. Therefore the mutual eonductance in 6+4=1.5. It is expressed in milliamps per volt, or ma/v. The Americans use the term micromhos. The mho, which is the reciprocal of an ohm, is measured by dividing the amplification factor by the impedance. It is usually expressed in micromhos-a millionth of an ohm. To calculate conductance in micromhos

multiply the amplification factor by @ million and divide by the impedance. This is sometimes referred to as transronductance. . Amplification Factor. THis is.the ratio of a set of grid and plate voltages. to plate current. For instance, referring to the eurve, we see that at no grid volts and, 150 plate volts our valve. takes 16 mamps of current. If now we add 5 volts bias, we reduce the plate current to 8. This is the same as 100 volts without bias. In other words, 5 grid volts are equal to 50 volts on the plate, therefore the amplification factor is 10. . Impedance. tHE impedance of a. valve is ascertained by the ohms law, which states that the value of current flowing in a circuit measured in ‘amps is equal to the ‘voltage dropped in that circuit divided by. the resistance (in ohms), or, putting it down more clearly, C=E--R. ' This can be applied to the valve, but we must take figures relating to changing current It will be seen that at zero grid volts an increase of 50 volts on the plate (for 100 to 150 volts) brings about an increase of 8 mamps. (8 from 16), so from rewriting our formula we get 50 x 1000-8, Which gives us an impedance of 6800, Choosing a Valve. HAVING briefly dealt with the main characteristics of the valve, it is necessary now to consider the points one has to bear in mind when a valve has to be chosen, First, there is the filament voltage, and this depends entirely upon the means at our disposal. If we have a six-volt accumulator we » will use 6-volt valves. If we have a _ 2.5 power transformer we will use 2.5. ac. valves. The next is the filament current. This is not at all important ' in a.e. valves providing the secondary of the power transformer is wound with sufficiently heavy wire to carry the current required. With d.e. valves. . one has to depend upon an accumulator and a dry cell, therefore it is important. Especially if dry cells are to be used, valves taking more than .06 to .1 should not be used. The .1 valve will place a very heavy drain upon dry cells . if more than two are used. There are very many ~.lves now with characteristics below .US filament amps. The last valve is rarely a .06 type, and almost invariably requires at least .1 (.06 type L valves can be obtained in 2, 4, and 6 volt filament voltages, and of Bnglish, Continental, and American makes). It

might be mentioned here that in replacing the 201’s in an American set it is not a bad plan to use the 221 valves, which take much less current than the 201A. A 201A can always be placed without any other changes in the cireuilt. The next consideration is plate voltage, and this again is governed by the B voltage available. Try to get as near as possible to the manufacturers’ recommended voltage. Some valves, particularly the battery valves, can be

Characteristic Curve of 2 Valve. worked-on very low plate voltage. A detector valve will work quite well, and sometimes better on 223 than on the higher voltages. A power valve and other audio valves will require very much higher voltage, and if you can give them 120 or 185 volts from batteries, so much the better. ‘It is not a bad plan in a set using screen grid r.f. valves to apply the same voltage to the plate of these as to the plates of the power valves. It is also possible to apply this high ‘voltage to the plate of the first audio valve, providing, of course,. that it is suitably biased. If it is not biased there will be too great a current flowing through the primary of the second audio transformer, and it is likely to burn it out. Amplification Factor. O not imagine because a valve uses a high amplification factor that it can pe used with safety in your circuit to give a bigger lift. It must be matched into the following circuit; that means to say, the valve impedance must be at Jeast at half of the impedance of the circuit in which it is being matched, Impedances, however, vary for different frequencies, and it is always a good plan to match at the low frequencies. itis interesting to note here that the dynamic resistance of a good coil at about 720 ke. is about 50,000 ohms, and the better the coil the higher the impedance. From this it is quite clear that low loss circuits should be used in conjunction with valves such as the screen grid type, which have a very high internal resistance. Increase in gain will also increase the selectivity. Power Valves. T is the function of a power valve te convert the audio voltages applied to its grid into variations of plate current: of sufficient strength to work satis- \ the type of speaker connected to it. A power valve rarely gives a high lift, and very often putting a power valve in a circuit will actually lessen the amplification obtained. The performance of a power valve ts indicated by its rating in watts. Two power ratings are applied to a power

valve, the plate input being the produce of the plate voltage, and the plate current in amps and the undistorted output, The calculation of the first is easy, but that of the second necessitates consulting the characteristics curve of the valve. When tuning a power valve the first thing to consider is the amount of battery power available. It is useless using a good power valve requiring, say, 200 volts, and expect it to be satisfactory when you only have a small battery delivering, say, 100 or 120. Furthermore, power valves require 2 heavy plate current, and if you use this in evnjunction with a light battery that battery is going to have its Nfe shortened considerably. Probably the best valves for the smull Sets to use are 605 type, with about a quarter watt output. These can be worked satisfactorily on 100 volts, and require plate current of about 6 amps.

_ OE eEeEOwOooreres, rere ee ee eee eee Oe Furthermore, they have a high amplification factor, which is also of a great advantage when using batteries.

Characteristics of the New Variable-mu Valves

Heater volts eceseoagcesaescod 2.5 Plate volts .....000000000 180 Grid volts ....00 occccce Screen grid volts .......0 Plate resistance, ohms 350,000 Mutual continctance (inicromhos) 1.200 Mutual conductance at -~40 volts bias & Mutual conductance at -50 volis biag — 4 Plate current, m/as. .... 5.8 Screen grid current, m/as, 2.5 Amplification ..cccccocoo 38S e. auien arn co

New and Better Valves oes

The Variable Mu INCE the very first days of broadcasting the aim of radio engineers has been toward high amplification before the detector, and when the sereen-grid valve was introduced it was hailed with delight, for it enabled much greater sensitivity than was hitherto possible. The use of the s.g. valve brought about the improvement in selectivity, improvement in tone, and the use of automatic volume control and these advantages led to the almost universal. use of this valve for high-frequency amplification. In using the:screen-grid valve, however, there are certain disadvantages, the most important of these being the cross talk and modulation distortion on powerful signs, resulting from the’ high amplification provided by the valves. Furthermore, the screen-grid valve can be easily overloaded. It is a valve with high mutual conductance and high amplification. However, it cannot ¥andle a big signal without cross modulation. and harmonic distortion. For this reason the control of volume on 4 powerful local station is "somewhat of a difficult problem. Local distance’ switches and preselection circuits are only two of the various methods that have heen employed to overcome this. If power valves could be employed in the first stages of the set we could (Concluded..on. page. 31.)

NEW and BETTER VALVES |

THIS will Surprise YOU

A. Valve's Life is Short and Merry

kk (Continued from page 7.) tune in the local station without overloading, but then we would not have sufficient amplification for the distant stations. Furthermore, the rate of current would be excessive. Engineers conceived the idea of uniting.a power valve and a high gain screen-grid valve in the one, and they have evolved what we now call the variable-mu valve, or as the Americans call it "the multi-mu tube." This type of valve has at the present time several numbers, though it is expected that before long it will be standardised. One maker uses the number "235" and at this, we will leave it. ¥undamentai Character. THE fundamental character of the multi-mu or variable-mu valve is the variation in the mutual or transeonductance with -grid-bias,: that providing the. valve amplification is controlled, not by varying the screen-grid voltage as before, but by varying the control grid-bias (the real "C" bias) between 8 and 50 volts. (Maximum volume is obtained with "C"* bias of -8 volts and a minimum at -50. Now to those who know anything about valves it will be apparent that when biased with -3 on the cathode, the vaive is operating as an ordinary sereen-grid amplifier, but when a bigger voltage approaching 50 is used, it is operating more in the manner of a power valve. .

. Mechanical Construction. EFORD considering the operation of this valve in more detail, we shall look for a few minutes at the mechanical construction. In ordinary valves the structure is uniform and a constant mutual conductance factor (mu) is obtained over the cathode area, bnt the new valve provides a mutual conductance factor which varies from point to point of the cathode area. The control grid is divided into two sections which are mounted with a gap between them, but for low ‘negative biases the entire cathode is in’ opera-

SE AE PE A EE EE 00 EE ee ee ee tion and the valve has about the same characteristics as it would if the gap were not present; in other words, like: an ordinary screen-grid valve. If the grid bias is increased negatively, the . electron current to the upper and lower . parts of the contro] grid are cut off, * leaving a low mutual conductance control through the gap. There are, however, very many methods in which" the control grid may be adapted. The variable mutual conductance factor, however, is generally brought about by a gap in the control grid. ,

‘ Cross Modulation. WN the ordinary screen grid valve the signal, in passing through, becomes distorted, because of an increase in the modulation of the signal. This distortion is most evident when a powerful signal ‘is tuned in. It is found that no more than .2 to .4 of a volt a.c. can be applied to the grid of the 224 valve, whereas in the 235 type, 1s large a voltage as from 4 to 10 may ‘be applied without introducing distortion..’ This new valve, then, can handle a signal without distortion some 20 times as great as that which the 224 can handle under similar conditions. Jn the ordinary valve the distortion becomes worse as the input to the valve increases, and this is particularly evident in the 224 type owing to sharp plate cut-off. Cross-Talk. (SROSS-TALK is commonly caused by the inter-modulation in the r.f. valves between the signals to which the receiver is tuned and a strong incoming signal of a different frequency, thus, when tuned, say, to 2BL on a modern set, operated in Wellington, we ean hear 2YA in the background, yet between 2BL and 2YA there is a large dead space. In order to diminish this cross-talk it is necessary to use one or more pre-selective circuits ahead of the first valve, and these cut down the incoming signals to a very large extent.

To compensate for this we must turn our valves up so that they are magnifying at their maximum. The result is that a strong hiss is heard in the speaker. This hiss can be kept down only by operating the valve well below its maximum. As the 235 valve can handle this strong current from the local station without this pre-selection direuit, a normal aerial is used and less valve hiss results. Application to Present Receivers. AS the variable-mu valve is of almost similar characteristics to the or-

dinary 224, it can be used in @ socket designed for that valve. However, best results cannot be obtained because of the provision of only a small amount of grid. bias, whereas bias must be variable between 3 and 50, if the best results are to be had. -- tn ot Experimenters can use the valve, for © it is necessary only for them to put in a higher valued resistance in the cathode of their screen-grid valve. The volume control for a tuner can be a 20,000 ohms wire-wound potentiometer. When using 285 valves bear in mind that the plate current drain is slightly higher than that of the 224 type, and that when the volume is controlled by varying the control grid the plate current will yary considerably. For this reason it would be really unwise to fit a set designed for the 224 valve with 235 valves without making any alteration to the circuit.

Advantages to be Untamed _ ~ Using the 235 Valves. 1. An-increase of the maximum alk lJowable input. voltage for distortionless operation, -- 3, Extension of the range of auto matie volume, control. : 8. Reduction of cross-talk by a, factor of several hundred times. ‘4, Improvement in uniformity of control over the entire range of volume control. Cone . 5. Reduction of hum or carrier due to incomplete power pack filtering. _ 6. Reduction ‘in receiver noise due ‘to the possibility of working the valve below its maximum point. The slope ean be varied from .08 mamps per volt and the low mu end to approximately 1 mamp per volt at the high mu end, or, expressing this in the American terms, 5-immbhos at the lower end and 1000 mmhoms at the high mu end. The 224 varies from 1000 at the high mu end to 100 at the lower mu end where the valve becomes inoperative.

His iabie will give some idea as to whether or not your valves want renewing. Glance along the vertical scale until you see the number of hours you use WOUR set daily, then travel along the line crossing the graph until the curve is met. Now go down to the bottom and see how long your valves should last. If they have heen in longer

than this they want renewing. ~ ¥or example, imagine that on three days a week the set goes on at 10 a.m., is shut of at 10 p.m., with four hours of quiet during this time. That leaves eight operating hours. During the re- °

maining hours: it is turned on at 6 and of at 10-four hours. The average for a day is approximately six hours. Referring to the chart, we notice that our valves want re-. newing after six months-that is if only the best is wanted of them. Of course they will: go far longer than that, but try another set and note the difference, _

Kew vatves are gooe aiter uriecn hundred . hours’ service. Just jot ‘down the hours YOUR set is in use this _week and divide the number into 1500 and see how many weeks your valvesare good for. You will be surprised. Just try it and see!