The Operation of the Five Electrode Valve

Radio Record, Volume II, Issue 30, 8 February 1929, Page 28

The Operation of the Five

Electrode Valve

EFORE discussing the advantages. of the pentode or five-electrode valve, it will be necessary to investigate the limitations and defects of the ordinary three-elec-trode valve when used as a power amplifier in the last stage of an audio-frequency amplifier. For a valve to function satis.factorily in this position, it is essential that it should be able to handle a large amount of power without distortion, and in order to be able to do this it must have a high mutual conductance, and must be able to ac.commodate a large grid voltage variation. The mutual conductance of a valve is the increase of plate current produced by raising the grid potential by one volt. It is, therefore, represented: by the slope of the characteristic curve of the valve, and the steeper the slope, the greater is the mutual conductance. English manufacturers usually express the mutual conductance or slope in milli-amperes per volt, whereas American makers use the micrémho — one micromho being equivalent to 0.001 milli-amperes per volt. Mutual Conductance. W!tH the three-eléctrode valve, the mutual conductance of the valve, measured under working conditions with a load (such as a loudspeaker), in the plate circuit, is considerably smaller than when measured under no load or statie conditions. It is the statie curve, measured with the "B" battery potential applied directly to the plate, that is usually published, and this, therefore, does not give a true indication of the mutual conductance measured under working conditions. Furthermore, with the threeelectrode valve, the greater the amplification factor of the valve, the greater the difference between the no-load mutual conductance and the true working mutual conductance. It follows from the above facts that to produce a_ three-electrode valve which will operate on a reasonable plate voltage, and yet have a high mutual conductance, the amplification factor must be sacrificed.. Thus it is that many of the usual power ‘valves have a factor of only three. The amplification factor of a valve is a ealculated factor that would only be obtained with an infinite impedance in the plate circuit, and under the best working conditions the amplification usually obtained from a power valve is about one-half of the theoretical figure. Such power valves,

therefore, add very little to the volume of sound, but act merely as a source of power to drive the loudspeaker. The Question of Overloading. ANOTHER point to be considered is that, since these valves amplify the signals to such a small extent, it is necessary that the second last valve should handle large variations in grid voltage. One of the difficulties of resistance coupling, with a valve of low

amplification factor in the last socket, has been the liability of overloading the preceding valve, owing to the relatively small grid variation that the high mu valves, usually employed, are capable of handling.

A still further disadvantage of the threé-electrode power valve is that the mutual conductance varies with the frequency of the note to which the loudspeaker is responding. The result is that the lower notes are amplified to a greater extent than the higher notes. In old-fashioned sets, with poor amplifiers and speakers, this fact may have been an advantage, but with modern apparatus, capable of even response over the whole musical seale, it is a decided fault. The Five Electrode Valve. FAviIne seen the limitations of the three electrode power valve, let us turn to the five electrode valve and consider how it compares with its ancestor. By the addition of two extra elements to the valve, the manufacturers have produced a valve with an amplification factor of one hundred, a mutual conductance of 1.8 milli-amperes per volt (which is greater than most power valves), and capable o. accommodating a grid swing at least as great as a 201A type valve. Furthermore, the mutual conductance on full load is just as great as on no load. This valve, therefore, is capable of handling more power than most other valves. In fact, it can deal with almost as much power as super valves of the 210 type,-and yet it can amplify the signais to equal or even greater extent than the best high mu valve previously produced. It cannot accommodate the same grid voltage variation as the usual power valve; but this is no longer needed when the amplification facwor is one hundred instead of three. for the valve can supply all the required volume without a large grid variation being applied to it. We saw that in the three electrode valve, the mutual conductance varied with the frequency. With the pentrode, the amplification is independent of thé frequency, and even amplification over the whole musical scale is obtained. Considered from all angles, the pentrode appears to be the ideal valve to be used in the last socket of any power amplifier, and a: detector valve, followed by the pentrode, will give results very nearly equal to the usual two-stage amplifier. To install the valve requires fio alterations to the wiring of the receiver, the terminal provided merely being connected to the B battery positive side of the loudspeaker. As is the case with any power valve, the pentrode re- quires ample’B battery voltage and current, and it is useless to install the valve unless these are available. Where the receiver is opérated from a battery eliminator the use of the petitrode in the power socket of the receiver will vastly improve the volume and tone of the set. The Sereen Grid Valve. ITHIN the last week, two or three correspondents have written in asking for particulars of the Screen Grid Valye, particular as to which terminals are which, so that we go to a little length to describe the operation of these valves and their method of use in a set.

In a recent issue we discussed fairly clearly the theory of the screen grid and the pentode. The following sum--mary from that article may interest those who are using this valve:in the sereened-grid valve plate-grid capacity is reduced to something so small that its effects, provided that the lay-out and wiring are suitable, are almost negligible. Between the control grid and the plate a screening grid is introduced. This is kept at a fairly high positivé potential. It acts as a capacity screen between the two elece trodes, preventing feed-back from 9g.

curring. Its presence has also another important effect: a very high amplification factor can be obtained in the screened-grid valve, an overall magnification of 30 or 40 from each H.F. stage being obtainable upon such wavelengths as those on the broadcast band. The screen grid valve was developed to fulfiill two requirements: very high amplification and stability. Numeroys methods of preventing oscillation "in high frequency circuits have been invented, neutralising methods being first tried for elimination of this, and whilst being successfu] to some extent, are far from perfect. Another method is to make the inter-electrode capacity of the valve almost negligible. In fact, this has been accomplished in the sereen grid valve. Types of Screen Grid Valves. T the present time, there are three different types of screen grid valves on the market: 1. The Osram 625 and Marconi valves, that need horizontal mounting, and have leads going to each end. The gsimall diagram shows such a valve. 2. The English and Continental upright type. These in appearance are almost the same as the ordinary 38-elec-trode valve, but have a small ca the top. This cap connects with‘ PLATDH of the valve. The four pins at the base connect with the filament as usual, the grid to the grid, as in the ordinary valve, but the PLATE pin goes to the GRID CONTROL OR SCREEN GRID. 3. The American type of valve, or UX222. The internal arrangement here is very similar to the Wnglish valve, but the terminal at the top of the valve is the GRID CONTROL, so that the four prongs at the bottom will go to their usual destination, that is, the filaments, the grid, and the plate. The diagram is that of the American UX222. a Constructors, when building from a systematic diagram, should bear these points in mind, as the valve they are going to use may be different from that described by the writer,