The Five Way Valve

Radio Record, Volume IV, Issue 34, 6 March 1931, Page 16

The Five Way Valve

~The Pentode’ in Action

by

W. M.

DAWSON

AMICE,, A.MIW.T:

NE of the greatest radio inventions is «certainly the penthode valve,. the appearance of which revolutionises receiver construction, and sets a new standard for quality of radio reproduction t Being a special type, intended pri"marily. for use as a power valve in radio sets and. amplifiers, the penthode | presents many advantages over normal alves. .It was first designed and deVWeloped during: 1926-1927 in the huge Tesearch laboratories of Philips Radio, where further outstanding improvements to it have since been evolved, The Name, Fr : word penthode is derived from the Greek word. penta, five, hodos’ way, so its significance-five-way | Walve. The word "way" should be understood as an electrical term, be"ing synonymous with "circuit." In ‘fact, with a penthode valve there are ‘five’ actual circuits associated with the walve. Not infrequently the word is i Spelt "pentode." Ina normal thermionic valve, there @re three fundamental circuits with three different elements; the filament, the grid, and the plate. When the ‘filament is heated electrically (by

means of an "A" battery of heating transformer) electrons are emitted, which being negative, are attracted by the positively charged plate or anode, and moved toward it, forming in this way the plate current. As the grid is placed between filament and plate, the stream of electrons must pass through its meshes, making it possible to control the electronic current by applying electric charges to the grid. In common with the normal valve, the penthode has the usual filament, which is surrounded by the control grid on which is impressed the input signal voltage variations. Surrounding this normal control grid is another called the screen grid. If the valve then contained only one more electrode (an anode or plate) it would ,be similar to the normal 4 electrode valve commonly known as the "sereen grid valve." The penthode does contain this anode or plate, but in addition it contains yet another element-a _ third grid, which is situated between the screen grid and the anode. Before discussing in detail the advantages of the penthode, it is necessary to have an understanding of basic operating principles. A clear conception should be had «' the real difference between the func-

tions of the normal 1st stage A.I. amplifier, and those of the power valve. The First Stage A.F. Amplifier. Pou first is purely, a voltage amplifier, which raises the applied gric voltage, such as a step-up transformer does. However, a transformer, having a sufficienfly high ratio to function without a valve would by no means give uniform or pleasing amplification. The normal first stage a.f. amplifier is thén necessitated purely by the fact that the Lf. voltage supplied by the detector valve is insufficient to operate the power valve. The Power Vaive. HE whole purpose of this valve is to translate the applied grid voltages into adequate anode current variations, so that the requisite amount of energy may be applied to operate the loudspeaker. As modern usage decrees a relatively high volume of sound from the loudspeaker, it follows that the input to the power valve must be sufficient to produce this, * Fidelity of Reproduction. AIS is an extremely important point, and put broadly means that all note frequencies occurring in speech and music (say, from 50 to 10,000 cycles per second) should be amplified equally, that there should be no distortion of the waveform, and that relative amplitudes are preserved at all signal levels, It will be shown later how the penthode obtains its outstanding fidelity characteristics. ° The Operating Principle of the Penthode. N the case of the normal three electrode valve, the electrons, liberated from the heated filament, are attracted at great velocity toward the anode, eventually striking it. Assuming no grid current to flow, the thermionic eurrent equals the anode current, Now, suppose the anode is of mesh construction; consequently its attractive effect is not large enough to gather all the available electrons on its surface, and a number of ‘them shoot through the nieshes into the space beyond. If now there is in this particular space a positively charged plate connected to the same anode voltage, this anode will then collect the electrons which have passed through the meshes. As the meshed element (actually the sereen grid) is closer to the filament than the real anode, its effect upon

the thermionic current is much great than that of the anode, and by spec construction of the screening grid the influence of the anode on the thermionie current can be reduced to a negligible value. As the amplification factor value is a measure of the relative (plate current controlling) effects of the control grid and anode, it follows that if the effect of plate voltage on plate current is negligible the valve must have a very high amplification factor.

4 Advantage of High Amplification : Factor. ITH three element valves, the ‘ energy supplied to the loudspeaker depends upon the plate voltége, and, in general, the higher this Yoltage, the greater the valve output. | It must be realised that the D.C. Yoltage on the anode of a power-valve is not equal to the battery or powerpack voltage, nor does it remain constant during operation, as there is inyariably a "load" in -the anode circuit of the power-valve. When, therefore, under the influence of grid voltage variation, the anode current increases, the voltage generated across this "load" increases (voltage equals current multiplied by resistance) and it is of such phase relationship that it acts against the supply and the actual anode voltage is &écreased. _ Conversely, if the anode current decreases (control grid made more negative), the voltage drop across the load decreases, thereby increasing the effective anode voltage, and counteracting the decrease in ‘anode current. It will immediately be clear that this must have a very unfavourable effect upon the speaker energy supplied ‘by the. power valve, and that if this influence of the varying anode voltage on the anode current could be eliminated a much larger useful output would be obtained. , The Third Grid.

O far the operation of the penthode is comparatively simple-there is, however, an important and unfavourable effect known as secondary emisKon, which must be taken care of, or e high quality of reproduction would y ruined.

This secondary emission is caused by the high velocity with which the electrons pass through ‘the grids, and strike the anode. There the greater part of their: kinetic energy is converted into heat, but due to collision with the metal anode, other electrons (called secondary electrons) are "knocked off," This secondary emission-which occurs also in normal valves-would not be troublesome were it not for the fact that at certain instants the screen grid is at higher potential than the anode, ‘and as such times a stream of secondary electrons would flow from anod2 to screen grid, completely. upsetting the normal operation. In normal valves the grid is always ‘at a potential so much lower than the e anide that this,does not occur. Ih the penthode this secondary emission current is eliminated by inserting yet another grid (the third) between the screen grid and the anode. This third grid naturally is connected to the point of the lowest potential in the valve, ie., the filament, and it thus ‘screens the screen grid from the anode, Due to the fact that the triode difiiculty of wastefully opposing voltages has thus been eliminated, it is -seen that the penthode supplies a larger quantity of energy than a triode operating under similar conditions. This, however, is by no means its only advantage, for, in addition to increasing the quantity of energy, it equally improves the quality of reproduction, as will be evident from the following explanation. High Internal Resistance, "THE sound volume produced by the loudspeaker is dependent upon tk

Upper-Showing the five different‘ circuits of the penthode valve. MiddleSome of the electrons are claimed by ‘the Secreen-grid, but the majority get through the meshes to the anode. Lower-Show- . ing the advantage of the penthode. ‘The steepness of the c aracteristic curve -remains for varicus frequencies much more eonstant than that of the ordinary valve. intensity of the alternating speech currents circulating through its coil, The value of these circulating currents is determined by Ohm’s Law (which states that current is proportional to voltage divided by resistance) applied to the anode circuit of the power valve. ~. ij As the total A.C. resistance in this anode circuit is.composed of the internal resistance (impedance) of the yalve, and the impedance of the loudspeaker, it will be understood that if one of these is variable as a function of the frequency (of the a.c. passing through it) the intensity of the speech currents will obey the changes in circuit impedance. This is the ease with loudspeaker impedance, which is low at low note frequencies, and may reach very high values at the highest audio frequencies. If, therefore, the valve impedance is comparatively low, the varying loudspeaker impedance will cause large

variations in the total circuit impedance, so that at the higher note frequencies the current intensity is much weaker, than in the middle or lower register. This high note loss may seriously impair the quality of the reproduction, -particularly as many transmitters and many receivers have already a marked tendency to suppress the higher note frequencies. For real quality reproduction it is essential to faithfully reproduce the note frequencies from 3000 per second upwards, as the intelligibility of speech and the "colour" of music is so largely dependent upon their presence. If we make the impedance of the valve very high compared with the loudspeaker at medium frequencies, the varying loudspeaker impedance will be a smaller fraction of the total impedance and thus exercise a relatively small effect upon the alternating current flowing through the cireuit. This is achieved in the penthode, where the internal impedance is of the order of 50,000 ohms. (To be concluded next week.)