The Construction of the Pentode Valve

Radio Record, Volume II, Issue 11, 9 November 1928, Page 28

The Construction of the Pentode Valve

A Power Valve With Unequalled Amplification

T has been stated by a wireless correspondent to a London paper that there are over 1200 difhg fi ferent types of valves tremestt4 on the market for ‘the amateurs to choose frqgm. Little wonder, then, that the man who has just dropped a valve is rather puzzled when he bas to choose one as a replacement from this selection. We are colstantly hearing of a new valve with improved characteristics being offered to the public and before we venture to speculate on a new valve with tremendous claims, we generally wait until a friend. has tried one out. There is one valve, however, which can be purchased without waiting to hear how Mr. So and So got on last night with his. This is a valve for the last stage called the Pentode, the one to be deseribed being the B443, made by Philips Lamps, Ltd. A similar valve is made by Mulards, PM22, and the following remarks as to’ general characteristics are equally applicable to it.

ABILSIBLBLALR ALAA LALO LALA RIAL OA ALAA AL ELA Lele e salar aeeiacdinimis’ " hh in Onn When the radio enthusiast reads of its characteristics he will wonder if the writer is often given to such hallucinations, and he will have every excuse when it is stated that here is a super-power valve with an amplification factor of one hundred. Much has been said of the screened grid high frequency valve. Tt has been tried and has proved its merit. Well, here is a screened grid power valve. The Internal Construction. AS§ its name implies, the Pentode has five electrodes. ‘To look at, it is the same as any other valve except that it has, at the top of the base; between the glass and moulded cap, a small terminal. If one could see inside a big difference would be noticed between its construction and the assembly of the triode. Instead of the usual single grid there are three grids. The necessary extra supports give the whole a very complicated appearance and one wonders at the workmanship displayed in getting the three grids and filament surrounded by the plate. The valve is supplied with the standard American base and is easily adaptable to any set using one or two stages of low frequency amplification. It has been stated already that there are three grids. The inner or con

trol grid acts in the same way as the usual grid in the three-electrode valve. Surrounding this is another grid with a lead brought out to the small terminal on the side. Between this grid and the plate is yet another grid. This is called the "earthed grid,’ and is connected inside the valve to the centre of the V-shaped filament. Its use will be described later. The plate surrounding the whole is connected to the usual plate pin in the base. Negative grid bias is applied to the inner or control grid as is usual with every other type of power valve and with 150 volts on the anode, 15 volts negative bias is required on the grid. To the second grid a high potential is applied, and this is done by connecting the small terminal on the cap by apiece of flexible wire to the B plus amplifier terminal. The earthed grid at first appears to have no value, as no connection is made to it outside the valve. Under working conditions the plate is undergoing a bombardment of electrons given off by the filament. So-

called secondary electrons are reflected by the plate and tend to return in the opposite direction to the second grid, which is also at a high potential. The use of the earthed grid is to prevent this return flow of secondary electrons, which if allowed to take place would upset the working of the valve. The earthed grid being at earth, potential absorbs these stray electrons and leads the charge back to the filament. Four volts are applied to the filament of this valve. Those ‘readers who are using 4-volt valves in their sets will have no trouble. Just plug tie valve into the last socket and connect, with a piece of insulated wire, the small terminal on the side to B plus amplifier or to the highest value used on the B battery or eliminator. Fitting the Pentode. ‘ LITTLE difficulty may be expgrienced in adapting this valve to a receiver using 6-volt valves. The Pgntode would soon be useless if 6 volts were run for any length of time through the filament, and a resistance will have to be used to bring the volt-, age down to four. As the valve takes | 15 amps the value of this resistance will have to be about 14 ohms. Unless a 20 or 30 ohm rheostat is used to eontrol the last audio valve only, . the additional resistance will have to

be put into the circuit. Perhaps the easiest way to do this is to incorporate another variable rheostat in the receiver. This can be done in the following manner :- Trace out the filament wiring to the last valve and break the negative lead close to the point where it. joins the valve socket. A baseboard mounting rheostat of 20 ohms can. be screwed to the baseboard, and the two terminals ‘connected Detween the spare wire and the valve socket. In this way a variablé resistance is in series with the last valve and can be adjusted to take the 4-volt pentode. With half the rheostat turned on a resistance of 10 ohms is obtained. Turned one-quarter on a resistance of 15 ohms is in circuit. A point just above this should be chosen and this rheostat can be left permanently in that. position. By inserting the resistan¢ée in the nevative lead an additional tw&4 volts grid bias is obtained, and this should be allowed for when adjusting thé C C battery. Instead of 15 volts CG at 150 volts B battery, only 138 should be used. As this is a power valve with a large amplification factor, care ‘should be Traken not.to overle1d the input. Under ‘correct working conditions, one pentode is equal in volume to two stage using ordinary valves. Using a gramophone pick-up plugged into the detector socket of a receiyer with two audio

stages, reproduction was given far louder and of better tone than when the same. pick-up was connected to a three-stage amplifier using a power valve of the ordinary type. Just as the screened grid is superior to the general -purpose valve for high frequency work, so is this new pentode better for power amplification. Research is giving new discoveries every month, and, who knows, perhaps we will have the cold or filamentless valve one of these days. BELow is *iven the characteristics of the B443 Pentode as supplied by the makers :--~ Fil. Voltage ...eceeees 4 VOItS, iz il. Current eeeseecenet 15 amp. Anode Voltage ........ 50-150 volts. Screen Grid Voltage ... 50-150 volts. Total Emission ...... . 5O ma. Impedance .....eseeee 53,000 ohms. Amplificatio Factor ... 100. Mutual Conductance ... 1.8 m.a./volts. Normal Anode Current 12 m.a. Grid bias at 150v. on ANOdE .ccesecesecees L5 VOItS.