VALVES FOR THE LAST STAGE

Radio Record, Volume I, Issue 35, 16 March 1928, Page 10

VALVES FOR THE LAST STAGE

} 3 There are several power-valyes suited for the last audio stage in addition to the UX11%2, 171 and 210 mentioned last week. The P3256 (5.5 volts) and PM254 {3.7 volts) are both excellent where large volume has to be carried. Apart from filament voltage, these two yalyes have the same characteristics, The impedance, which should be low in a power-valve, is 3500 olims, filameut cutrent 4 amp., and plate curreut 16 milliamperes at 125 volts, with a grid bias of 17 volts. At 166 volts, they pass 21 milliamps. with a grid bias of 22 to 24 volts. Then for dry A battery operation there is the PM2 (1.8 volts), passing 5 milliamperes at 100 volts H.T. ‘There will soou be on sale the 11252, a 2-yolt power-valve with similar characteristics to the PM256 and 254, , In the Philips series we have the B605 (6 volts) with an impedance of 3100, the B406 (4 volts) with impedance 4300 ohms, and B403 {4 volts) capable of carrying large volume, with an impedance of 4100 ohms. In the 2-volt series there are the B205 and B203, the latter a special lust-stage valve. The De Yorest DV7 and DIL14 are suitable valyes for the last stage, and the DL2, DL5, and DLT for the first stage. The foregoing valves meutioned are suited for transformer coupling. in resistance-coupled amplifiers special valves are often used iu the early stages.

MILLIAMPERES AND IMPEDANCE, Those who are not familiar with the meaning of "milliamperes" and "impedance" should get used to considering the suitableness of valves by taking these important factors into account, Impedance, expressed in ohms, is the total of several factors opposing the passage of high-tension plate curreut from plate to filament in a valve. The lower such resistance, the more the’ plate current that may be passed at. suitable voltage, wud this increased plaie voltage allows the handling of greater volume without distortion, which is the object in utilising powervalves with an impedance cs low as 2000 or fauQ olims, and passing 20 or more milliamperes of current, A "milliampere’ is the thousandth part of an ampere.

Thus when we wiv to select a new power-valve io carry more volume than the ons previously in use, the above Characteristics of the old one. being known, a valve with a lower ianpedance and greater plate current «may easily be selected. But care must always be taken that the valee is suited to the B battery power avaiiable, unless it can be conveniently increased. Superpower valves require both a higher voltage and a larger current, and whilst extra voltage may easily be provided by adding another section to the B battery, the drain in milliamperes must be carcfully considered, as it is this demmand fer quantity of current (not voltage that determines the useful life of the battery.

The UN2O1A as detector passes 1.5 milliamps, and as amplifier 2.5 milliamps, so that a five-valve set with this ~=-enee valve in every stage would require less" than 12 milliamps from the B battery, which would be a reasonable demand upon a battery of ordinary capacity (not voltage’. By substitution of the last valve for one requiring, say, 15 iilliamperes, the total drain on the battery would be increased to 24 milliamperes, or double, so that the same battery would last for half the time. Where a B accumulator is in use, this aspect of power-valves requires much less consideration, especially when chargitig is carried out at home, and where a B €liminator is employed, one might almost say, the more consumption on the last valve the Deiter.

] | ' ' FIRST AUDIO VALVE. sat in giving consideration to the last valve, we must not lose sight of the first audio valvc, which may in many cases be satisfactorily replaced by one of lower impedance. In some cases when a larger power-valve is placed in the lust stave, ihe supplauted valve, if of lower impedance than the one in the first audio (transformer) stage, may well take its place. : ADVANTAGE OF B ELIMINATORS. If fou are situated where electric power is available for a B eliminator, it will prove a good investment whether bought or home-constructed. A fullwave eliminator will run an eight or nine-valve set, and a half-wave one will run four or five valves, but where the plate-current demand is likely to be large, only a full-wave should be con4 sidered, as plenty of margin is neces-| sary for quiet operation. A demand too. nearly approaching the total emission of the rectifier is bound to cause objectionable hum, but an eliminator that has a capacity well above the work to be done, will be just as good as an acenimnulator,