A.C. VALVE OPERATION

Radio Record, Volume I, Issue 26, 13 January 1928, Page 12

A.C. VALVE OPERATION

A RESUME OF THE POSITION

The principle that allows of alternating current being used for filament heating is that of "thermal inertia." The degree of temperature variation depends upon the heat-storing capacity of the filament in relation to its heatdissipating ability, so that in operating ordinary valye filaments with unrectified and unsmootlied a.c. the most efficient will be those filaments that have sufficient "thermal inertia" or bulk, which is almost its equivalent, to retain full emission heat between one cycle and the next. It has been | shown that the amount, of temperature hum is -greatest from thin, low heat capacity filaments such ¢s the 199 type, and smallest in the heaviest filaments such as those in the 112 type of tube. For this reason the heating of the filament of the last power valve With raw a.c. is_a simple proposition, the chief precaution being to twist the leads together to prevent the hum reaching other parts of the cirenit by radiation or induction, It has also been shown that the amount of temperature variation depends not alone upon the ratio of surface or radiating area to the mass of the filament, but also upon the actual operating temperature of the filament itself. As radiation losses are the chief factor in lowering temperature during the periods of small er no current, it is seen that very low temperatures, that is dull emission yalves, greatly facilitate temperature stability, It has also been found that a V-shaped filament conduces to hum more than a straight one.

CONCERNING THE FILAMENTS. Coming now to actual a.c. operation of a whole set, there are two outstanding types, first those in which the a.c. is rectified and smoothed and then used to heat filaments of ordinary valves, and second those using special ralves cither of rugged filament or heater element type, either of which operates with raw alternating current. In either type, however, the voltage is usually reduced by a step-down transformer, and this enables the filaments to be wired in parallel in the ordinary way. When ordinary valves are used, some systems necessitate the filaments heing wired in series, Which is usually a more economical scheme, as the amperage required by the filaments is thereby much reduced, although the voltage required increases for each valve used according to the filament requirement, which must be the same for each. . As ordinary power-valve filaments nay be run quite well on raw a.c., one of these may be used in the last stage instead ofa special a.c. valve. A number of firms make filament heating transforinets, including Amestran, Dongan, Modern, National, Silver-Mar-shall, Thordasson, and General Radio. The voltage taps on all these except the last are 1.5, 2.5, and 5.0 volts. The General Radio transformer taps are 2, 3.5, 5.0, and 7.5 volts. In some cases the voltages of two taps are connected in series, so that they add together to suit certain valves. *

The filament type of a.c. valve costs less, and has a longer life than, the heater type, but the latter has a lower inter-electrode canacity, which is desirable in a neutralised circuit. Grid bias is essential on the R.F. valves, when the filament type is used, and should he a little more negative than on the AI® side, but if a slightly Jower plate voltage is employed on the R.F., then the saime bias can be used as on the ALT’, Any type of audio amplification can be employed with excellent results. THE DETECTOR. Hither heater or filament type may be used as a detector, but the UY227 type of heater has several advantages over the filament type, from which there is a slight but not objectionable lum. The heater tube may he used with cither grid-leak condenser arrangement or with C hias, and although the filament type will function quite well with the same arrangement, is better suited for plate rectification Plate rectification, however, is not as sensitive as erid-leak condenser, andieads to other complications. The Killogg a.c. tube is an excellent detector for cirenits such 2s the Browning-Drake. A bias up to 40 volts must be available for a heater type detector. LEADING MAKES OF VALVES, The following are the leading makes of a.c. valves at the present time :-

nhe plate voltace Om a1l #ho ahota 7c

90 volts for both types aud the grid bias minus 4.5. In adapting a.c, operation to the Browning-Drake a different method of neutralisation has to be adopted by which the plate voltage is fed to the R.F. yalve through a cored choke coil instead of through the primary of the R.F. transformer. For the audio stages is tecommended an impedance incorporating an R.I, choke, second stage | resistance coupling, and last stage a special arrangement of resistance and impedance to eliminate any tendency to. alee when used with a B power unit. The T1¥227 and CY327 are listed at six dollars in America, and the CX826 at nine dollars.

| The above brief particulars have been compiled by ‘Megohm"’ with the idea. of giving readers an outline of the system emploved and the valves used for a.c. operation. ‘The first complete a.c. operated receiver in New Zealand arrived in Wellington a few weeks ago. It is probable that in a short time some of the makes of valves mentioned above will be on the market here.

Tilamelt A tnplifi- Volts, Amps, cation. (23327 2,5 1.75 7,8 Ty 27 25 1.65 8.7 McCullotgh 3.0 1.0 8.6 Sovereigu 1.5 8.5 Marathion 6,5 1,0 7.3 Arcturus 16.0 0.35 10.5 Magtatron 2,5 1,50 9.

HEATER TYPE.

(x326 1,6 1,05 8, {2g 1 205 8,8 Van Horne 1 2.0 Ce Co 40$ 1.05 88 9 Magnatron # 1,6 1,05

FILAMENT TYPE.