HEATING POWER-VALVE FILAMENTS

Radio Record, Volume I, Issue 49, 29 June 1928, Page 13

HEATING POWER--VALVE FILAMENTS

WITH ALTERNATING CURRENT in is quite a practicable proposition to heat the filament of the last power-valve with unrectified alternating current of the correct voltage, such as may be obtained from the filament or low-tension winding of an eliminator transformer. In using an ordinary valve in this way, the only important condition is that the filament must be of a thick type rather than a thin one. Thickness of filament is fairly well shown by the amperes taken, so that valves taking less than 25 (4) amp. could well be ruled out for the purpose. ° Bexring this in mind, we find that in the six-velt class the ‘suitable valves are 112, 171, M256, de Forest DV7 and DL14, and also the UX210,which latter carries exceptional volvue, but in order to obtain full benefit from it, more A and B power is required than is available in the average radio installation. In the fourvolt class the same conditions would apply. The writer has run the last power valve in this way for many months, and no harm has been introduced into the receiver on that account, although the grid-bias of 22-volts has been applied in the ordinary way. UT there are circumstances con: nected with a.c. heating of filaments that may cause the appearance of hum until steps are taken to remove the cause, When operating on direct current the negative end of the filament is considered as zero voltage as far as grid and’ plate are concerned, and if the grid is connected to the negative leg of the filament, it may be said that the grid is positive to no part of the filament. ‘That. being 60, no gvid current will flow, It may here be explained that grid current is an unwanted current set up between grid and filament when the grid becomes positive, and it is such current that causes distortion, If the grid is connected to the positive end of the filament, grid current will flow. T® is, therefore, seen that the negative end of the filament, which is eonnected to the grid, must be kept

negative continuously, Now, if we apply alternating current to the filament, the end to which the grid is connected will be alternately positive and negative, (rid enrrent will then be produced in step with the positive half of the eycles, and a powerful | hum may he heard in the receiver. | 'N such a ease there is an easy remedy. It consists in providing a paint which is equivalent, and this is deve by placing a potentiometer across the filament terminals, ‘The grid and B negative leads are then connected to the arm of the potentiometer, and wdjustment is made wutil the hum disuppears. Even after this has been done, there

is auether point, which is that during «ach half cycle one end of the filament or the other is negative in respect to the grid by a voltage equal to hulf. the full a.c. voltage applied t¢ the filament. This meang that it is necessury to provide a grid bics of sufticient yoltage to ensure that half the filament voltage added to the: sigs nal yoltage will at no time make thé grid positive. From this it is cleag that rather more than the manufactua rers’ recommended grid bias must bé applied, so that with valves taking 5 to & volts on the filament, from 3 to 43 volts additional bias should be provided over that which would bé required when working with direet current. . . Coe