For Beginners

Radio Record, Volume II, Issue 38, 5 April 1929, Page 13

For Beginners

Plate Voltage of Valves. A CORRESPONDENT this week has had trouble presumably because the voltage on his detector was too high, and this raises the all-important point of the voltages on the plate of valves (anode voltage). In general, very high voltage is needed only in the last, or at most, in the last two stages, for it is only in these valves that a great amount of current has to be handled, and where this is the case adequate voltage has to be provided in order that the set work perfectly. Most power valves require 150 volts on the plate, and if true reproduction is to be obtained, and the bass notes brought out with all fidelity, full votage is necessary. This is best supplied through a battery eliminator. If the 250 type of valve can be obtained, and about 350 volts applied to its plate, a tremendous output of 2350 milliwatts can be obtained. This would permit of almost perfect reproduction providing it were used with a suitable amplifier. With the detector, however, the case is different, and if good reproduction is wanted, voltage should be kept from 20 to 40 volts, and this should in no

wise be exceeded. Wxceeding this Waly mean that the set is difficult to neutraliiag and the tone is harsh and unna em Signals may, of course, be slightly iondy Be but loudness without tone has little-imity¥ favour. a. The voltage on the radio fr valves should be intermediate, fred screen grid type is used; 90 volts wit out grid bias is a safe medium, and J5¢ that usually recommenced by makers used in the R.F. stages. ‘ Push Pull Amplification, HE purpose of push-pull amplification is to obtain a great increase of volume without overloading the valves. ‘The necessity for push-pull amplification has been lessened with the advent of power valves, for they were able to furnish great volume without distortion, and a single: yalve only is required, whereas with pushpull two are used in one stage. They are not connected directly in parallel, but are used with transformers of a special design so that one of the valves amplifies one-half of the signals wave, and the other valve amplifies the other half, For real quality tone, however, there is very little to excel this type of amplifier. Whereas the power valve requires a very high voltage on its plate, the push-pull amplifier will give equal if not better, results on & very much lower ‘voltage, Thus, for the owner who is operating his set from dry batteries, the push-pull amplifier will still have a great appeal. With 135 volts on the plate of each of the valves in the last stage, tone and volume equal to that of 180 volts applied to’ a single valve can be realised. A push-pull amplifier can be made to replace the existing audio amplifier of the set, or it can be made up separately and used to boost up weak signals. Again, it can be used in conjunction with the gramophone pick-up, and will give reproduction that is equal to that of % very expensive machine.