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Vacuum tube amplifier in class A

     This vacuum tube audio amplifier has two tubes per channel: triode E1 (preamp) and Pentode E2 (power stage). As E1 you can use almost any small triode, as E2 a power pentode. Better, of course, use tubes designed specifically for sound amplification (it would operate with RF triode and pentode for horizontal deflection, but the audio quality suffers a bit). The amplifier is powered by a DC power supply 150 to 250 VDC. I believe that everybody can do supply with transformer, rectifier and C-R-C or C-L-C filter. The output transformer can be salvaged from old TV or radio, buy a new (new transformers manufactured specially for this purpose can be bought, but they are quite expensive) wind it yourself, or make some adjustment of available transformer. The output transformer must have an air gap, therefore, never use the mains power transformer as the output transformer (there is a DC current). Pentode current can be adjusted by changing the 270R/2W resistor. The current is always chosen not to exceed the maximum value of the catalog. Lower anode current contributes to a longer life of vacuum tubes.

     Warning! Operating voltage of the vacuum tube amplifier is dangerous! Prior to manipulation, it is always necessary to disconnect it from the network and make sure that the capacitors are discharged.

The schemtic of the Class A Vacuum tube audio amplifier (one channel).

Added: 2002