High-Quality Single-Ended 8W Amplifier

Design Goals: Claus Byrith said:
I want to build an SE amplifier capable of producing 8W, because this is what my transformer is intended for.

I shall try to use widely available standard valves in order to keep costs reasonable, but I am prepared to make a series of investigations to make it possible for me to choose the most suitable solution to the very important question: the configuration of the output stage.

Valve tables are not very helpful here. Their application suggestions are always limited to the standard pentode coupling with emphasis on maximum power.

I shall try to make the circuit simple and understandable, and I want to make it possible to use the amount of negative feedback that suits the user best, ranging from 0 to 10dB, and I shall limit myself to two valves.

Since no mains ripple cancellation takes place in the transformer as it does in PP amplifiers, I am prepared to use a filter choke in the power supply.

These considerations lead me to choose EL34 as my output valve. I want it to behave as closely as possible to a triode without sacrificing more than 25% of the power that the valve can provide in a normal coupling.

I tried the five circuits shown on the next page with two different supply voltages, 275 and 375 Volts. The circuits with a tap on the primary for the screen grid were tested with taps at different points.