Power amplifier schematic – 100Watts, using 6L6 (Audio output) and 12AX7 (pre-amp) tubes

6L6 can be substituted by 6N3C tube (Russian), provided a very small change with the heater voltage.
The heater voltage for 6L6 is 6.6VAC and for the 6N3C is 6.3VAC.
The substitute for 12AX7 is ECC83
Matched pairs of 6L6 or 6N3C should be used at the output stage. Otherwise, some bias adjustments should be done to compensate the gain of the output valves. (R42, R24, R25, R26 and R27) are the adjusting resistors for this purpose. (R42) adjusts the common bias, and the rest are meant to adjust individual valve bias.
Use good quality valve sockets to avoid distortion. Some sockets may have poor contacts, and should be avoided. Sockets with gold plated contacts are recommended.

Pre-amplifier schematic

Push-pull power amplifier schematic Pin numbers are indicated with one of the 6L6 valve at (top-left) in the circuit. Pin configuration of 6L6 and 6N3C are similar. 

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