Simple 5 Watt SET with cathode feedback.
(About cathode feedback)
Compleate schematic with power supply
The amplifier can run with output tubes like 6550, KT66, KT77, KT88, KT90, KT100, 6L6, 5881, EL34, etc.
Max power output (onset of clipping):
5.3W (6.6Vrms @ 8.2 ohm)
Harmonic distortion, Vout=5Vrms:
400Hz: 2nd: 2.5% (-32dB), 3rd: ---% (---dB), 4th: 0.18% (-55dB)
30Hz: 2nd: 3.5% (-29dB), 3rd: 0.6% (-45dB), 4th: 0.2% (-53dB)
Distortion pattern 5Vrms, 400Hz: