Unity coupled output amplifier (ala McIntosh) with 2 pairs of EL86 as output tubes.
One of two mono blocks:
Working conditions are 260V B+, 190V Bg2 (regulated) and –90V for the negative rail. The amp runs near to class B, biased with 4 x 10-20mA in the output stage. Max power output was estimated to be about 30W and I measure the onset of clipping at ~15Vrms into 8.2R (~28W)
Schematic: front end and output stage:
The output transformer is a custom transformer (3x720R) from Sowter UK. Global negative feedback is kept low, about 10dB, but there are no problems to apply more feedback to get even better technical results. Personally I am not a big supporter of global NFB, but used in proper amounts it does good things.
Results achieved with 10dB global NFB:
o Zo : 0.8 ohm
o THD: ~0.6% (50Hz, 1kHz, 10kHz, @ 25W Po) (2nd and 3rd harmonic dominant)
o Frequency response: 10Hz-100kHz (+0/-0.5dB). -3dB at 230kHz.
30kHz square wave response (4V pp) is not that bad: :-)
Some historical papers:
o Pen-Tung Sah 1936
o McIntosh 1949