"Patrick Turner"
The center of pot gives the original amp's amount of NFB. But a problem
occurs where GNFB exceeds 6dB, with the cabinet speaker connected as a load.
Above 6dB NFB the amp breaks into HF oscillation once past the class A to AB
threshold. It was to be expected because the Fender OPT is not much
interleaved like a Hi-Fi OPT. The oscillations only occur as HF waves
appearing on positive going peaks of Vo, and the amplitude grows larger as
clipping is reached and the oscillations have amplitude larger than the LF
part of waves even when over driven.
So I can only guess that in class B during cut off for one 6V6, the circuit
changes to that of an HF oscillator.
The solutions tried were 15r & 0.27uF across Vo, 680pF & 8k2 across anode to
anode, placing small C from Vo to pot wiper for phase correction of NFB.
None of these worked. Then I remembered the right solution, TWO R&C zobels
across each 1/2 primary.
** Take a look at the schems on this page for Deluxe Reverb:
http://www.webphix.com/schematic%20h...om/fender.html
Many of them have 1200pF caps from grids to ground on the 6V6s.
I believe this is for the same problem that happens with some combinations
of OT and speakers.
Using caps across the OT primary is dodgy, high voltage ( ie 2 kV ) ceramics
will survive - 600V film caps will not.
275VAC caps " class X " caps should be OK too.
..... Phil