If the amp has a sf/bf tone stack, I'm pretty sure there
should be a 10 to 25uF bypass cap on the cathode
(depending on whether it is a vibrochamp or not).
This is also the spot where the NFB loop is connected.
If you rewire the amp to the old tweed specs, don't
connect any cap here because that will short the NFB
loop right to AC ground.
If you leave V1b the way it is, then it's OK to leave the
bypass cap in or play with different values.
 
My Soulkicker Jr. uses a FB loop similar to the SF
Champ using a 1k5 resistor on the cathode of V1b,
sitting on top of a 100 ohm with the NFB inserted at
their junction.
 
The 6V6 should have a 25uf cap bypassing the 470
ohm cathode resistor (pin8) but if it's an old one,
replace it with a new cap and rated at 35v to 50v
minimum.
You might want to raise the cathode resistor up a little
to 510 ohms too.
 
Bruce