No bias tap

[g1]

Ah, but with that leg of the pot grounded the NFB loop can go from stock to zero nfb. And I see no harm in it's implementation.

I'm not sure I understand what you mean. Zero NFB would be if the loop were disconnected. Having the pot grounded that way would give stock NFB at one end of the pot, with 50K in parallel going to ground. At the other end of the pot you would have 56K in parallel with the cathode resistor, but yes, the NFB signal would be grounded. I'm not sure if this is a common method, maybe it is standard.
My concern was when he said "Minus the 56K resistor obviously", such a circuit without the 56K in series would allow the pot to short out the cathode resistor, big no no.

[Chuck H]

When he said "minus the 56k" I took it to mean that the 56k resistor was the part of the circuit that WOULDN'T be removed.

And I agree that having the grounded NFB circuit parallel to the tail would be very slightly different from the stock circuit, but it's really mice nuts. This is my first time seeing a variable NFB grounded like this. I have never implemented such a circuit so I never considered it. I thought it was clever. A lot of players seem to like variable NFB. I personally feel that each amp has an ideal amount, or lack of NFB and making it variable just mucks thing up. But that's just my take. I don't see a drawback to the grounded circuit, it makes it possible to eliminate NFB with smaller circuit values and doesn't alter the stock circuit resistances much. My kind of circuit. Eloquent.

[tedmich]

could be worse, there could be no Spinal Tap!

[Steve A.]

A lot of players seem to like variable NFB. I personally feel that each amp has an ideal amount, or lack of NFB and making it variable just mucks thing up.

For your own builds it can be a good idea to wire up a pot to the NFB line just to see what value works best for you. Once you determine that you can remove the pot and optionally install a switch if there were several values that you liked.

Steve Ahola

P.S. The schematic as posted looks like a cool project!