Fender Champ 600 questions

[thermionix]

What would changing the NFB circuit in the 600 to that of the 5F1 do?

IMO it would leave you with a very dull sounding amp.  You would be combining the tone stack losses of the blackface/silverface champs, and the greater NFB of the tweed Champ.

I'm not really sure how you have your presence control wired, as the 5F6-A is a different setup.  If you want to experiment with a bit more NFB and hear a difference between "some" and "none" you could increase the value of the 47r.  Maybe start with doubling it, see where that gets you.

[PRR]

> explain why there is so little NFB?

NFB sacrifices gain for other benefits.

There's just three stages here. It takes almost 3 stages to bring guitar up to speaker level. So you can get some NFB. But then Leo added the lossy tonestack. Now there's just a trace of excess voltage gain. More at speaker bass resonance where load Z is high so 6V6 gain is higher; so it may moderate the gain-bump and reduce cone flapping.

If you want "serious NFB" with a champ-like power stage, you really want three triode gain-stages in front. But that is an odd number.

BTW: I keep saying "tonestack" but the 600 lacks tone knobs. The tone-stack is IN there, with fixed resistors for a fixed sound. If you want more control(s), hack the PCB and bring them out to pots. Everything here is stolen from the AA-Champ, use those pot values and connections.

[Electron Tornado]

> ditched the tone stack altogether

Then you will have "too much" gain, and can re-purpose it for NFB to smooth the output tone (New for 1961!) or just waste it off between stages.

Looking at the 5E1 circuit, could you bypass the tone stack with a 0.02uf cap and a resistor in series to keep the signal increase from being "too much"?