NFB Switch For Peavey Classic 50/50

[HeavyMetal69]

Hello!

This being my first post I thought I'd introduce myself as an actively gigging (in two bands) guitar player with a lust for modifying everything. I'm no expert in Amp electronics however, so I need help that's somewhat simplified for me, pictures work very well (schematics are fine however).

My question is:

How can I install a NFB switch (on/off) for my Peavey Classic 50/50 power amp. I believe this power amp is somewhat more complicated than the average power amp to work on and am having trouble pin pointing where to disconnect and connect...if that's the route I'm even supposed to take!

Thanks ahead of time

[HeavyMetal69]

And another question:
There's a preamp in my power. I obviously already have a preamp so what's the purpose of this one? What happens if I disconnect it?

[Enzo]

What do you mean there's a "preamp in my power?"

There is no preamp in the Classic 50/50. The small tubes there are phase inverters (12AX7) and power tube drivers (12AT7).

You want to disconnect the NFB. Then you ask if that is the route to take. I can tell you how to insert a switch, but you have not specified what you want to accomplish, so i can;t offer alternatives.

R310 and R311 on the output jack board are the feedback resistors. They each are connected to a four wire - plus key space - cable back to the front. Somewhere in ther, you could wire in a switch to open the path.

[HeavyMetal69]

I see, do the tubes in this stage have a large impact on the tone of the amp?

I am very interested in what I've read about removing the NFB loop, but understand there may be circumstances where it's not recommended. Since I'm no electronics whiz, I didn't know if it came as not recommended for the Classic 50/50.

I play 80s metal and like the idea of a more driven and raw power stage. I also like the idea of this being switchable.

Thanks for the help, I'll wire it up tonight

[Enzo]

If you just want to see what it sounds like, unsolder one end of those two resistors and lift them off the circuit board. Now the NFB is gone. You may or may not care for it, but it is as simple as pushing the resistor back down and soldering it back in place to revert to stock.

The resonance control would now do nothing. The presence control would still be in the circuit but wouldn;t work as before, not sure it would do much. And i don;t think it would have to be disabled electrically.

[HeavyMetal69]

The components are not marked on the board. Is there a schematic somewhere that I can use to find R310 and R311?

Thereīs a 100k resistor that looks like a prospect, but I donīt know for sure.

This circuit is a pain in the ass with how itīs laid out

[Enzo]

ANy PV schematic can be had for the asking from customer service at PV.

The schematic is too large to post here, so I posted it in the Enzo gallery at ampix - Home

[HeavyMetal69]

I can't thank you enough for your helpfulness.

Thank you!!

[HeavyMetal69]

Those were the ones I thought they were too!

[HeavyMetal69]

Well I did it and made it switchable. Itīs not as big of a difference as I thought it would be, but I think I like it better with the NFB loop turned off. Itīs smoother with it on but ballsier with it off.

Thanks again!