Negative feedback loop mod for Pro Reverb

[Lydian]

Hey guys, I'm looking to get some more edge, growl, from my clean tone. AA-165 scheme

If I use the ground switch as the NFL switch will there be significant hum from the wires traveling to that part of the amp? If so I believe I'll just throw a resistor in series of the speaker Jack. Spec is 820 ohm resistor, so I was going to add 1500 ohm resistor to start out.

The Amp is biased a little high right now, I could see needing to bring it back down once this mod is placed. Just not quite sure since I haven't heard anything yet.

Small it's: I've replaced the Neg feed resistor before because it read 1/10thits value. I put a known 820 in its place, it will also read 82 ohms. Any explanation for this?

Any advice or recommendations are appreciated. Thanks!

-Dalton

[Lydian]

I switched it out! Sounds great, really has that warm gain sound i like. Still a bright sound, but if you use the volume knob slot like I do it's perfect.

Note: I used a 1500 ohm to add in series.

[Enzo]

The explanation is simple, it is in a circuit, and that circuit has parallel paths. Look at the 820 ohm resistor. The left end has a 100 ohm resistor to ground, the right end connects to the output transformer secondary winding. That winding has an extremely low DC resistance. SO essentially, the right end of the 820 is grounded. The net result of all that is the 100 ohm is in parallel with the 820. That gives me about 89 ohms on my calculator. Close enough to your 82. Whatever resistors you use there, they will always be in parallel with the 100 ohm, so taking measurements in-circuit will never reveal the true resistance.

You replaced the original 820, did you measure it when it was removed to see what its resistance really was?

I am not sure why you would need to rebias the amp when you change the NFB resistance.

Does lifting the NFB give you the tone you seek?

[Lydian]

Well said about the resistance measurement. That makes a lot of sense.