info on adding a negative feedback pot in a pro reverb.

[sidehatch]

Hey Guys, Its me again. Screwing around with my pro reverb.

Today I disconnected the negative feedback and liked the tone a lot. I 'd like to have the ability to blend it back in though.
its a 74 pro thats been Bf'd and adjustable bias added.

Does anyone know of any text on the internet (preferable with photo's) that would show how to do this? I'm new to working on amps and a lot of the time the talk is over my head.

I plan on using the Hum balance hole in the back because there is a disconnected pot there i can remove.

I really want to try this but have no clue on where to start.

Thanks!!!

Shane

[Firestorm]

No pictures, but here's a description of the mod that Jack Price at Priceamps does for $18:

"Sole Control
(a.k.a. Feedback/Distortion Control).
This mod controls the amount of feedback that is fed back to the inverter stage. Normally a fixed value of 820 ohms is used on Fender Black/Silverface tube amps. The mod uses a 5K ohm wired in series with the fixed feedback resistor and lets you adjust the feedback. The extension speaker jack is most commonly used to insert the pot. Thus the mod is visually hidden from the front view of the amp. As the pot is turn up to 10, less feedback is allowed to return to the amp and thus the amp will breakup up more easily at lower volume The variability of this pot allows the player to adjust the distortion characteristics unique to the player's style and desire."

That's pretty much the deal. Find the wire that goes from the speaker jacls to the 820 ohm feedback resistor and unsolder it. Run the wire to the righthand tab of a 5K linear pot (righthand with the pot shaft towards you). Take another wire and solder it to the other end of the pot and the middle (wiper) tab of the pot. Connect the other end of the wire to the 820 ohm resistor (where you initially unsoldered the feedback wire). Wired this way, the amp gets dirtier as you turn the knob clockwise. You can also wire it the other way so the amp gets cleaner as you turn it up. Your choice.

[sidehatch]

I totally understand. I need to find a 5k linear pot and I can do this.

Thanks for the help!!!

[jaysg]

fwiw, I recommend the hole where the "GROUND" a.k.a. polarity switch goes. It's closer to where you want it and that switch should pretty much go anyway.

[sidehatch]

I cant find a 5k pot locally .

I have a 10k pot. Will that work?

I can order one in the meantime but would like to use this 10 k if it will. It also has an on/off. I wonder how that would play into this?

[sidehatch]

Mod accomplished!!! I found a 5k mini pot locally. Its linear taper.

I want to change it to a full size one eventually but this will work for now.

I'd leave it but I had to use really skinny wire like I'd use in an fx pedal build.

Does anyone know if its ok to leave it with that skinny wire for now?

It does work though and I would say its the most useful simple mod I've done. Clean for my Jazz gigs and dirtier for other stuff.

I have another question but I'll start a new thread because it regards the polarity switch.

Thanks for everyone's help as always!

Shane

[oldhousescott]

10k will work, but you'll have most of the usable adjustment toward one end of the pot. You could solder a 10k resistor in parallel across the outer legs of the pot to bring the total resistance down to 5k, but that will mess with the taper. Doug Hoffman at www.hoffmanamps.com carries a 5k linear full-size pot. He ships fast and there's no minimum order requirement.