Slight problem with my Fender Twin Reverb.

[Humbucker]

Well, I just finished retubing my 1976 Fender Twin Reverb combo with a brand new quad of JJ 6L6 tubes. I set the bias and everything is running smoothly and sounds wonderful except for one thing, the amp might be a little bit unstable now and I think I might know why.

While I was in there, I replaced a bunch of cheapy caps and stripped out the push-pull master volume too. I also decided to install a 25K linear pot in series with the 820 ohm NFB resistor(I think this is my problem). When the pot is fully cranked, the pot is at zero resistance, so the amp is basically stock at this setting.

I had the 25K NFB pot set at appx. 5K, so the total NFB would have been appx. 6K at that point and all of a sudden, as I hit the "D" note on the fifth string, 5th fret, the amp began to howl. It was like a low note feeding back. I quickly reached over and turned down the volume knob on the amp and after a couple of seconds the howling stopped.

I tried it again after that but it didn't feedback this time. I'm guessing that this problem might have been caused by the lack of NFB? The amp gets a LOT louder when decreasing the negative feedback.

The only other thing that I did to my amp is to cut loose a small cap in series with a 220K resistor in the PI section. This is something that Fender started doing in the 70s, I think it was a snubber cap, maybe that's got to go back in? The pre-CBS amps didn't have this junk in it.

[MBSetzer]

when disconnecting the master volume, you need to really look at the schematic & layout drawings for your particuar model, and make sure to do a clean and/or complete job of removal.

Make sure you do not have any un-necessary wires still connected, especially do not just unsolder one end to break the signal path while still leaving the other end of the wire connected.

Then in case there is shielded wire, you need to pay attention to how the shield is grounded and if you remove it, sometimes provide an alternate source of ground. Some places need ground to be so superb that minor differences in soldering competence make an easily audible difference.

for that matter, make sure your replacement caps and other parts are not only fundamentally as good or better than the factory originals for this exact service, but the soldering must be as good or better, otherwise a truly better cap soldered into position less-than-ideally can end up sounding worse than a truly cheap cap soldered in outstandingly.

is this the schematic for your amp?

http://www.ampwares.com/ffg/schem/tw..._100_schem.jpg

Mike

[Humbucker]

Hey Mike,

Thanks for taking the time to respond to my post!
8^)

Yup, that's the schematic for my amp. BTW, I removed that 25K pot that I had put in series with the NFB resistor and so far, I'm not getting any weird stuff happening. Before I removed this pot, I was occasionally getting a Tremolo effect happening, probably due to a funky pot. The NFB loop is back to stock condition.

As far as the master volume wiring is concerned, I know that I removed every bit of it and it should be fine but if I get any more weird stuff happening, I'll re-check my soldering.

Thanks again!

[isaac]

again, I know this is an old post, but if anyone gets this, I did the same thing to a UL twin I had, I basically blackfaced everything in teh preamp. I didn't change the PI or power section, but it sounded amazing, I wish I still had it. If I can find it, I'll post the schematic I edited... Couldn't find it, oh well.

[Bruce / Mission Amps]

When you get the other problem sorted out, 820 with the pot is OK but 25Kb in series is too big for this to work really
well.... use a 5Kb instead.