Hi People,

I just discovered this place and this is my first post. I'm hoping you folks can help me diagnose a feedback problem that I'm having with my Fender ProTube Twin-Amp.

I just bought this amp off eBay. It's a used amp, but it looks almost brand new. According to the folks at Fender the amp was manufactured in April of 2003.

When I got the amp the tubes were in a cardboard box, individually wrapped in bubble-pack, so I carefully unwrapped each tube and installed it in the amp according to the tube chart inside the amp. Unfortunately, two of the 12AX7 preamp tubes were missing. Since some of the tubes were missing, and all the ones I had appeared to be the original Fender Groove Tubes (relabeled Sovteks), I decided to go ahead and replace all of tubes. So I ordered a complete set of JJ tubes from Eurotubes.com. The retube kit for the Pro Tube Twin-Amp consists of a matched quad of the JJ 6L6GC's, one ECC81 to replace the 12AT7 reverb tube in V7, and seven ECC83S's, with one of them balanced for the phase inverter in V4 to replace the 12AX7's.

I ordered the tubes on Friday and they arrived on Saturday afternoon. The first thing I did was install two of the new ECC83S tubes to replace the missing 12AX7's. Then I gave the amp a quick test. With the mix of old tubes and new tubes the amp seemed to work okay. Unfortunately I didn't test everything. I was just relieved that the amp worked at all. Up to that point I didn't know for a fact that it would work. Then I pulled out all of the old Groove Tubes and replaced them with the new JJ tubes, then set the bias and balance using a bias probe and meter that I bought from Eurotubes

With the new tubes installed, I plugged in my '06 American Deluxe strat and started checking things out. The 100w/25w switch is a nice feature. The clean channel sounds awesome! I love it! The distortion channel sounds great too. The tremolo works great a sounds terrific. The reverb however has a problem. With the amp in 100w mode, if I turn the reverb knob up past 5-6 or so it immediately starts feeding back. A high pitched tone starts out low and quickly builds until it blows out your eardrums. I verified that it's not the guitar. It will feedback like this even with no guitar attached. It also feeds back with the amp in 25w mode. It just takes a bit more reverb knob. It feeds back in both the clean and distortion channels.

After speaking with the guy at Eurotubes, I tried swapping out the reverb tube in V7 with the original 12AT7 tube. That didn't help. Then I tried swapping out the phase splitter in V4 with one of the original 12AX7 tubes. That didn't help. Then I tried swapping all of the preamp tubes around. I'm not sure, but I think that might have helped a tiny bit. I can turn the reverb up to 6-7 now before the feedback starts. Then I bought a can of contact cleaner and cleaned all of the preamp and power tube sockets. That didn't help.

That pretty much brings us up to date. I love the amp. It sounds awesome... as long as you don't turn the reverb up too much. But the reverb is one of the main reasons why I bought a Fender Twin in the first place. Would someone help me find out what's wrong?

What causes feedback like this anyway? Is it only caused by microphonic tubes, or is there something else that could be causing it?

If it has to be microphonic tubes, would it be preamp tubes or power tubes, or could it be both types?

Anyone?