Fender ProTube Twin-Amp reverb feedback
Hi People,
I just discovered this place and this is my first post. I'm hoping you folks can :confused: 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?
Possible cause of reverb oscillation (squeal)
Do you have a schematic for this amplifier? I was looking at the Pro Reverb schematic at the Fender web site and see that this is an all tube amp including the reverb drive and recovery circuits with an 8 ohm input tank, nice. From what you have posted the reverb will start to feedback with no signal into the amplifier just turn up the reverb level pot. By looking at the schematic for the Pro Reverb if the signal level out of the reverb is high enough it could feedback across R76 to its input and then oscillate (squeal). This is probably at a frequency greater then you need for the range of the guitar. One way to prevent this would be to limit the bandwidth of the recovery amplifier the 12AX7 V5-A. If you add a 300pf 500volt cap with 10k ohms in series across the plate load resistor R71 this will roll off the maximum frequency from the recovery at approx 5 kHz and may stop the oscillation. Because this oscillation happens with no mechanical excitation (no input to amplifier) I kinda don’t think it is a microphonic tube that is the cause.
Have you tried changing tube V3
Quote:
Originally Posted by Mick
Dude, I'm afraid to even look at a capacitor! :eek:
If it comes down to modifying the circuitry the way you describe, then I'll take it to someone like you that knows what an ohm is.
Well thought it might help. On the chance changing a tube could have an effect changing V3 may have an effect. This is used as a cathode follower for the clean signal to the reverb and the output. If its cathode impedance was higher then normal it could cause a problem like this. Hope this helps.