Fender Twin amp problem...

[ubernerdsteve]

I have a fender evil twin amp ('94 twin amp) and have been having problems as of the past week. I changed my tubes about a month ago, rebiased and balanced the power tubes (slightly colder than factory specs but I had it that way before anyway...takes a little bit of the edge off). At band practice last I absolutely hated my sound and couldnt figure out what the heck was going on... I have never been quite satisfied with my choice of power output tube so when I got home I changed it back to the old power tubes thinking thats what I was hearing. Listening to it now in the quieter environment even with the old tubes it sounds like crap (to explain crap a little more in depth it sounded remeniscent of clipping but worse) whenever I run distortion (either outboard pedals or onboard distortion channel). I moved my pedals to the FX loop and it sounded better but did not fix the problem. If I run only clean FX on it (chorus, flanger,etc) it sounds great but as soon as I hint at distortion the sound breaks up and you can barely distinguish different chords. This does not happen when I play right away, takes about 15-20 minutes of the amp being on.

My gut feeling is that it is the output transformer because it has a reason to be unhappy with life...during transportation to my last gig my amp must have gotten bumped into or something happened that made one of the wires from my speaker driver to the terminal block break. Unfortunately I did not notice this when setting up my amp and powered it up in standby mode while setting up everything else up. I flipped it off of standby and nothing worked, no buzz, nothing. I changed cables and got nothing so I immediately powered the amp off. I then after a more complete inspection noticed the broken lead and resoldered it. Because of this my amp went probably about 5 minutes in standby mode with no output load (the speakers are in series so cut one and you cut both) and about 30 seconds with standby off. My question is if it was a problem with the output tranny would it be a problem that happens only after warmup or would it sound like crap all the time? I have thought that the another source of the problem could be with the rectifier tubes because I have not yet had a chance to change them and frankly I do not want to keep powering up my amp and potentially damaging more components until I have a second opinion. My thoughts against it being the rectifier tubes being faulty is that it sounds like a very solid state harsh even harmonics distortion.

This is what I have done so far for testing and thinking...

I rechecked the bias and balance after it started sounding bad.

I changed the power tubes and the 12ax7's on the three preamp channel ( I also don't think it is one of the preamps because it only happens with distorted inputs and on all 3 channels and it even happens when I run my distortion through the FX loop)

I doesnt seem to be the FX loop because it happens even if I dont use the FX loop so it has to be something POST FX loop (based on elimination of the preamps above).

Reverb cicruit causes no change to problem.

presence circuit causes no change to problem.

EQ filter circuit causes no change to problem with the pots zero'd or maxed

The volume I set the amp for minimally effects the problem. Even at very low levels it still sounds like its clipping.

No fuses are blown or have blown on me ever.
No visible solder lifting/cracking from leads.

speakers seem to be operating fine and like I said it happens after the amp warms up for like 15 minutes and I would not really think that speakers would typically have a problem of that nature.

Suggestions? Questions?
sorry for the novel above but I figured more info would be better so that you can fully understand the situation.

thanks in advance

Steve O

[ubernerdsteve]

don't know why I said rectifier tubes above, meant to say phase inverter tube (it has a ws bridge rectifier).

[Andrei_T]

It's a long shot but did you check the speaker cones? maybe what damaged the wire got to the cones also...

[Andrei_T]

Or you biased the tubes too hot and after 15 min they start red plateing.

[pdf64]

I suggest that you definately eliminate the speakers as the cause by plugging into a different cab for 30 mins the next rehearsal, it's amazing what weird symptoms bad speakers can cause.
If substituting known good speakers doesn't solve it, one possibility that comes to mind is that the power amp is on the verge of ultrasonic oscillation, maybe caused by a dodgy 12at7 phase inverter or a ribbon cable routing problem. Pretty difficult to diagnose and solve unless you got the experience and test kit, so if subbing in a known good 12at7 doesn't help then take it to a tech.
Hope that helps - Peter

[ubernerdsteve]

Thanks for all of your help guys. I pulled it open and metered everything to try and find any odd voltages or current draws and it turns out one of the solder connections to the phase inverter was pulling off the board and as it heated up would lift more. This is turn screwed up the phase inverter tube. Soldered it and put in an ax7 i had laying around and everything is back to normal.