fender twin reissue - tremolo / reverb circuit causing overheating.

[tibbar6]

Howdy, I've run into a peculiar problem with a 2003 twin reverb reissue that found its way on my bench. I have a good amount of experience with amp repair, mostly with older point to point wiring and im trying to get this project wrapped up for a client of mine. Any suggestions are appreciated.

Heres whats going on - the amp runs fine without reverb or vibrato tubes are normal temp, and bias is at spec. However when you turn the reverb knob up it makes a terrible low hum sound even with the tank unplugged. When the Tremolo knob is turned up, sound becomes weak and the power tubes begin to overheat. Supposedly this problem has been bad enough to even blow fuses at a gig. I probed the bias when this overheating occurred and watched the bias rise way out of spec.

Tubes on the amp have been replaced with new JJs and it didnt make a difference. I have inspected the boards and components and i cant see any cracked or burned components.

Does anyone have a schematic for this? I can't seem to find any online that match this make/year. Let me know if you've run into this kind of thing before, thanks in advance!

[nevetslab]

Fender 65 Twin Reverb Service Manual

I have a 65 Twin Reverb Service Manual here:

65_Twin_Reverb_SvcMan.pdf

I haven't run into this particular problem on the Twin Reverb, re-issue or vintage. It sounds like a power supply problem. Do you have a power analyzer to monitor AC mains current/wattage that you can plug the amp into? One can build a safe-and-sane AC mains current measuring box to adapt your DMM to for such purposes. It does sound like mains current is increasing.

With regards to the Reverb hum, I sometimes find someone has gone in and replaced an open reverb tank, installed it 180 deg from where it had been, which puts the internal tank transformers into a higher hum field. I'd first rotate the tank and see if that makes any difference, or remove the tank from the cabinet, so it's further away from the power transformer magnetic field. The normal orientation of the tank has the RCA connectors facing the front of the amp)

With the tank disconnected, turning the Reverb (return level) up, it will ALWAYS add noise/hum since that input is unterminated.

Since the tubes are getting hotter, sounds like the bias circuit voltage is decreasing, which increases the plate current. May be a filter cap going bad on you, both in the bias circuit as well as in the main supply filters located in the covered housing between the choke and output xfmr. You can monitor the DC supplies that feed the C/T of the output xfmr, as well as the bias voltage. You'll see those test points on the PCB layout in the manual

[Tom Phillips]

Also scope the output of the reverb circuit and the output of the phase inverter to check for high frequency/ultrasonic oscillations. A bad decoupling cap or connection at the cap can be one cause of oscillation.