Old Fender Twin Buzz in Reverb

[dstiner11]

My buddy has an old Silverface Fender Twin Reverb (1984) that sounded fantastic until a few weeks ago when it started buzzzing until you switch off the reverb. It sounds like a nasty mosquito. It only buzzes when you play. When you switch off the reverb, the buzz is gone. I just put new sprague atom caps in today because the originals were visually leaking. I was hoping somehow that was causing the buzz but it wasn't. Still buzzes. I tried replacing the 12at7s and 12ax7s one at a time but no luck. I cleaned all the reverb connections. I looked at the reverb tank and the everything is intact and the springs are not broken. I'll have the chassis out again on Thursday and I was hoping someone might have some suggestions. Thanks in advace. I love this site!

[Enzo]

Unplug the foot switch completely. ANy help?

[pdf64]

Nothing to do with the reverb, but it's good practice to replace the bias cap every few decades. I'm not one for replacing parts unless they're bad, but if that goes bad your amp is toast. Plus the original part was generally underrated anyway. So if the reservoir or filter caps are leaking then it's time to change that one as well.
It'll likely be on a little board near the power transformer / jewel light fitting, about 50uF at 50V. Replace it with a long life 50 or 100uF at 100V rating, as it has about 60 to 70 volts across it. Remember that the positive end goes to ground.
The other parts that have a hard time on that amp are in the reverb driver circuit, the tube itself and the cathode resistor and it's bypass cap. The SFs had a way too low value at 1k, the BFs were 2k2 or 2k7, which puts a high dissipation on the tube and resistor, and so too much heat on the cap. Replace the resistor with a 2k7, replace the cap, and the amp can breath a sigh of relief. Or leave the cap off for a less boingy reverb.
As Enzo suggests (I think), the buzz is probably due to old age and wear on a cable causing the screen to fail, either on the footswitch cable or the return cable from the reverb tank.
Good luck - Peter

[dstiner11]

Thanks guys. I'm at home today and took the advice to replace the bias cap. There were two caps there and I replaced both. They were both 50 Mfd 70V and I only had 47Mfd 450V caps so I used those. I know 450V is overkill but they fit just fine and it powered up good but of course, it still buzzes on reverb. Unplugging the footswitch made no difference and I am really suspecting the old cables from the reverb tank as suggested by Enzo.

As soon as I get some carbon comp resistors I'm going to replace the cathode resistor and cap in the reverb driver. I have some thick shielded RCA cables that are meant for a VCR but I assume these won't work because of the voltage. Is that correct? I'll look online and see if I can find the real replacements. Thanks for all of the info. I think I'm getting close to resolving the buzz.

[Enzo]

There is no "voltage" going to the reverb pan. ANy cables will work.

If you think the pan cables are at fault, unplug them. NOW does the amp make hum when the reverb control is advanced? If that stops the hum, you either have a bad cable or a bad pan.

[dstiner11]

Hey guys. Thanks for helping a novice amp troubleshooter. I truly appreciate it. I fiinally did what I should have done up front. After trying new cables, I connected the reverb tank from my Vibrosonic and the buzz was gone. I'll look for an exact replacement since his amp sounded freaky good before the tank failed. Now I can start troubleshooting the vibrato problem on my Vibrosonic Reverb.