Fender Twin Reverb (ab763) motorboating?

[Lifto]

Hi all,

Been a long time since I posted but I haven't been away (just out of sight). After rebuilding a '73 Twin Reverb I've completed all sorts of other projects and recently build my first amp from scratch (a 5e3 Tweed). I must admit I couldn't have done it without the ton of information that I found on this site. Thank you all for sharing your knowledge with the rookies like myself.

Although there is a lot of information available, there is a problem I can't find the answer for. My '73 (100w) Twin I rebuild to AB763 specs is working perfectly and I'm loving the tone, there is only one problem. When I turn the reverb past 8, out of nowhere a incredibly loud and low pitched rumble occurs. It seems like it feedbacks only difference is, it is a low pitched tone. The sound is suddenly there and (very loud) doesn't slowly swell up. I tried different reverb cans and swapped tubes (to no avail). Is there a point where I should start looking? Maybe the Reverb coupling caps? I build the the circuit on a turretboard using orange drops and f&t caps.

Thanks!

[g1]

Try pulling the reverb tank out of the bottom of the cab and further away from the speaker magnets. Any difference?

[pdf64]

If not that then try replacing the cathode bypass cap on V4.
Pete

[Lifto]

This weekend I had some spare time and looked further into it.

I had another twin around so first thing i did was (double) checking if it wasn't a hardware or tube problem. I tried the cables+reverbtank from the other Twin and the same problem occurs (at the same setting). After that I tried the using the preamp tubes from the other Twin to rule out any tube issues. I also checked the values of the resistors int he reverb circuit and changed the cathode bypass cap (the cap connected from pin 3/8 on V4 to ground). This didn't help as well.
Is it possible that the problem I'm having might be a ground loop/wiring issues? Which isn't really an issue unless I dial the reverb up past 8?

[Jazz P Bass]

The low frequency tells me it's in the power supply.

Have you recapped it.

If not, I would look at the plates of V3 & V4 A for ripple.

[Lifto]

The low frequency tells me it's in the power supply.

Have you recapped it.

If not, I would look at the plates of V3 & V4 A for ripple.

Yes. THe amp has a completely new board. All resistors, capacitators, pots, tubes are new. The only salvaged parts are the chassis, transformers and tube sockets. The amp is built on a new turretboard. Come to think of it, the power supply is newly built on the original eyelet board using the original resistors that were still there. I will look further into the power supply section. The amp works perfectly well and almost no hiss or noise. This revebr problem is the only issue to be resolved and I would be 100% happy with it.