Fender twin reverb (ab763) PI trouble

[Chuck H]

Seems like I'm addicted! I'll definatively will hang around on this forum!

Te he... It's all over now bud. You USE TO be a guitarist that wanted the right tone. Now your just an amp tweak. It happens to the best of us (and that ain't me). Welcome to the forum. I cut my teeth here asking all my dumb questions almost two decades ago!!! My playing has suffered a bit (though not entirely because of my passion for amps) but I've never had better tone than since I joined this forum those many years and three incarnations ago. At the very least your certainly in for a fun ride.

[Lifto]

Hi all,

first of all happy holidays to you all!

I have been enjoying my amp tremendously over the last few weeks/months. The amp performs perfectly apart from 1 aspect. When the reverb knob is turned past 7.5 the amp starts oscillating extremely loud (window shaking loud) and very low pitched. From 0 to 7 there is a noticable increase of noise (which might be expected from a reverb).
I swapped V3 and V4 for other tubes to see if this was a tube problem and replaced the coupling cap. The amp will still do this when i disconnect the reverb unit. It seems that V3 gets very hot when compared to other tubes but voltages on the tube seem ok.

Also sometimes it seems like there's a very light (but noticable) tremolo on. Especially when the amp is played 'loud' and in the lower registers.
Does anyone have an idea where to look next?

[Chuck H]

Is the noise like a growing rumble or an even hum? And is this light trem effect associated with the reverb setting or does it happen even with the reverb off?

There are a couple of things that it could be and it's possible the two issues are related. It could be reverb pan feedback. Or it could be an improper ground scheme causing low frequency oscillation and hum.

The reverb driver tube is working very hard. It'll be hot for sure.

[Lifto]

Is the noise like a growing rumble or an even hum? And is this light trem effect associated with the reverb setting or does it happen even with the reverb off?

There are a couple of things that it could be and it's possible the two issues are related. It could be reverb pan feedback. Or it could be an improper ground scheme causing low frequency oscillation and hum.

The reverb driver tube is working very hard. It'll be hot for sure.

I would describe it as a growling rumble rather then an even hum. But it really loud so I usually don't leave it on. When turning up the reverb from 0-7 it sounds like the amount of hum is gradually increasing and when turned up further it'll feedback but on a very low pitch. I disconnected the reverb pan but the amp would still do the same so I think the reverb pan can be dismissed as the culprit.
The tremolo effect also occurs when the reverb is turned to 0.

I will recheck the grounding scheme. I will compare my grounding scheme with grounding as on the ab763 layout.