Oscillation at pin 3 on the 6L6 output tubes

[Normybaby]

I just completed a Weber 6A14HP amp chassis and everything seems to be OK except for a kind of after tone when I hit an open A note on my guitar (5th string). I checked the primaries at pin 3 on the 6L6’s on both output tubes and I’m getting a 3.5V saw tooth ripple (oscillation) at about 400 Hz. I think this might be the cause of the after tone; anyone have any ideas on how to stop this.
Thanks
Norm

[Alex R]

- you'd normally see a sawtooth waveform there at 100/120Hz (ie rectified mains); the ripple remaining after the first stage of filtration. It can be a volt or two or even three, there being no choke on this amp. If it does check out to be rectified mains (bet it does) you could have a look first at lead dress - try preamp plate and cathode preamp leads close to the chassis, grid leads in the air, making sure there's some distance between plate and grid leads. Speaker leads and OT/plate leads radiate a lot - keep them as far away from the preamp as poss.

More likely though this is down to the design. I note that all four preamp stages are on the same filter cap (junction labelled D). That could be enough to cause oscillation all on its own. I'd be tempted to put in another power resistor (say 10K) and 20uF cap for the plates on first two stages (ie pins 1 and 5 on V1) - decoupling them from the next two might well help. If the voltages on the first two stages then looked a bit low you could try a 15K to replace R46.

Or wait for someone wiser than me to reply...

[Normybaby]

Right on!

Thanks, I think you're right on the mark with your response. Yes, I miss-read the OS frequency; it looks to be more like 120Hz. Thanks for all of the fix-it suggestions. The amp doesn't really have much hum at all just that a buzz overtone at 440Hz. I'll try fooling with the tone control and/or plug it into a different speaker cab; it may be that the speaker/cab on my 65 Deluxe reverb (re-issue) that I’m using as at a resonating frequency. Otherwise, this thing sounds great!
Thanks again
Norm

[Alex R]

No problem, glad to help, as I receive plenty of help on this forum. Karma payback! (does that age me?)

Just to clarify: the extra filter suggestion wasn't to stop hum but to decouple the preamp stages, as the following note effect you describe is a common symptom of oscillation, which lack of decoupling can cause. This is a strikingly un-decoupled circuit!

Also btw: the reason you don't hear the rectified sawtooth hum on the plates of the power valves is that the push-pull arrangement cancels it.