I've seen these things happen due to bad grounding or problems with the HT powersupply too. The way to discover this, would be a scope with the ground clip at the powersupply ground and the tip at the other ground points one at a time and the vertical "gain" set to max (turned all the way to right). In a perfect world you'll only see a flat line .... in the real world there's at least some noise + hum there. If you see the same signal as the one you're trying to get rid off, you just might have found the reason.
I'll agree with SemperFi, you badly need to put some gridstopper resistors on the two EL84 output tubes - I have very good experience with 1K8 used there. And most guitar tubeamps have a 68K gridstopper on the input tube, in this case on pin 2 and pin 7 on V1.
I would also consider adding gridstoppers on V3 - if you hope to keep the sound, then use 1-5K here.
I do not agree that Carbon types are preferable (not that I will oppose using them), I NEWER had any bad experince with Metal-Film resistors - neither soundwise nor with reliability - but I have MANY carbon resistors fail over time .... Other than that I'm aware that this easily can become a Hot Topic and I'm not particularly interested in such a debate, but to me reliability is an extremely important parameter in a instrument amp - believe me - NO fun having the amp crapping out in the middle of a great solo.
If the solution keep eluding you - remember that the requsition for "creating" a oscillator are : 360 degree phase shift at the osc. frequency AND a system gain of one at that frequency.
The tone stack might be a important part of the phase shift, but as SemperFi said, it's not to blame (unless wrongly assembled or poor conections).
I repaired several thousand Guitar tube amps over the years and the last Fender actually had a similar problem to what you have, turned out to be a bad cap in the HT supply in the preamp section.
I'll agree with SemperFi, you badly need to put some gridstopper resistors on the two EL84 output tubes - I have very good experience with 1K8 used there. And most guitar tubeamps have a 68K gridstopper on the input tube, in this case on pin 2 and pin 7 on V1.
I would also consider adding gridstoppers on V3 - if you hope to keep the sound, then use 1-5K here.
I do not agree that Carbon types are preferable (not that I will oppose using them), I NEWER had any bad experince with Metal-Film resistors - neither soundwise nor with reliability - but I have MANY carbon resistors fail over time .... Other than that I'm aware that this easily can become a Hot Topic and I'm not particularly interested in such a debate, but to me reliability is an extremely important parameter in a instrument amp - believe me - NO fun having the amp crapping out in the middle of a great solo.
If the solution keep eluding you - remember that the requsition for "creating" a oscillator are : 360 degree phase shift at the osc. frequency AND a system gain of one at that frequency.
The tone stack might be a important part of the phase shift, but as SemperFi said, it's not to blame (unless wrongly assembled or poor conections).
I repaired several thousand Guitar tube amps over the years and the last Fender actually had a similar problem to what you have, turned out to be a bad cap in the HT supply in the preamp section.
- Status
- Not open for further replies.