Odd noises in my amp!

[Mostro]

Hi guys, I want to ask you about a noise that recently appeared in my DIY tube amp (or at least, I recently became aware of it). Itīs a push pull 6V6 amp, sort of a mutant Deluxe. One pre gain stage, Fender type tone/vol controls, another gain stage, and into a copied-from-the-Deluxe phase inverter, into the power tubes. The biggest differences are, itīs cathode biased, it has a pot for variable negative feedback, the power tube grid resistors are quite large (around 100K, if I remember well), a passive loop, a cap to ground after the last gain stage to tame a little the highs, and some values slightly modded here and there. I can post the schem later if itīs helpful. Oh, and itīs a head, not a combo.
Hereīs the noise:

Il Mostro noise by True Mostro on SoundCloud - Create, record and share your sounds for free

This is a low E, palm muted, guitar>Fuzz>amp. Then a low G, A and finally a low B. All on the 6th string.As you go up in freq, the noise eventually goes away, so itīs frequency related. The amp doesnīt add distortion in the recording, I just used the fuzz so as to be able to record it clearly. It also does it just with guitar>amp, though itīs less obvious. And it takes a little while after the amp is on for it to start. So heat related?
I tried it with different cabs, so itīs not speaker/cab rattle. Is this motorboating? I donīt have experience with that.
Apart from this noise, the amp works very well, healthy amount of volume, all controls work, no hum, buzz or radio stations... Another thing: Itīs biased quite hot, around 85%-90% of the rated dissipation of the 6V6īs. That are not actually 6V6īs, but the russian equivalents (6P6S I think).
Well, I wanted to show you people this before I pop it open, to see if it sounds familiar to you. Itīs quite strange to me!
Any help appreciated. Thanks!

[Chuck H]

It sounds like a parasitic oscillation to me. This usually happens due to layout, lead dress or ground scheme.

Does anything change the issue? Changing the variable feedback, tone control settings? Does it only happen above a certain volume setting?

Can you post a schem and maybe even a photo of the inside of the chassis?

[Mostro]

Thanks for the answer!

hereīs the schem:



I forgot to mention that I tried the guitar plugged directly into the power amp in, so bypassing the pre, and the noise itīs still there, so I believe itīs a power amp issue.
I donīt have any photos (or camera, for that matter...) but Iīll see if I can make some half decent ones with my cell phone.

[Chuck H]

If it happens with the guitar straight into the power amp it's probably not a parasitic oscillation. Since you tried different cabs it's not speakers. Things it could be:

Low batteries in active pickups
Mechanical vibration of the power tubes or even the output transformer
Mechanical vibration of something in the room
Mechanical vibration of something on the guitar

[Mostro]

Update on this guys: After testing various things like swapping tubes, I replaced the 270K (!!) power tubes grid Rīs with 2K7īs and the problem went away! Quite possibly I was exceeding the max R allowed in the grid of the 6V6īs, or too close to the limit. So the thing went into blocking, or grid current, or something like that (not too sharp on the inner workings of tubes). Still have to do more testing but it looks as if that was the thing. You might wonder why it had such high value Rīs in the first place. It was an attempt to reduce harshness when the amp was first built, by taking advantage of the Miller effect to reduce highs.
Well, live and learn! The moral of the story: Donīt go over the top on those grid Rīs.

Thanks to all for the help.

EDIT: when testing, I became aware that the recto tube (5Y3GT) makes a rattling noise when powered (not in standby). Strange, right? It works OK, and has for some time. No red plates or anything strange besides that. Tried a GZ34 and no rattling. Both are Mesa branded. Will it explode soon?

[J M Fahey]

I heard 2 "noises" mixed with your guitar signal.
One, that is constant all the time, a slight buzz, is normal, and not bad sounding.
The other one, is a "chirp" which sounds as an oscillation caused by instability.
Its not oscillating continuously but appears for a short time during each cycle.
It also appears whan playing some notes and not on others, because the complex impedance load presented by a real world guitar speaker, full of resonances.
Iīm sure that if you hook a scope there, you will see a "normal" waveform most of the time, which during a small part of the cycle has a small parasitic oscillation "mounted" over the main audio frequency waveform.
Probably caused by the way too high series grid resistor.
You tried to cut some high frequency, but it probably caused an unexpected phase shift which worsened things.
Oh well.
Glad that you could solve it.
The rest of the circuit is conventional and should give no surprises.
If any, itīs certainly because of a layout or grounding problem.
Good luck.