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). It´s 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, it´s 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 it´s helpful. Oh, and it´s a head, not a combo.
Here´s the noise:

http://soundcloud.com/arifig/il-mostro-noise/s-ghrub
 
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 it´s frequency related. The amp doesn´t 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 it´s 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 it´s not speaker/cab rattle. Is this motorboating? I don´t 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: It´s biased quite hot, around 85%-90% of the rated dissipation of the 6V6´s. That are not actually 6V6´s, 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. It´s quite odd to me!
Any help appreciated. Thanks!

[Cups]

Maybe a schem would help. I got a similar sound when I opened the loop on a champ clone I built. So maybe add more resistance to the feedback?

The reason I'd like to see a schem is maybe it's related to grid values or maybe you're clipping a stage or something.

[Mostro]

Thanks for the answer!
OK, here´s the schem. Some final values might be tweaked in the real amp, but nothing too drastic.



The amp is essentially a clean thing, no distortion by design. I could also post voltages if it´s any help.

[plexi50]

What effect is the 270k grid resistors having on the over all tone of the amp? Never seen grids with that high a value before

[John]

would they be forming a voltage divider with the R21 and R22? (asking dumb questions helps me learn stuff)

[Mostro]

Those high value grid resistors were meant to tame excessive brightness in the amp. Hard to say if that idea really works, cause there are other high end reducing measures taken, like the cap to ground at the pre out.

[tubenit]

meant to tame excessive brightness in the amp

That is an odd noise.  I'd make sure the guitar strings aren't too low and hitting a metal fret?  IF you have not already tried different preamp and power  tubes, then I would try that.  I had a Sovtek 5Y3 rectifier that was so noisey rattling that it was unuseable for me.  You could have oscillation issues from the layout or poor lead dress also?

Regarding the brightness to the amp, I've drawn in some values that I would be tempted to try based on Tweed BluezMeister and Tweed Overdrive amp  clean channels that I have.

Just some thoughts.  Do move that 47p snubbing cab after the coupling caps and prior to the 220k resistors to ground, please. It will help with the transparency of the tone, I think.

I also would be inclined to raise the .001 going into the LTPI to at least .0047 and the post LTPI coupling caps to .02 or .03 (as shown). You could carefully try clip wires and caps paralleled to do this possibly?

The enhance cap across the 82k plates resistor has worked out well for me on all 3 of my amps. You can use clip wires and try some different values there.

Anyway, these are some things I'd be inclined to try.     With respect, Tubenit

[67polara]

I had a noise like that in my TOS it turned out to be a bad speaker.

Tony

[Mostro]

meant to tame excessive brightness in the amp

That is an odd noise.  I'd make sure the guitar strings aren't too low and hitting a metal fret?  IF you have not already tried different preamp and power  tubes, then I would try that.  I had a Sovtek 5Y3 rectifier that was so noisey rattling that it was unuseable for me.  You could have oscillation issues from the layout or poor lead dress also?

Regarding the brightness to the amp, I've drawn in some values that I would be tempted to try based on Tweed BluezMeister and Tweed Overdrive amp  clean channels that I have.

Just some thoughts.  Do move that 47p snubbing cab after the coupling caps and prior to the 220k resistors to ground, please. It will help with the transparency of the tone, I think.

I also would be inclined to raise the .001 going into the LTPI to at least .0047 and the post LTPI coupling caps to .02 or .03 (as shown). You could carefully try clip wires and caps paralleled to do this possibly?

The enhance cap across the 82k plates resistor has worked out well for me on all 3 of my amps. You can use clip wires and try some different values there.

Anyway, these are some things I'd be inclined to try.     With respect, Tubenit

It is an odd one, isn´t it? That´s why I wanted to show it to others. No string rattle. It´s not that easy! The cap parallell to the 82K intrigues me, what does it do? I´d assume it reduces high frequencies, right? And also the move of the 47p cap to after the coupling caps... the difference I see is that it would not have DC on it, only AC, right? Will it have a different effect on the tone?
Thanks for the suggestions, as soon as I have time I´ll try them.

[jojokeo]

For the noise issue, I've had this happen w/ power tubes. After spending a bunch of time messing around w/ things it was a last ditch effort as they were new too. I thought it couldn't be that for this reason. Save time and try replacing them if possible.

The cap is just another way to kill some more high end.

[Mostro]

Update on this guys: After testing various things like swapping tubes, I replaced the 270K power tubes grid R´s with  2K7´s and the problem went away!  Quite possibly I was exceeding the max R allowed in the grid of the 6V6´s, 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. Well, live and learn! The moral of the story: Don´t go over the top on those grid R´s.

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?