6G3 build: oscillation solved?

[Bieworm]

Hi fellas!
I have a question about oscillation.  When I dimed the tone or volume on the bright channel I got this loud buzz.  It was clearly oscillation of some instability problem. I tried a lot of chopsticking, changing ground points  ..
I finally got rid of it by bridging a 10pf cap across the plates of V2. It's totally gone. And this solved another strange problem.. I found this 20W 6G3 a lot less loud than my 18 watt amps and my cathode biased deluxe reverb. This made no sense.. even when I swapped the jensen P12Q for a Celestion Alnico Cream it still wasn't loud. Now it is!!!
Can I call this case closed or is the problem now masked, but still there?

[SILVERGUN]

added schematic

[Bieworm]

added schematic

Sorry.. and thanks

[shooter]

And this solved another strange problem..

if it's loud as expected then;
if you see no problem, hear no problem, measure no problem, there is no problem 

[Bieworm]

And this solved another strange problem..

if it's loud as expected then;
if you see no problem, hear no problem, measure no problem, there is no problem 

I wonder if a grid stopper isn't the better solution...

[pdf64]

… got rid of it by bridging a 10pf cap across the plates of V2. It's totally gone…

So V2 not V3?
If V2, do you mean 10pF between terminals 1&6?

[Bieworm]

… got rid of it by bridging a 10pf cap across the plates of V2. It's totally gone…

So V2 not V3?
If V2, do you mean 10pF between terminals 1&6?

V2.. so across the plates of the 2nd preamp stage and the trem oscillator. Trem is stronger now too.
Should I rather be trying a grid stopper on V2A? That's the standard first step in troubleshooting oscillation when chopsticking changes nothing, no? 😃

[pdf64]

Whatever works, works, I suppose 
If you want to carry on tinkering, I suggest to move the 220k mixer resistors to the V2 socket terminal, with some heatshrink or tagstrip. Then they can properly act as grid stoppers.
Grid stoppers for the output valves seem a good idea too.
Photos of the build would be useful.

[Bieworm]

Whatever works, works, I suppose 
If you want to carry on tinkering, I suggest to move the 220k mixer resistors to the V2 socket terminal, with some heatshrink or tagstrip. Then they can properly act as grid stoppers.
Grid stoppers for the output valves seem a good idea too.
Photos of the build would be useful.

Forgot to take photos of recent changes.,but I'll have to open it up anyway, since I haven't checked voltages now the oscillation is over. Iran imagine they changed since the volume increased substantially.
BTW, I added 8k2 grid stoppers and 1k G3 resistors on the 6V6 tubes from the start.

[pdf64]

... I added 8k2 grid stoppers and 1k G3 resistors on the 6V6 tubes from the start.

1k g2 resistors 
But bear in mind that g2 is a grid, has gain, and the resistors may act as grid stoppers as well as current limiters. So are better mounted at the valve socket terminal.
But looking at your build, with all that screened cable, oscillation seems to be well mitigated. So in the absence of scoping it for a verified diagnosis, who knows what's going on 

[Bieworm]

... I added 8k2 grid stoppers and 1k G3 resistors on the 6V6 tubes from the start.

1k g2 resistors 
But bear in mind that g2 is a grid, has gain, and the resistors may act as grid stoppers as well as current limiters. So are better mounted at the valve socket terminal.
But looking at your build, with all that screened cable, oscillation seems to be well mitigated. So in the absence of scoping it for a verified diagnosis, who knows what's going on 

Thx. I meant G2... typo😉
When I have the time I will put my Siglent on the bench and scope the circuit to find the oscillation

[glass54]

Hi Bieworm,
In regards to your photo, that's really impressive, ie almost every lead/wire is dressed 
I fuss a little (too much) but you must have had a bit of time on your hands (Covid??)
Seriously, that's impressive 
Kind regards
Mirek

[Bieworm]

Hi Bieworm,
In regards to your photo, that's really impressive, ie almost every lead/wire is dressed 
I fuss a little (too much) but you must have had a bit of time on your hands (Covid??)
Seriously, that's impressive 
Kind regards
Mirek

Thanks?😉 I do my best to make it look good. Build time was ok.. 2 weeks of evenings and weekends,  including the cab build.

[Tone Junkie]

Dumb question but we’re did you get your unused socket whole covers from. Thanks Bill

[Bieworm]

Dumb question but we’re did you get your unused socket whole covers from. Thanks Bill

Those are simple electricians supply grommets for (more or less) rain water resistant cable-throughs.