Odd Distortion Noise

[nickb]

I replaced the coupling caps to the power tubes when I added the grid stoppers. They were modern Nichicon radial metal film, but like you guys, I figured it was worth a try. I'll try scoping the PI outputs again to see how symmetrical they are. I have replaced all the resistors on the PI already. We'll see. Thanks!

I can't help thinking of "Car Talk" and the "Stump the Chumps" spot.

[Daver]

OK, I took some time of from this thing for the holidays, other projects and to clear my head. The PI outputs are very symmetrical up until they start clipping, of course. The background overdrive sound is still there. Even if I just lightly play the strings with the amp on "2" you can hear it. I'm starting to wonder if it wasn't always there to some extent and I and the owner never really focused on it. I don't know where to look. I've ruled out a ton of things. I'm getting tunnel vision or tunnel hearing. Any other thoughts?

[TimmyP1955]

Split the amp and preamp sections, and try them with the sections from another amp, so you can isolate it somewhat.

[Daver]

Timmy, I already know the noise is coming from the power tubes. See post #16. I just don't know why yet. Thanks.

[überfuzz]

Is the 300 V stock? Seems a bit low for 6L6..? Have you checked for unwanted jumpers to ground or bad grounds in the power section?

[g1]

Is the 300 V stock? Seems a bit low for 6L6..?

This is a very old amp, around 1940, when 6L6 were a 19W tube and had lower voltage ratings. Plate voltages at that era's line voltage were a bit less than 300V, so with modern line voltage 300V ballpark sounds about right.

[TimmyP1955]

Maybe I missed this too: Have you changed the cathode bypass cap? Checked that the ground is good on it?

[Daver]

There is no bypass cap. I did try adding one, but that did nothing. Thanks for the suggestions and help.

[g1]

Can you get the bad sound with a sine wave? Then post a picture of the scope display?
Maybe someone will recognize something.
Or if the signal can be eliminated as the source, that leaves the speaker, cab, or sympathetic vibrations/rattles etc.

[Daver]

The distortion doesn't show up on the scope anywhere through the circuit with a sine wave. I've even scoped it with a guitar plugged in. I've already tried several different speaker cabs. All I know for sure is that it's created in the power tubes and is always there at any signal level. The OT has been replaced too. Thanks!

[g1]

Ok, I re-read and saw where you used a signal tracer and did not hear the problem at power tube grids but it was on the plates.
Can you hear the problem with sine wave, or only with guitar?

In case this will give anyone any ideas, this is supposed to be a class A amp, thus the high idle current and lack of B+ drop when pushing signal.
Would the fact that the original 6L6's were 18W tubes have any bearing with regard to class A as far as using 30W 6L6's goes?

[Daver]

I never tried listening to the sine wave. Painful! I'm not sure it would show itself. When playing the guitar the distortion is kind of in the background and most easily heard when a note or chord is played and you let it sustain. As the original sound begins to fade the distortion is more evident. You can hear it along with the guitar as it's played but not as easily as when the sound trail off a bit. Thanks.

[Jazz P Bass]

That description (especially that a sine wave will not cause it) sounds to me like a bad interstage coupling capacitor.

[Daver]

Yet all of them have been replaced. And, again, this is happening in the power tubes and not the phase inverter so that should eliminate any coupling cap. Thanks.

[gbono]

Try a different guitar.