Odd Distortion Noise

[Jazz P Bass]

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.

I do not understand why you cannot see it on the scope.
If not, then how do you know it's 'in the power tubes'?

Is there any change by going in the 'Microphone' input? (obviously a hotter stage)

[Daver]

I used a probe I built to output the signal to another amp while the test amp was hooked up to a load. I started at the preamp and worked my way through to the power tubes. No distortion at the grids but it's there on the plates or at the OT secondary.
Yes, I tried other guitars........

[Jazz P Bass]

No scope, huh?

Maybe the probe is masking the issue.

[Daver]

Yes, I have a scope and have used it with both a sine wave and a guitar. There are NO unusual artifacts that show up. No spikes, flattening of peaks, duty cycle changes or anything that looks out of the ordinary. That's one of the things that has made this so difficult and frustrating. Using the probe has been the only thing that has helped narrow it down other than the ton of substitution I've done.

[Jazz P Bass]

I don't know.
Maybe it's the 1940 ish design.
That PI circuit sure looks odd.

[Daver]

That's not like the amp I have. There are links to the schematic in post #4. I have the 6L6 version. It's a pretty standard paraphrase. And it did work at one time.

[Jazz P Bass]

Oh.
It still looks goofy with all of those meg resistors.
Almost like they where trying to squash 'something'.

What is the Mic input like?

[g1]

It still looks goofy with all of those meg resistors.

Those real old schems use M to designate K, but Meg means meg.
So, for that 6N7 PI, the cathode is 1K, the grid is 100K.

[Tom Phillips]

…I fabbed up a box with a blocking cap and a pot and some jacks. I made a shielded test cord with a couple of alligator clips on one end and a 1/4" jack on the other. I hooked up the amp to a load and used another amp to monitor the output from various points in the amp. The distortion is coming from the power tubes. It is not present on their grids/phase inverter output but is on the plates of both tubes. Now I have already subbed out the output transformer as well as the power tubes and have changed the cathode bias resistor from 150 ohms to 500. None of that made any kind of change…

…The amp was all good after I serviced it six months ago.

…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…

The information you provided sure rules out many potential faults. Even if the problem was always there to some extent it should be fixable. Perhaps the distorting is riding in on the B+ line to the OT center tap? You could listen to that node with your home made signal tracer. You could also scope that point using AC coupling. You could even drive the amp with a pure sine wave and listen with your signal tracer because there should be little to no AC signal at that point. If there is then I’d suspect bad 1^st or 2^nd stage filter cap(s) or their hot/ground connections. It will be interesting to know what you see & hear at the OT CT node.

[olddawg]

This is a 1940 amp. What was the ORIGINAL speaker? Does it have the original speaker? It had to be a field coil type, right? You say you are using extension cabinets for tests? Are these modern cabinets? Has the amp been modified to use modern speakers with permanent magnets? If so, what was done? Choke or resister in place of the field coil? I didn't look at the schematic. I'm on my phone, lol. But something is odd here.

[Daver]

Yes it had/has a field coil. It has been mounted on the chassis and is essentially taking the place of a choke. The owner tells me this is a common mod for these amps when the original speaker dies. I have subbed it out for a resistor with no change in the distortion. Again, it was working in its current configuration. Thanks.

[Jazz P Bass]

What are the chances that one of the E caps, that were installed last repair, went belly up.

They sure are using some low value caps.
Have you clipped a known good one across each cap?

If the preamp decoupling cap is not up to snuff, the signal can actually ride on that supply node.

[olddawg]

Yes it had/has a field coil. It has been mounted on the chassis and is essentially taking the place of a choke. The owner tells me this is a common mod for these amps when the original speaker dies. I have subbed it out for a resistor with no change in the distortion. Again, it was working in its current configuration. Thanks.

I only mentioned it because you said earlier that you were not really sure if you had the problem before and didn't notice it. My notion was that it might be an artifact caused by the modification. Especially since the problem seems to be in the output amp.

[g1]

Agree with above, it will have a major impact on troubleshooting which of the following are true:
a)the problem was definitely not there 6 months ago
b) the problem may have been there before and been unnoticed

[olddawg]

One thing I would do move that field coil from off of the chassis and away of the cabinet for tests. (You might get some weird static coupling through the chassis) If I had an inductance meter I would be tempted to measure it and sub in a choke that is close. What does the ripple look like on either side of the choke/field coil?