Odd Distortion Noise

[Daver]

OK. I did just that. 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. So I'm not 100% sure where to go next but though I'd report back. Thanks for all the help and suggestions. Wish me luck!

[g1]

Like Juan, I immediately suspected the speaker, and missed that part of your original post. Upon re-reading I deleted my original response.
But I would suggest you revisit that swap, and also the filter caps and tubes, which you also covered. Don't know what else it could be, so triple checking is for good measure. Defective replacement parts/subs can be your worst nightmare.

[nickb]

OK. I did just that. 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. So I'm not 100% sure where to go next but though I'd report back. Thanks for all the help and suggestions. Wish me luck!

Well you've narrowed it to the power tubes and that is a big step forward. I wondered if it could be an HF oscillation but then I would have expected you to see that on your 'scope.

What is the bandwidth of your scope?
How about you send a scope capture with a sine wave going in?

Or you could just put in a couple of 2.2k grid stoppers in series with the power tube grids (tight to the grids) and see if that does it. If it does then the question is why, what changed? Did the lead dress change?

Going back to the filter caps I take it that everything was AOK after the recap and this started recently?

After that I'm all outta ideas...

[J M Fahey]

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.

I still suspect the speaker is involved.
When you scope the plates you are also scoping the speaker terminals through the transformer.

Please:

1) scope the speaker terminals .
if you hear something you must also see something.

2) when you do see the problem, turn amp off , disconnect leads to speaker terminals and connect them to another , external speaker.

Turn amp on and retest.

Is the fuzzy still there?

Thanks.

[Daver]

I have scoped the speaker terminals. There are solder tabs on the OT so I'm able to put the scope probe right at the output transformer. Besides the internal speaker I have tried two other cabinets. So it's not the speaker or some interaction with the amps' cabinet. I agree that I should see something. That's what makes this so weird.
I tried subbing out the 20K power supply to ground resistor and tried yet another pair of power tubes. I re-cleaned and re-tensioned the sockets again. I'm going to try adding grid stoppers to the power tubes and add screen resistors. I'll report back. Thanks!

[Jazz P Bass]

There are not any screen resistors installed from the factory are there?
Hmmm.

[g1]

Perhaps as nickb suggested, there is some kind of oscillation that is in turn modulating the signal. The oscillation may not be visible, and the modulation might require slowing down the scope trace to see it, similar to the effect ripple has on the waveform.

[Mick Bailey]

A shorted turn in the output transformer primary can cause distortion, though mostly the output level drops too. Worthwhile checking though, in case the insulation is breaking down.

[g1]

He tried swapping in another OT, and pretty much everything else too.

[Daver]

No, there weren't any screen resistors. Remember this amp is from around 1940. The screens and OT center tap are supplied from the same node. Like a lot of old amps the screen voltage is a little higher than the plates. I put in some 470 ohm resistors on the screens. That didn't change anything. Maybe they're too small? On to the grid resistors. EDIT: I added 5.6K grid stoppers right on the socket. No change. D'oh!
nickb, to answer you previous questions; my scope is a 30MHz. The amp was all good after I serviced it six months ago.

[nickb]

No, there weren't any screen resistors. Remember this amp is from around 1940. The screens and OT center tap are supplied from the same node. Like a lot of old amps the screen voltage is a little higher than the plates. I put in some 470 ohm resistors on the screens. That didn't change anything. Maybe they're too small? On to the grid resistors. EDIT: I added 5.6K grid stoppers right on the socket. No change. D'oh!
nickb, to answer you previous questions; my scope is a 30MHz. The amp was all good after I serviced it six months ago.

Welcome to... the Twilight Zone.. Dee Dee Dee Dee, Dee Dee Dee Dee,...

Edit: I feel like I'm throwing out random ideas now but we don't have much left.
I wonder if one of the coupling caps to the power tube grids has gone slightly leaky? Check the DC voltage on output grids.
How about asymmetry from the paraphase stage? Shove a sine in and scope the power tube grids for symmetry.

[Jazz P Bass]

Random thought #3.
The output tube grids may be masking a bad interstage transformer problem.

[Daver]

Unfortunately there is no interstage transformer. It's the second schematic posted above, not the first. But I do really appreciate the help! Believe me.

[Jazz P Bass]

As posted,it may then be the coupling caps.

[Daver]

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!