Tremolux Amp repair FINISHED

[martin manning]

The signal out of the PI with no power tubes looks good, but the crossover notch in the output suggests this may be blocking distortion. Try increasing the power tube grid stoppers to 5k6 or 8k2, or lowering the bias feed resistors (the 220k's) to 100k.

[JazzGuitarGimp]

Xtian, my previous comment was based on the assumption you measured the PI output (second photo), looking at both PI outputs with both input channels of the scope, and the scope in differential mode. Or are we seeing just one of the PI outputs? If it's just one output, have you confirmed the second output looks identical?

[Milkmansound]

I find it easier to go the other way. Grab signal from the output of the PI and put it into a mixer or powered speaker

How? The PI has only half the wave on either plate.

They are out of phase with each other but your test jig won't care. I would check all values in the PI - if that 470 on the cathodes (I assume that's what is in there) drifts too high you'll get some interesting distortion.

[xtian]

Xtian, my previous comment was based on the assumption you measured the PI output (second photo), looking at both PI outputs with both input channels of the scope, and the scope in differential mode. Or are we seeing just one of the PI outputs? If it's just one output, have you confirmed the second output looks identical?

Yes, you're just seeing one leg, and the other looks identical, though phase flipped.

Thank you very much, everyone! I'll probably run the preamp channels out to an external power amp to confirm they sound OK. Then I'll replace the aging components in the PI and see where that gets me.

[Firestorm]

I think you've got a problem in the PI. Notice how the negative peak bulges down out of what the waveform looks like on the positive peak (second photo)? I've seen this in Merlin's preamp book. It's an unwanted distortion, if I recall correctly.

That gain spike Merlin talks about occurs when the tube is already cutoff, so no problem unless the tubes are horribly mismatched or biased so hot they're still in Class A.

[Firestorm]

Xtian, my previous comment was based on the assumption you measured the PI output (second photo), looking at both PI outputs with both input channels of the scope, and the scope in differential mode. Or are we seeing just one of the PI outputs? If it's just one output, have you confirmed the second output looks identical?

Yes, you're just seeing one leg, and the other looks identical, though phase flipped.

Thank you very much, everyone! I'll probably run the preamp channels out to an external power amp to confirm they sound OK. Then I'll replace the aging components in the PI and see where that gets me.

Certainly clean up the PI components. And maybe try reducing the grid loads as Martin suggested. (Maybe start with 150k rather than 100k).

But running the pres out to an external may not tell you anything if the problem is parasitic coupling between the pres and the PA. If you have an external pre you could drive the PI with, that might tell.

[xtian]

Replaced cathode R (had drifted up to 520) and matched 1Ms (read 1.2M each when removed). No change.

Next up: 220K bias pair to 100K. 1K5 grid stoppers to 8K2.

[martin manning]

Next up: 220K bias pair to 100K. 1K5 grid stoppers to 8K2.

Go slow here, one thing at a time, and I'd try 5k1 or 5k6 on the grid stops first. You'll lose some bass and volume with reduced bias feed R's.

[Baddog01]

Try putting a spacer between to two boards. My fender Bassman 50 was making a frying bacon sound until I installed some nylon washers between the boards. She's quiet as a mouse now.
The board also became conductive so I grounded all unused eyelets which also helped quiet the amp even more.

Just throwing out ideas here.

Keith

[xtian]

Changed 220K bias pair to 100K. 1K5 grid stoppers to 5K6 (didn't have 8K2 on hand). No change. Must be barking up the wrong tree.

Keith, I like your idea.

[Baddog01]

I got my washers from my local hardware store but I suppose you could just wedge something between the board to test it on the bench. 1/8 " thick nylon washers worked great in my amp with the original screws.

I hope it works.

[cbass]

I wonder if you put the amp in a room with a dehumidifier if it could pull moisture out of the board .at least maybe temporarily

I know it makes a huge difference in plywood thickness when I run one in my shop

[Baddog01]

My amp must have been kept in a basement most of its later life. The board looks like a wave on the ocean and almost looks like it has mold on it. The Baseman is quiet now and sounds darn good so I'll leave her alone for the time being.

You shouldn't store your amps where you wouldn't sleep .

Keith

[xtian]

I removed the screws and floated the whole fiberboard. No contact with the backing board. No change, same old fizzy distortion.

[martin manning]

Might be getting down to trying an OT swap.