Marshall TSL 122 strange sound at louder volumes

[beedoola]

Using the combo as a head. The sound occurs through different cabs so I know it’s not the cab.

I swapped out 3 of the 4 preamp tubes with newer/
known to be good JJs. I swapped the 4th around work other tubes I had. El34 are new EHX. I installed the stable bias module mod on this. Bias stays stable at 90MV.

Here is a clip. Again if only occurs at louder volume. Anyone have suggestions on what else I can do to troubleshoot it?

https://www.dropbox.com/s/f0aaqrgzln...ounds.m4a?dl=0

[Jon Snell]

Which part of the distortion from too much drive are you talking about?
More information is required to form an opinion I think.

[Mick Bailey]

That sounds to me like it could be blocking distortion. Did your amp do this before the bias mod?

[beedoola]

I didn’t have it long before the mod and and I never cranked it that high but from the little I played previous if did not do that.

[Mick Bailey]

Scoping the signal would tell you where the distortion is coming from. Outside of that, does the bias mod change the effective PI coupling cap, bias feed resistor or grid stopper values? A schematic of the mod would be helpful.

[beedoola]

Which part of the distortion from too much drive are you talking about?
More information is required to form an opinion I think.

What do you mean?

I think you’re asking is it an issue that seems to be coming from the pre gain or master? It’s the latter.

The second half of the clip was the clean channel with the gain maybe on 11-noon. It’s when the mater volume is up higher.

The first half of the clip is on the lead channel. That is where I first noticed the issue.

[Jon Snell]

I would inject a 1kHZ sine wave to one of the inputs and with my oscilloscope probe on CON2 pin 1, as a starting point, measure the signal with my oscilloscope, probing either way until the distortion becomes apparent.

[beedoola]

Do I need to monitor the audio output or can I use a dummy load while I test with the scope?

[The Dude]

You can use a dummy load. You are just looking to find where the distortion starts.