2204 fade out issue

[ledvedder]

What could be causing this as the notes fade?

 

[glpg80]

Bypass the loop, does it still occur?

 

[311splawndude]

swap in some different power tubes too

 

[burger]

is this an old Marshall or your new build?

 

[ledvedder]

is this an old Marshall or your new build?

It's my new build. I think I may have fixed it by putting a 470pF across the cathode follower. But I'll have to test it some more.

 

[burger]

It's my new build. I think I may have fixed it by putting a 470pF across the cathode follower. But I'll have to test it some more.

That essentially sets up a low pass filter, probably just masking the issue which is still there.

 

[burger]

It's my new build. I think I may have fixed it by putting a 470pF across the cathode follower. But I'll have to test it some more.

Have you dialled in your bias at idle. What are the readings?

 

[glpg80]

Have you dialled in your bias at idle. What are the readings?

Yeah I’m thinking the same thing. Bias related. Or grounds/solder joints related to the power tubes.

 

[SpiderWars]

How loud is that clip? If it was a cranked plexi I would tend to think power tubes but in a MV it seems it could be V2 as well. V2 is a 'hot' and sensitive area in Marshalls.

And just to be sure ..this is NO clipping diodes right?

 

[ledvedder]

Thanks for the responses. I haven't had an opportunity to contine testing over the last few days. The bias is set to 35-36mA. Clipping is not engaged. That clip is very low MV. Maybe between 1 and 2?

And this only occurs when I have it switched to the JEL mode. It doesn't happen in the stock or the #34 mode.

 

[SpiderWars]

I would check voltages throughout and do that in each mode. It does sound kind of bias related (preamp tubes have an idle current too) but the fact that the cap cured it points to instability...which can also muck with the idle current.

 

[burger]

Thanks for the responses. I haven't had an opportunity to contine testing over the last few days. The bias is set to 35-36mA. Clipping is not engaged. That clip is very low MV. Maybe between 1 and 2?

And this only occurs when I have it switched to the JEL mode. It doesn't happen in the stock or the #34 mode.

Check the grounding on that 3k3 resistor and 0.68uF bypass cap.

 

[glpg80]

What order did you put the diodes in?

 

[ledvedder]

What order did you put the diodes in?

How do you mean? I have them on a 3 way switch like this diagram. I'm using a 0.22 cap instead of 0.47.

 

[ledvedder]

I tried recreating the issue this morning. For the life of me, I can't get it to happen. Not sure what changed since the other night. I checked grounds and voltages. Plates are 447v, 321v at the PI. Bias is set to 36mA. The effects loop voltage is 313v.

Intermittent problems are so difficult to diagnose.

 

[FourT6and2]

Hard to say what the problem is without the amp in front of me. But I'll say just from those few chords, the amp's tone sounds gooooood.

But yeah, could be a tube issue. Could be a cold joint. Could be filtering. Could be any number of things.