2204 fade out issue

SpiderWars

Well-known member
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

Well-known member
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

Well-known member
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

Active member
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.
 

ledvedder

Well-known member
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.
Screenshot_20220912_132137.jpg
 

ledvedder

Well-known member
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

Well-known member
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.
 
Top