Super Champ XD Mod Sounds Great...then it doesn't

[JustMike]

I found the DSP board bypass mod and did it to a dead amp. This little amp sounds really good! But after about 5 minutes of playing at full volume-which is a little loud for in the house, but probably not loud enough to play with a rock drummer- it gets a little distorted and the volume fades to less than half. I don't see any red plating and if I turn it off for only a couple of seconds and back on, it plays at full volume.   Any ideas what could be causing this?

[tubenit]

When I have had that happen it has been an oscillation issue. It was resolved with new power tubes. 


With respect, Tubenit

[JustMike]

Hmm...I have new JJ6v6's in it. Grid stoppers?

[tubenit]

Not saying power tubes are the issue in your case.  It could certainly be something else.


I will say the first time I had oscillation problems were with brand new EL84 tubes right after I installed them.   


With respect, Tubenit

[JustMike]

 Could it have anything to do with the 5u tone cap? I didn't have a regular one, so I used an electrolytic. Tied the - to gnd.

[tubeswell]

Maybe heat-related part failure

[glass54]

Mike
What is the DSP bypass mod that you invoked. I could think of one BUT what did you do?
It might help us narrow down the culprit 
There is a Mute circuit (Q3) between 2 Op Amps which may be contributing to your worries??
Regards
Mirek

[JustMike]

 I think it's been discussed here before.

[JustMike]

I have it on the bench and Im monitoring plate voltage. Pretty steady at 377v. When I touch just the tip of the input cable, The plate voltage drops to 317. This happens only when Im not grounded and the amp squeals. Is this normal? When im grounded (touching both tip & sleeve of guitar cable, plate voltage is steady).

[JustMike]

 So this probably isn't the most efficient way to troubleshoot, but I put in a power amp input jack. I've been running an external preamp into it and it's not acting up. So this points to the op amp preamp stage. I guess I could get the old can of freon and start spraying components. A friend who I do repairs for has offered me a scope for my services. He has no idea what model it is, but we'll see.

[glass54]

I think you have done good 

So this probably isn't the most efficient way to troubleshoot, but I put in a power amp input jack

That's probably the way I'd attack the situation, test and prove the PA is all well.
Then....
Regards
Mirek