Champ 600 sometimes gets real bright

[tmenss]

I've got a champ 600 reissue (~4 years old) that's been lightly modded. (~3yrs ago) Bypassed the tone stack and NFB, rebiased for the current power tube. The tubes (JJ replacements) are about 1 year old. Sometimes when I first power it up and play for a while it will be really bright and clear, more so than its regular sound, later it settles down and the tone becomes mellower. Its like it has a bright switch that has flipped off. Sometimes it cycles and goes bright for a few minutes. Actually I'd like to have both tones switchable when I want it though. Any thoughts?

[Jazz P Bass]

First thing that comes to mind is the power tube.
Got a spare?
That is about all you can do from the outside.
Swap power & or preamp tubes.
Short of that you will have to open it up.
Basic trouble shooting from there.
Check the power supply.
Look at the output.
Look at the input.
Divide it down until you can locate which stage the issue is in.

[Gtr_tech]

Sounds like a resistor breaking down under working voltage to me.

[jmaf]

Or a bad solder somewhere.... Poking around with a chopstick may reveal the issue.

[tmenss]

I swapped out the power tube and pre with some I had, but the song remained the same. Since theres no volume change would NFB be a possible cause? It looks like I'll need to get back in there and muck around a bit...with chopsticks of course. While I'm in there I'll put a proper R10 resistor...I paralleled a couple 2 watt resistors to achieve the value I needed from salvaged components. Guess I'll need to cram my Jet City amp into my practice space while the Champ is benched. Thanks...I'll get back when I figure it out.

[Mandopicker]

I agree with jmaf...check those solder joints in the areas where you replaced any parts. I would even suggest taking the board out of the chassis and look closely at the solder pads to make sure they are well done.

[Jazz P Bass]

While you have the chassis out, just resolder the amp.
That will eliminate that part of the equation.
You have something goofy going on.
Resolder it.

[tmenss]

Thanks for the replies, Luckily it is a very simple layout to troubleshoot. I"ll post back what I found.