Ampeg B-18 issue/debug

[aborgman]

I've been using this B-18 head for the past 4 months or so. It's been performing well, but just in the last couple shows I've noticed it seemed like it was starting to get dirty before it got loud enough.

I thought it might be the room we were playing, or my guitar players new amp drowning me out - but I thought I'd haul it into work and stick it on the 'scope and see how it looked.

Up to ~20 watts - it looks just fine. The signal across the dummy load look just great.

Above that, things start to get ugly. The top half of the wave looks fine all the way up to the point it starts flat topping - but the bottom half has some odd noise burst/oscillation thing going on that gets worse with increasing power levels above ~20 watts.

Just above 20W:


A bit higher:


Just before flat topping (~50W):


...and a close up of the oscillation - it's at ~95kHz.


I pulled all the tubes and cleaned/tightened the sockets.

I swapped power tubes - the burst didn't move.

So it looks like it isn't a bad power tube.



Any thoughts/suggestions before I get around to probing everything?

[aborgman]

So I've managed to trace it all the way back to the first gain stage - above 20-25W it's starts showing up on the output of the first gain stage feeding the tone stack.

It effects both channels, so it isn't the tube... it's got to be coupling from somewhere or a power issue.

All the power looks good - both voltage wise and noise wise. The first stage out of the rectifier is showing normal 120Hz ripple, but no high frequency noise. All the other stages look wonderful.

Bias voltage is good (-51V) and shows no noise.

Chopsticking around the wiring from the input jack->first gain stage->tone control-> second gain stage (which all happens to run back and forth right under the output transformer) showed no change.

Bypassing the the PS caps with a .01uF cap shows no change in the oscillation.