69 Marshall Super Lead blowing fuses

[krhnyc]

Hi,

I have a 69 Marshall super lead that is blowing (mains) fuses. It doesn't blow them instantly, rather it must warm up first. I thought it might a bad power tube, so I pulled the tubes and powered it up as a test, still plugged into a load. I left it in the basement, and when I returned the fuse was blown and there was a smell, like ozone or smoke. I looked the circuitry over and nothing stood out as burned.

Any ideas as to what I'm up against? Where I should start to look? Bad filter caps? The smell was somewhat fishy... I would appreciate some guidance. I know the precautions about working with tube amps. Curiously, I metered the filter caps after the fuse blew and there was no measurable voltage stored.

Thanks in advance for your help.

[txstrat]

Filter caps would be my first trace to follow.
If it wasn't the filter caps or the tubes there would be the power transformer to suspect. But since you measured no voltage on the filter caps, and I assume there are no bleeder resistors, are there?
The amp is quite old. Do you know if somebody did a cap job on this one? If not I would change the filter caps and see what happens. It wouldn't be bad for the amp anyway.