Marshall 2555 Reissue JCM 25/50 HT Fuse Blown

[misterc57]

This is a newer reissue version. Amp sees a lot of usage. Customer states the HT fuse is blowing.

I tested all tubes (test good), tested OT (test good).

Ran the amp in 50W mode for a few minutes with no issue.

Switched to 100W mode, played for a minute and then heard a subtle noise (like something switched) and then saw the current spike. Powered down before HT fuse (T1A) could blow. Waiting to hear back if this mimic the original issue.

Tested again, chop stick everything in sight. Wiggled all tubes, no issue.

Amp is drawing around 1A at the wall plate until it spikes.

All tubes appear to be original Marshall branded. I suspect a tube(s) may be failing after a few minutes of usage.

Looking for suggestions before I try new tubes.

I have attached a schematic but I do not think this is the latest/correct version as I see some rectifier diode numbering not matching the circuit board. Anyone have a schematic?

Thank you, Mark

marshall_25aniv_silverjubilee_50_100w_2555.pdf

[Jon Snell]

I would replace the output valves. One is probably arcing over when hot.

[g1]

Agree with Jon, a tube tester will not detect this kind of tube failure. To paraphrase what Enzo used to say, a tube tester can tell you a tube is definitely bad, but can't tell you a tube is definitely good.

[Chuck H]

Agree with all above.

And I'll add that if you have a set of tubes on hand it's easy enough to try them in the amp to see if the problem persists. If not then it could be an intermittent open fault or intermittent short in the PS or bias circuit. I've chopsticked before without finding cold solder joints that existed in my own amp. It would only show itself at certain frequencies after the amp warmed up. That was a real corker.

[misterc57]

New output tubes installed and biased. Issue seems gone. Thank you all!