Marshall 4100 Blowing Fuses

[hot_tube_tone]

I have a Marshall 4100 100 watt head that is blowing the output fuses. I have replaced the fuses, and checked the bias. If I let the amp set turned on, or play it at some point (maybe 15 minutes, maybe 4 hours) the amp will start to make an increasing humming noise and then pop the fuse. This is the 500ma tube protection fuse. Until the humming noise starts the amp sounds normal as if it doesn't have a problem. Do you guys have any ideas where I should start to look?

[John Phillips]

Almost totally certain it's a power tube. It will have an intermittent short.

The two 500mA fuses only apply to each pair of output tubes, so any other fault would either blow the internal HT fuse, or the main fuse.

You can tell exactly which tube for the cost of another fuse. Swap round the two tubes at the same end. If the same fuse blows again, the bad tube is in the pair you didn't swap. If the other one blows, it's the tube you moved. You can tell whether it's the inner or outer tube from the tube numbers for each fuseholder.

It's best to get a whole new set, and keep the three good tubes as spares. If in doubt, just change all four anyway.

You might be able to 'induce' the failure by knocking (gently) on the tubes with the amp running, if it won't do it quickly and you need to use the amp.

[mark norwine]

I might also suggest testing the decoupling cap that separates the PT grids from the plates of the inverter. If one of those caps starts to leak DC, you might get the same results.