Help diagnosing monoprice 15 watt tube amp fuse blower

[Justamook]

I’ve got a monoprice 15 watt tube amp on my bench. Customer reports it blows it mains fuse which should be a 500ma slo blow. I connected my 60 watt bulb limiter across the fuse holder. The bulb glows bright with or without all tubes installed. The bulb is totally dim if I disconnect the power transformer by disconnecting its wiring harness. I tested all high voltage caps and diodes for shorts with amp unpowered. Nothing seemed to be shorted. Not sure where to go next. Was thinking of isolating power transformer circuits one by one by cutting all but one pair at a time, with the goal of isolating the problem area. Before I go there are there other possible suspects or better ways to go about trying to isolate the problem? Has my logic been faulty?

Any help would be greatly appreciated.

[The Dude]

The schematics I've seen on the web show that those amps have some SS components (op amps, etc.), which would require a low voltage supply- probably +&-15V. If you can't find shorted rectifiers or caps and the unit still draws excessive current with tubes removed, I'd look for a shorted regulator- maybe 7815 or 7915 or possibly shorted zener diodes. If you can determine what and where the supplies are, I would check resistance from each supply output to ground and see if there is a short on one of the supplies.

[Jon Snell]

Disconnect SK4. Still short = mains transformer. Short gone ... I would check the HT rectifier, smoothing cap C14 and after it if the diodes were not at fault.

[Dave H]

The Monoprice 15W circuit is almost the same as the Laney Cub 12 (the inputs are different)

[Justamook]

There are no voltage regulators. I checked all the diodes, none were shorted.

[Justamook]

Neither was c14. There is a square tan component labeled x30 UF 600. I gather that is some kind of resettable fuse.

Anyway, I’m stumped.

[Jon Snell]

If you are UK I offer repairs to all valve equipment at reasonable cost.
At this point, I recommend you send it to a qualified valve amplifier repair workshop. It will be a simple fix for a competent engineer.