Carvin X100b problem

[squimbolio]

Non-modded x100b 6L6, no bias adjustment pot. PCB id is 30-x100-4b.

The problem: Fuse blew while playing. The amp was functioning correctly during play, until the fuse blew. Yes, it is the correct fuse.
Replaced fuse and tried again. This time the amp hummed when taken off standby and then the fuse blew again.
The only sign of possible damage from a visual inspection is C64, which has what could be some electrolyte showing on it's surface.

I figure on replacing the cap, but I am more interested in any ideas concerning possible causes of this problem.

Thanks!

[Enzo]

Yeah, THE most likely cause of fuses blowing in tube amps is failed power tubes.

Pull all your power tubes and power up without them. Still blow fuses? Or not?

[squimbolio]

No, the fuses didn't blow when powered up without the 6L6's. However, even with different tubes - humming on standby (!) which increased dramatically when taken off standby. I powered down before the fuse could blow. Not liking this very much.

[mozwell]

Schematic for posterity
Bias would appear to be fixed, R159, D6, C64, R127, R126
The note says to "select R127" for bias current of 100mA for the 100W model.
Is the bias voltage correct, does C64 look ok ?

Maybe if one electrolytic is leaking, its worth changing all of them

[Jazz P Bass]

And when you get her all fixed up, put an in line fuse (10A) on one of the heater wires.

[squimbolio]

Why?

[Jazz P Bass]

One of the worst case failures of a power tube is plate voltage arcing over into the heaters.
In the case of the XL amps, there is not any fuse protection on the heater circuit.
I had one of these come in with a very charred output tube board because of such a failure.
Luckily Carvin had a spare board.
In questioning why there was not heater fuse, the Carvin tech advised adding one.
The 'newer' 100 watt amps have the fuse on the PCB.

[squimbolio]

Thanks for the info. I am probably just going to take it to Soldano and see if they can fix it.