JCM900 4100 blowing mains fuse

[shaun]

I have a '93 4100 head, converted to EL34's in 2006.
A few years ago, it kept blowing the mains fuse (running 240V), with fairly new matched valves. I figured one of the valves was bad, but I don't remember the bad valve indicator light coming on. I didn't bother to get it checked out, and havn't played it since.

On the weekend, i put in another set of valves with low playing time on them, and as far as i know they are good. It sounded great but the volume was way down on what it should have been, and again no bad valve indicator lights. After about 10 minutes i switched it totally off to change speaker cab, and when i turned it back on the mains fuse blew straight away.

There was no burning smell or anything, and on the low power mode i could turn the clean channel master and preamp both to 10 and it was about as loud as my JCM1 on full...?? (This should have been about 50 watts cranking)

Any ideas, and also is there a way to check a valve using a multimeter?

I know how to solder, but electronics and amp schematics etc. are not in my range of knowledge.

Thanks guys.

[Anitoli]

Shaun, There could be numerous things wrong with your amp and i'm not trying to discourage you but if you have limited experience working on these i would reccomend taking it to a reputable technician. This is the best course of action.