Valve Gtr Amps

[russ russell]

anyone have a sound knowledge of valve gtr amps?

i have a marshall jcm800 that someone borrowed from me and the fool fired it up with no cab plugged in and "booof" its dead.
i'm pretty handy with fixin stuff and can follow diagrams and instructions, can anyone get me started on fixin it or direct me to somewhere i can find the info i need.

big thanks - russ

[orange]

was there an signal going through the amp when the amp was on...should be ok if nothing plugged in...

could be the output transformer ($$$) blown but first check fuses (mains and high tension/low tension)

are all valves lit ?

try replacing output valves (big ones) with known good ones - most likely fault.

report back with more info

si

[Geoff_T]

Hi

I agree with Orange... if there was no load on the output transformer the output tubes will swing beyond their normal operating range and I'd guess they might be victim of the twit's forgetfulness.

There's a teeny chance the output transformer is damaged but I'd suspect the tubes first.

Or, of course, it may have blown a fuse but you should be able to see the tubes light up.

I often wondered, back when, why tube amps makers didn't wire a dummy load across the normalling contact of the output jack to prevent this. Probably because it would add a couple of dollars to the price!

[russ russell]

thanks guys,

i wasnt there when it happened, apparently the drummer from the band i was working with (dont get me started on drummer jokes!) decided it would be a good idea to take my amp home with him to do some 4 track recording. the bass player said he fired it up with only the direct out connected and was playing for just a couple of seconds when it went off. when i got it back i checked and the ht fuse was blown, when i replaced it and tested it blew again so i havent had a chance to see if the valves light up or not.

i'm reluctant to go ahead and replace the tubes if there is some other problem causing the fuse to blow. if it may be the transformer how can i test for this?

any furthur thoughts?

big thanks again for your input on this,
russ

[orange]

try the output valves first - can you 'borrow' some - or better still try your valves in another amp.

si

[russ russell]

i'll try that, thanks orange. might be a while, i'm kinda busy right now but i'll report back (eventually!)

-russ

[orange]

FWIW thomann have el84s for just over a fiver

http://www.thomann.de/thoiw2_sovtek_..._prodinfo.html

plus 5 quid delivery to the UK

might not be the 'king of valves' but useful as spares

PS thomann are great....I love those guys...shipping to the uk is usually about 2/3 days. Much of their stuff is stupidly cheap - and service is as good as anything you get in the UK - and they all talk the english better than what I do
Prices for accessories such as mic stands/clips/bits etc are first rate.

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