Amp woes Part 2

angry_young_poet · Post by **angry_young_poet** » Thu Apr 28, 2005 7:31 pm

I've got a friend who has a Marshall JCM900 1x12 combo and yesterday as soon as he turns it on.. there's this rumbling noise and he says the tubes were suddenly very very hot (he felt the top of the amp and it was very hot..)

and so he plugs in his guitar, all the volume knobs are at "0" and there's sound coming out of the amp (from the guitar) so he increases the volume.. nothing happens.. it stays quiet.. so he puts it all the way to 10 and nothing happens.. and all the pots are suddenly scratchy.. but the rumbling noise gets louder.. kinda like the sound of water boiling...

weird?

would this be a major fix? i was thinking of buying it.. he'll sell it reasonably cheap but then would the symptoms lead to bigger problems and added costs?

Red Fred · Post by **Red Fred** » Thu Apr 28, 2005 9:49 pm

Sound like the amp is hungry, feed him! FEED HIM!

Sorry couldn't help myself....

ash · Post by **ash** » Fri Apr 29, 2005 12:03 am

Its haunted. You need a young preist and an old preist....

Or call an amp tech for a diagnosis. I think these symptoms sound like a specific issue.

B45-12 · Post by **B45-12** » Fri Apr 29, 2005 12:49 am

My advice is turn it off and pronto take it to SEE A TECH! - without being there or examining the amp this sounds very like the dreaded short across the ht line where basically the rectifier tube(s) glow red hot thanks to an effective short across its/their output.

Can be caused by a variety of things such as filter caps going short circuit - effectively means there is no sound output frorm the amp until the rectifier tube explodes (no I'm NOT joking).

Hope I'm wrong and even if I'm right it's not usually a too expensive fix - usually a new power cap/choke or limiting/bleeder resistor (if that's cooked up and gone low) BUT don't leave it on until it is fixed. Not soemthing you can put off.

angry_young_poet · Post by **angry_young_poet** » Fri Apr 29, 2005 7:53 am

found the source!!!

while putting the amp together after re-tubing..

one pre-amp got hit and was bent and partially out of its socket.. and some of the pins were bent..

after carefully unbending the pins.. the pre-amp tube was put back in and voila.. no more noise rumble..

lots of hum though at hugh gain settings..

ant · Post by **ant** » Fri Apr 29, 2005 9:39 am

If it hums more than it used to it might be a filter cap has failed in the preamp (which may be possible with a tube unplugged causing higher voltage at that section - making the filter cap blow)

B45-12 · Post by **B45-12** » Sat Apr 30, 2005 3:17 am

Still have a look at the rectifier tube when the amp is on and if it is glowing a nice cherry red colour beyond the normal filament level - see above advice.