JCM 900 Problem

TBotskor · Posted: Sat Oct 11, 2008 5:17 pm Post subject: JCM 900 Problem

Hi...im a long time reader first time poster Very Happy

I own a JCM 900 100W Hi Gain dual Reverb head (4100) with a 1960 Quad..
The tubes were replaced about two years ago...
After heating up the amp i switch it on to play then about 30min of playing the amp starts to make a humming noise and then it gets louder and louder and my guitar signal gets weaker. If i dont turn it off at this stage it blows both valve fuses..
i also noticed that when the humming starts the two power tubes closest to the input are super hot and glowing bright red..
could someone please help...it would be very much appreciated

Herb · Posted: Sat Oct 11, 2008 7:05 pm Post subject:

If the tubes are glowing red, they are bad, replace them - immediately before you burn up Transformers!
_________________
Yer guitar pickin' friend,
Herb
"It don't mean a thing if it ain't got that swing!"
Doghouse Jazz

TBotskor · Posted: Sun Oct 12, 2008 9:32 pm Post subject:

could it be anything else cause i pulled out the two tubes that were glowing red and put one of the tubes that was running cool into the side with the problem and i set the amp to half load....and now the tube that was originally running cool on the good side is running hot and humming after 30mins or so.....i think it could be something else...

jamez · Sea Monkey Joined: 22 Oct 2008 Posts: 13

sounds like the bias circuit for has failed- Id have a look at that. check voltages at the grids with valves pulled. They should be negative and the same for each grid.
Good luck