Marshall Jcm 2000 Dsl100 Tube Failure. Help!!

[avatar87]

Ok so i traded my peavey ultra plus straight across for a jcm 2000 a few weeks ago. good deal right? i thought so. anyway i started having problems right off the bat. after about 30 minutes of playing time, the volume would cut out and make a sound like static. two of the power tubes got real hot and the other two turned very dim. if i shut the amp off and turned it back on, the amp worked fine. one guy said my bias was off (i don't know what that means). another guy just told me i needed new tubes. 2 weeks later i was playing on it at practice and it stopped making sound completely. one of my power tubes had gone completely out. the amp still powers up and all the tubes light up except for the one blown power tube. does the failure of one power tube cause the amp to make absolutely no sound? or did the failure of the tube take something else out with it? i'm not very familiar with the insides of tube amps so any advice is greatly appreciated. thanks!

-Dylan

[rf7]

Did you check the HV fuse? The dead tube may have taken that fuse with it. But before you power this amp up again, take it to a shop and get it checked out. The 2 tubes running hot and 2 tubes running cold issue is not a good situation. You probably need new power tubes along with a bias adjustment for starters. And there is a chance that the dead tube took some other stuff out on the inside too. Have a tech check it out for you and get it running right. Don't just pop in a fuse and try to use it as it is. You could be causing more damage.

[Enzo]

When tubes are getting red hot with static and so on, and you can restore it by turning it off and on, your bias is not the issue. If the tubes just sat there and ran hot, then maybe. You have a bad tube. And when it cools down a little from the power going off, then it might work a while longer until it fails again. The more often this happens the more likely the tube is to blow that high voltage fuse. THE THE THE most common reason for blown fuses is bad power tubes.
Somehow I doubt the guy bought new tubes for the amp before he gave it to you. I suspect it was ready for new ones anyway already.

When you get a new SET of power tubes for it, THEN we can adjust the bias.

Yes, one bad tube can affect the others. If one tube has a grid shorting to another element it will kill the bias for that tube, and the one next to it as well.

ANd when tubes finally fail, not only will they blow a fuse, they can also burn out screen reistors, which means the amp will need repair.

[avatar87]

i'm getting the amp checked out as we speak. the guy is supposed to call me with a quote. what kind of power tubes do you suggest? i was checking out mullards. i believe the amp still has the original tubes. the guy i got it from seemed to know nothing about amps at all.

thank you!

[voodooeye]

I have the exact same head and had the exact same issues. at first i tried new tubes and rebias, but the bias wouldn't go any higher than 85 on one side, the other side i could bias to the proper 90. so I biased both sides to 85. WRONG, i shouldn't have done that. one side not going up was a sure sign that something esle was wrong. two of the new tubes burned out with a few hours of play time. I took it to a tech and found that a resistor had gone bad. He replaced the bad resistor and put in new power tubes.

Mullards sound great in this head. a little pricey but worth the dollar.

on another note, while you have it in the shop, go ahead and pay the tech to retouch all of the solder points. Someone on this board gave me the same advise and it was well worth it. I'm finding out from various sources that these were very mass produced which made the quality suffer. One of those being weak solders.

[John G]

Hi,
Don't forget to get the Tech to check for bias creep. It will show up usually only with one pair. Attach two meters to the bias pionts and let it run for a period and observe meters for drift, this can take quite some time. This is all done with the amp running and no signal.
Cheers
John G

[avatar87]

you guys are awesome, thank you so much. i'll go talk to the guy tomorrow and get it worked out.

thanks again!

-Dylan

[Amp Kat]

You are talking about NOS Mullards right ? The new Mullards are nothing but relabled Sovteks.