jcm 2000 dsl blowing tubes!

[serious dude]

hello glorious readers of this post. i have a problem and hope you can maybe help me with some advice.

I lent my wonderful JCM 2000 DSL to a friend, and being a good friend he decided to replace the tubes, because the amp hadn't been used in a while and was a little dusty. he still has the old tubes by the way. he tells the story from there:

"Not long after getting the amp back, the tubes blew out. So I took it back to main drag (a guitar shop) and they gave me new tubes for it and adjusted the bias (I think this means, roughly, that they adjusted the amount of power that goes through the tubes so they won't blow).

The tubes blew out again, like a couple weeks later. I took it back again and they replaced them again and made more adjustments. But this time they were like "you're killing us with this. We'll fix it this time, but please don't bring it back again. These JCM 2000's have notoriously shitty electronics and are well known for having problems like this."

so now my friend and i are quite concerned. what can be done for a jcm 2000 that keeps blowing tubes?

thanks in advance for your help and advice,

jesse

[stokes]

Obviously,they didnt "fix" anything.Replacing tubes that keep blowing without finding why they are blowing is plain stupid.I wouldnt trust these guys to fix anything.Merely lowering the current draw via the bias adjustment is not a fix,and I have to doubt they actually did that.Afterall if you are opening the amp anyway,it makes sense to have a look and see what is causing the problem while the amp is open.Its impossible to tell from here why the tubes are blowing,but take their advice and dont bring the amp back to them.Find someone who knows what they are doing.The first thing I would check is the screen resistors,but if it is a screen resistor,it would be obvious when they opened it to do a bias adjust,burnt screen resistors usually look burnt.

[JerryP]

Replacing a shorted tube is the fix for a shorted tube. A screen grid resistor won't cause a blown fuse. Since the shop installed the tubes and it was weeks before it failed again I would say they did all they could at the time. Some problems are intermittent and not easy to find.

The first things that come to my mind are:
You're really unlucky and had 2 sets of junk tubes
You're running the amp into the wrong load
The output transformer is shorted and when playing loud it causes the fuse to blow
Jerry

[Jazz P Bass]

Tube failure

Can you identify the brand name of the tubes that you are dealing with?
There are a number of ways that a tube can fail.
There are also a number of things that can cause a tube to fail.
(Just stating the obvious)
It would be nice to know what exactly was the failure.
As far as checking resistors, correct me if I am wrong, but the components are "under" the board, as seen from inside the chassis.
You may have to remove the power tube board to visually check for burnt parts.
Or you could use a DVM & check the parts from the backside.

[Steve Conner]

Tubes don't go stale sitting around. You can take a vacuum tube that's been kicking around a drawer for 50 years, plug it in, and it'll probably work just the same as the last time it was used. Why would your friend replace them because the amp was unused and they got dusty?

As others have suggested I'd check for a blown output transformer (did your friend forget to plug the speakers in? do those old tubes he replaced for no apparent reason still work? )

[stokes]

Replacing a shorted tube is the fix for a shorted tube. A screen grid resistor won't cause a blown fuse. Since the shop installed the tubes and it was weeks before it failed again I would say they did all they could at the time. Some problems are intermittent and not easy to find.

The first things that come to my mind are:
You're really unlucky and had 2 sets of junk tubes
You're running the amp into the wrong load
The output transformer is shorted and when playing loud it causes the fuse to blow
Jerry

Sorry,I missed the part where he said the fuse blew.What I see is he says the tubes blew.A shorted screen grid resistor should cause the fuse to blow.I didnt say it absolutely was a screen grid resistor,I said that would be the first place I'd look,I actually said it is impossible to know for sure from here.It would have to be an extreme impedance mismatch to cause a tube to blow that quick.His tech stated "These JCM 2000's have notoriously shitty electronics and are well known for having problems like this." If thats the case then why doesnt he know how to fix it?They surely didnt "do all they could at the time" they did nothing but blindly replace a blown tube without finding out why it blew.Sure,some problems are intermittent and harder to find,but telling a customer not to bring the amp back,because "you're killing us" is just bad business practice.He cant fix it,so the customer is killing him?Take it to someone that knows how to fix an amp.

[Enzo]

I would suggest making sure the speaker cord is 100%. And make sure it is an actual speaker cord and not just a guitar cord. I would play some sound through the system at a low level, something steady, and then shove the speaker cord plugs side to side and up and down. DO this at the amp and at the jack on the speaker. We are looking for any intermittant conection that might open up while playing from vibration.

Likewise flex the impedance spelector switch - I forget, does that amp have one? - and see if it is iffy at all.

Anything that can interrupt the path from amp to speaker can kill tubes and/or transformers.