Dead JCM800 . . need help . . .

[steve_e_nixx]

So my wife was gigging with her 50 watt JCM800 (4 input), when it started cutting in and out. By the end of her set, the amp was completely dead. We took a look at the back and saw one of the power tubes was not lit. Id like to take a crack and trouble shoot this problem to see if I can fix it myself before sending it to my amp tech. Im hoping its just a dead tube.

Can someone outline some basic trouble shooting to find this problem? Ive built SE class A amps from scratch and have done mods on Fender amps, but I have not jumped into a Marshall amp yet. If it turns out to just be the tubes, I could use some advice for setting the bias as well.

Thanks,

>S<

[R.G.]

Go read the "Tube Amp Debugging Page" at GEO.

Then ask any questions you have left.

[Mars Amp Repair]

well,
first of all as always, be careful of the potenially deadly voltages in there. Since you've build amps before, you're probably aware of the precautions.

Now,
if the tube is not lighting, it could just be a tube. It could also be a fuse in the filiament circuit as many Marshalls have. Either way, the fuse (if it has one) doesn't go bad unless the tube has shorted at some point.

You can swap the two power tubes & see if the dead filiament goes to the other tube socket...in that case it's pretty certain the tube is the issue. I wouldn't hit the standby switch on as if the tube is shorted, it could cause further damage.

So....any which way the pair of tubes should be replaced with a new matched pair. If the filiament is still out in the same socket...there might be the filiament fuses under the chassis or simply a bad contact in the tube socket.

That's if for the easy stuff....glen

[steve_e_nixx]

JCM 800 not dead, not alive. .

Go read the "Tube Amp Debugging Page" at GEO.

Then ask any questions you have left.

I did some trouble shooting and found out one of the tubes was dead. I replaced it with a new tube that was part of the matching quad set (its a 50 watt amp, I had two spare matching tubes). I found one of the fuses was blown as well. Everything fired up fine, I did a quick negative voltage bias adjustment and things went fine for about 4 minutes . . . then the amp cut out. I turned it off, checked the fuses, and they werent blown. I turned it back on, standby off. checked the bias again. Checked the tubes. Everything was ok. turned it on and got output again. sounded great. 4-5 minutes later, the output cuts out again. The tubes were still lit. I switched it off. Both fuses were still not blown.

Any thoughts on what I could check next?

>S<

[acorkos]

clean and tighten your power tube sockets. if your tubes are known good, then it sounds like you have an intermittent connection somewhere....check the tube sockets first.

[Enzo]

You are checking the bias and fuses after shutting down and powering up and it is working. But did you check the voltages and stuff WHILE IT WAS IN THE FAILURE MODE? If it fails after a few minutes, we would want to know what happens to the voltages at the grid, the screen, the plate.

Quit worrying about the bias. If it went away completely, you'd still get sound, though the tube innards would start to glow red hot. If the bias shot up to 70 volts, it would clamp down on the amp for sure, but this is a simple measurement to detect.

It certainly could be loose or dirty tube socket, though in that case, just wiggling the tube a bit should wake the sound back up.

It might have nothing to do with the tubes. Something like an effect loop jack can kill the sound of an amp.