JCM800 2210 bias problem

Have you had a look at www.marshallforum.com/workbench?  You should find some good pointers by scanning through the threads in this forum. There are a few threads on JCM800 bias and overheating problems.

I'll search the Marshallforum when I have some time. Looks like a good resource.
The amp sounds great, perhaps a bit grittier. What worries me is the top of the cab gets much hotter than before. I don't understand how the mA values of the tubes can vary so much. It's a perfectly matched quad of JJ EL34s. We tested them on AVO valve meter (nice piece of 60s machinery, btw). Especially the last power tube V9 runs away. As it's obviously not the tube itself, it must be something inside the amp. If anyone has experienced or heard of this before, let me know. Cheers.
BTW, the amp is 100% stock, no mods whatsoever and in pristine condition. First owner.

Thanks for that detailed answer, Rick. I thought about caps, too. They all look like new. We definitely haven't noticed any leaks. Do caps dry out even when the amp is not fired up? The amp hasn't been played for almost twenty(!) years, hence its pristine condition. The tech used a probe measuring various bits and bops and gave the all clear. I might get new power tubes and have all caps checked again.

The coupling cap that Rick mentioned could very well be the culprit but make sure and swap each pair over and see if the high bias current follows the pair.  If it does, then the tubes have drifted which is very common no matter how well matched when out of the box.  Your tech can also tell if the cap is probably leaking if the hot pair of tubes has a less negative voltage at the grid resistors.  I say probably because a thermal run-away tube can cause the grid voltage to go up.  It usually doesn't stop though until it is glowing cherry red taking its neighbor with it.

As far as bias level, I have a 4212 which is the 50 watt version and HV is around 460-465 so at the max idle dissipation of 17 watts for an EL34 you can set the tubes at 38mA.

And your amp is much hotter because .015mA X 4 X ~470 = 28 watts before and ~.034 X 4 X ~460 = 62 watts now.

As far as your caps, I'd have them changed out.  They are over 20 years old and the clock is ticking down on them even though you haven't used the amp.  Occasional use is better for caps than storage or constant high temp use.  Mine are still good but I have a leakage tester, cap meter and ESR meter to confirm their condition.

BTW, I long ago gutted my 4212 and built an SLO into it as a prototype before building a stand alone SLO 100 clone head.  I again gutted it a few weeks ago and whipped up a P2P 2550 board, stuffed it into her and she likes it!  I have to say, it is a perfect mod because all the transformers are the same as the Jubilee, the controls are labeled close enough and it sounds sooooo much better both clean, crunch and lead.

Joe L

Amps built, no kits - Mesa Mark III, Caliber 50, SLO 100, X88R preamp, Trainwreck, Vibroverb, Various Marshalls and of course, a 2550.  A Deluxe Reverb is in progress.

Thanks Joe and Rick for your expert advice -- I learn more and  more about the technical side of amps which is a good thing.
I might change the caps as my next project but this will have to wait for a while because I've ordered a Silver Jub clone, the Ceriatone 2550. 

Sounds interesting. How does your 2550 sound? I have to wait for at least 4 weeks, so tell me more. Should be pretty close to the original Silver Jub, is it? What speakers do you use? Perhaps stuff for a new thread?