Marshall JCM2000 (TSL100) - Tubes (almost) on Fire, Bias is Weird now!!!

[nickk678]

Hi guys,

Allow me to preface this post with while I am an Engineer (Thermal Automotive) and tinker a lot with PC's, home automation, etc, high voltage amplifiers are not something I know about. Therefore, I've come here for help.

Last week, I was playing my TSL100 and I heard no issues at all. It runs through an Avatar 2x12 (8ohm) cab. I've been playing it this way for ~15 years and am on my second set of JJ tubes. Anyway, while playing, luckily my brother happened to be behind the stack and noticed the two right power tubes were RED hot and smelled the wonderful aroma of electrical burning. He pulled the plug immediately, but I was able to slide around behind and see how hot the right two power tubes got (it wasn't normal!).

Anyway, the last time I replaced the tubes was ~2004 (albiet I only play ~40-50hrs a year on this amp), so I picked up a new set of matched ECC83=S and EL34L's and received them today. I pulled out and replaced all the tubes (with proper care and gloves), then fired the amp up to let is stabilize in stand by (with the cab plugged in of course). I allowed it to stabilize for ~10 minutes, and all 8 tubes had what appeared to be a normal "glow" to them. Finally, with no instrument cable plugged in and the volumes at 0, I turned off standby so I could check the bias on my Fluke multimeter. Here's where the fun starts - the left bank read 1.5mV and the right bank read 0mV. Adjusting the pots on either side made no difference whatsoever. I did not plug an instrument in to see if it worked as I wanted to check in here first.

Anyway, what do you all suggest I do next? I'm in the process of building, furnishing, etc. a new house, so getting this amp professionally repaired is at the bottom of my to do list (I have others I can play with since I don't gig). If you think the problem is severe and over my head, it will go in a closet for a year or two until I have the time/resources to get it fixed. Otherwise, I'd love a list of checks I can do to help diagnose the issue.

Thanks for your help all!
Nick K.

[g1]

Despite your background, I'll mention this just to be sure. Make sure your meter is set to DC volts (not AC).
If still no reading, perhaps the H.T. (or other) fuse has blown. Not sure if it is external or internal for this model.

[nickk678]

Despite your background, I'll mention this just to be sure. Make sure your meter is set to DC volts (not AC).
If still no reading, perhaps the H.T. (or other) fuse has blown. Not sure if it is external or internal for this model.

Hi g1,

Believe it or not, I spent a good hour (and a few beers) playing with the multimeter because I couldn't make sense of it myself. My Fluke 87 is very simple in that it auto adjusts the range based as long as the correct measurement is selected (in this case, DC V). After I got the same readings over and over and over again, I went to my car battery and grabbed a solid 12.2V. Then, went back inside and got the smae readings off the amp. After that, I grabbed my brothers Craftsman Multimeter and checked the amp - voila, same reading.

I should have mentioned in first post, but I did check the 2 fuses on the back (one was labeled mains fuse and the other I believe HT fuse - both looked OK).

[nosaj]

Hi g1,

Believe it or not, I spent a good hour (and a few beers) playing with the multimeter because I couldn't make sense of it myself. My Fluke 87 is very simple in that it auto adjusts the range based as long as the correct measurement is selected (in this case, DC V). After I got the same readings over and over and over again, I went to my car battery and grabbed a solid 12.2V. Then, went back inside and got the smae readings off the amp. After that, I grabbed my brothers Craftsman Multimeter and checked the amp - voila, same reading.

I should have mentioned in first post, but I did check the 2 fuses on the back (one was labeled mains fuse and the other I believe HT fuse - both looked OK).

I know that in some instance autoranging can be a pita! If possible switching to the highest range then working down can help in some instances.

nosaj

[g1]

You can try this, but caution, there are high voltages present.
Assuming the power tubes are glowing as you stated.
Remove one of the power tubes. Check DC voltage at all socket pins working around clockwise. Start at any pin, we should be able to figure it out. Black probe to chassis. Power on and standby in 'play' position. Speaker cab connected.

[Enzo]

Check the basics. Is there B+ on pins 3 and 4 of each power tube? What is the bias voltage at pins 5? Is ther continuity to ground from pins 8?

[nickk678]

You can try this, but caution, there are high voltages present.
Assuming the power tubes are glowing as you stated.
Remove one of the power tubes. Check DC voltage at all socket pins working around clockwise. Start at any pin, we should be able to figure it out. Black probe to chassis. Power on and standby in 'play' position. Speaker cab connected.

Hi again g1,

Thanks for the info - I'll try to give it a go within the next few days and report back. On top of the new house stuff, we just had our 3rd baby 1.5 weeks ago (the others are 1 and 4) so we have our hands full.


Enzo - Your tip sounds same as g1 - I'll let you guys both know when I have some free time to get to it!