Marshall DSL JCM2000

[Ant]

Hiya all,

Just a question to the experts on here (of which I am not one):

I had a DSL 100watt head a friend left in for me to look at. It was completely dead (he said). When powered up however I noticed that V5 wasn't lighting up but the other 3 tubes (power section) looked fine - pre amp tubes didn't look like they had much life either. Anyway, I did the usual with these amps and re-soldered everything in sight and then I noticed that F1 was blown (heater supply to V5, T6.3 Amp). I replaced F1 and put the amp back together and up she came fine. I checked the bias on the power tubes and it was reading 120+ mV on each side! In fact both pots were screwed as far as they could be. I pulled this down to about 78mV per side and the amp looks to be ok since. I suspect someone was fiddling with the bias pots in some vain attempt to change the amps performance without actually knowing what the consequences were. My questions are (1) Would the increased bias make the tubes heat more than they should cause they seemed very hot to me and no so hot now? (2) Could excess bias pop the F1 fuse or is there possibly something else wrong that will come back and bite? (3) Would the excess bias have damaged the tube life of the power tubes - apparently these were only replaced recently. (4) Finally, the power tubes had some kind of fancy looking springloaded retainer/heatsink clips on them. Never seen these before, has anyone else? what's the purpose and why do they not exist on subsequent amps (this was a 1997 model).

Thanks in advance for any opinions.

[Twist]

Yes, the excess idle current will run the tubes very hot and will cause damage. It is not uncommon for a tube to function fine for a while and for whatever reason arc or short and blow a fuse. I've found some bad tubes like that by letting the amp warm up and flicking each output tube to see if I can get it to arc. If your lucky you'll see which one before it blows. I would pull all the tubes out but one and turn the amp on to measure that tubes idle current then pull it out and repeat the process with the other three. This gives you an idea of how the tubes rate to each other. Make sure you have a speaker plugged in while you do this and no signal at the input. Also since something blew the fuse I would measure the voltages at each tube socket to make you don't have a blown screen resistor or something. Also if if memory serves me right I think the bias trim pots are opposite to side they control. Maybe somebody will jump in here and correct me if I'm wrong.

[Amp Kat]

The tube retainers were probably installed to keep them from falling out as I can't remember those that early but I may be wrong and they could be stock. The answer is yes to all of your other questions. Inspect the power tubes good and see if there is some discoloration on the logo. If the tubes get too hot from a hot bias they will sometimes bake the logo's to a dull color. Not good either way for anything from biasing it that hot so you did good bringing it down. Funny how these amps always have power section problems yet they are also one of the only amps out there with bias pots in the end users grasp ! Consider yourself lucky.

[Ant]

Thanks for your comments guys, much appreciated