Marshall DSL 100 Redplating and POP goes the fuse!

[Steve Casas]

Ok... a friend called me to say that his marshall head was fizzling out on him so I told him to buy some new winged c el34's and bring it over. I swapped power tubes, plugged in a speaker, warmed it up, turned it on, adjusted the bias test-points to 90mV, turned it off, took it out to the big shop to plug in a different cabinet, turned it on and while I was moving stuff around I accidentally unplugged the speaker cable, and immediately turned it off. After plugging it back in, and turning it back on, plugging in a guitar, I had a real weak signal. So I started swapping ECC83's, V3 was bad 'cause it came to life immediately, but only for about 15 seconds and POP went the fuse. My daughter said the two tubes on theright turned blue and bright right before the pop. I replaced the fuse and same thing; redplate and pop after about 15 seconds, but only the two on the right. I took it back to the shack, and while I was thinking, I noticed a few suspect solder joints on the tube sockets, like REALLY suspect! After re-touching those, I pulled the two outer tubes, warmed it up and turned it on... no pop, played it for a few minutes. Put in the other two tubes and pop, same thing.

Ok the question is: can he run this with only two tubes for a 50 watter like on a Fender? Do I need to adjust the bias like a 50W DSL50 (45mv)? Any idea what the problem is? Couldn't have been the OT from when I accidentally unplugged the speaker, right? I am tired from work and I have little time (big gig tonight) and I need help! Thanks in advance!

[Jazz P Bass]

I seriously doubt that the amp itself was damaged from having no speaker connected for a short period of time.
If there was signal applied, no harm.
I think that you may have a few bad tubes.
Try one at a time.
Then try two at a time.
Yeah, the soldering on some of these amps is a real hoot!

[Chuck H]

OK... There are some misunderstandings here, I think. To run an amp on only two of four output tubes (sans impedance adjustments) at the very least it must be either the two inner tubes or the two outer tubes that are run OR are pulled. You can't just pull any two tubes. But I agree with JPB that this is probably not your problem. I'm betting that the stresses caused something else to give. And I'll wager it's a marginal board connection or the bias feed resistors.

[Euthymia]

You should check the bias feed resistors, screen resistors, and the tubes themselves to make sure that they are still good after a fireworks display like that.

[Steve Casas]

Thanks for the response, all.

Jazz P Bass: Yep, it was the tubes. They were from Antique Electronic supply and he didn't get the 24hr burn in option like I told him to.

Chuck H: I knew it had to be the two inner tubes or the two outer tubes... I have done this on Twin Reverbs before... I just didn't know if a modern Marshall could do that as well without adjustments.

Euthymia: I kept looking for signs of cooked resistors, etc. I did see a ceramic cap that looks discolored (c46).

So, thinking about the fact that V2 (not V3 like I said earlier) was definitely bad, I put the old EL34's back in, adjusted bias, fired it up and SUCCESS! I am going to assume that at least one new EL34 was bad out of the box and it took out it's neighbor, not it's partner, but its neighbor, because they were both redplating and flashing and I saw fireworks inside the tube once right before the fuse blew. Crazy!

I played through this amp for 30 min. with no issues and he came to get it. He will try to return the tubes and bring me a new set, preamp too, and I will check for any other damage (bias feed resistors, screen resistors, C46, etc.) Btw, what is the function of c46? Only V8 has it???