JCM 900 combo 4102 all tubes blew at once

[stoma]

Hi - and thanks for having me in your forum

I have a 1996 jcm 900 4102 just back from voodoo modification and retube with 6L6s. They do a break in process on new tubes and I also played it gently at first. Took it up to 6-8 for brief periods after 1 week of no problems--and then left the room with the amp off stand-by...when I came back the room smelled like electrical fire and the back of the amp was smoking hot. Sent the chassis back to Voodoo and they told me all the tubes were fried! They said they'd never seen that before...nothing wrong with the amp / cicuit board itself that they could find. Interestingly the fuses did not blow.

Any of you experts with ideas? Does this seem like a biasing problem?

Thanks for your help

Stoma

 

[Chickenwings]

Re: JCM 900 combo 4102 all tubes blew at once

i have a 900 2101 combo that runs enormously hot. Its biased correctly (according to the dudes at the marshall forum) , but always smells hot and actually has a kind of toasted/burned mark on the inside of the wooden back panel. They are hot amps, and being upside down in a combo is not ideal.
It could well be that they were biased too hot and have cooked themselves, but im only really guessing.
Mine is biased at its lowest setting to make the range and its still freakin hot fwiw.
Im sorry to hear this has happened to you especially after you have gone to the trouble of having it modified etc.
Let us know how it works out.

 

[ErikH]

Re: JCM 900 combo 4102 all tubes blew at once

i have a 900 2101 combo that runs enormously hot. Its biased correctly (according to the dudes at the marshall forum) , but always smells hot and actually has a kind of toasted/burned mark on the inside of the wooden back panel. They are hot amps, and being upside down in a combo is not ideal.
It could well be that they were biased too hot and have cooked themselves, but im only really guessing.
Mine is biased at its lowest setting to make the range and its still freakin hot fwiw.
Im sorry to hear this has happened to you especially after you have gone to the trouble of having it modified etc.
Let us know how it works out.

Curious. What were you told was the bias setting for your 2101? What tubes are you running in the power amp?

 

[DankStar]

Re: JCM 900 combo 4102 all tubes blew at once

Just curious - do you plug into a wall directly or into a surge protector type outlet or bar-type with a fuse? I actually fear plugging an amp right into the wall sometimes.

 

[zzmoore]

Re: JCM 900 combo 4102 all tubes blew at once

Curious. What were you told was the bias setting for your 2101? What tubes are you running in the power amp?

Yeah....weird.
Like the bias ran away BIG TIME all at once, or the B+ spiked huge....blew the plates and screens....I do not know the stars that would have to align for this to happen.
I will defer to the opine of Jerry Glass on this one.....
Best

 

[ErikH]

Re: JCM 900 combo 4102 all tubes blew at once

I would get it in to a tech and have the power transformer checked as well as the output transformer. Check the fuses too. Back in 2006 my MkIII 2100 blew the power tranny. It took the tubes and some bias components with it. Finally got it fixed a couple years ago. Get yours checked before doing anything.

 

[glassman]

Re: JCM 900 combo 4102 all tubes blew at once

Four tubes don't fail like that...they were taken out, likely by a bias supply that failed. The 900s have fuses that each protect the amp against a fault in a pair of tubes...that should have blown and averted any sort of fireworks show...but it didn't. Makes me wonder why someone would defeat that function of the amp.

Something ain't right.

If they attempt to charge you extra for the "second pass" I would suggest you contest it. Personally, I've "forgotten" things in the past and didn't charge for going back in. In a vintage amp, a repair can aggravate things that were ready to bow out anyway and I generally explain that up front. A relatively modern amp such as your 900 don't fall into that category though and I wouldn't suspect an unexpected breakdown unless someone had a brain fart. It probably would have been better to take it into someone local and have it assessed first before sending it off. It's your word against theirs at this point. Hopefully it gets sorted out with minimal pain on your part.