marshall jcm 900 4102 - 4 tubes blown at once!

[stoma]

Hi - and thanks for having me in your forum

I have a 1996 jcm 900 4102 just back from voodoo modification. 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 and then left the room...when I came back the room smelled like electrical fire. 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 itself that they could find. Interestingly the fuses did not blow.

Any of you experts with ideas?

Thanks for your help

Stoma

[Gtr_tech]

Bias failure....

[stoma]

Bias failure....

Thank you for the reply---Voodoo is saying that the tubes aren't in the warranty (strange because here they replace the tubes and then they all blow). Do you feel that their biasing after the re-tube may have been inadequate?

Thanks again
Stoma

[exclamationmark]

Depending on the amp, you could possibly bias the tubes into an unsafe region. But the more likely scenario is that the actual bias circuit failed, leading to over dissipation of the output tubes.

[km6xz]

Why are the tubes not covered under a warranty? Besides, as mentioned, the tubes did not all decide to fail individually at the same time. I would agree that the bias circuit seems most suspect but it could easily have been misadjusted by VooDoo. If have it back, tell them to burn it in for a day or two before sending it back. It ought to be under warranty, all of it.

[Amp Kat]

I agree that the tubes should be covered under warranty. Come on Trace man own up. haha ! Trace is actually an old Ampage poster but when he built Voodoo he was gone for the ages. On the flip side if the tubes were old then he has a point but if they were replaced by Voodoo than they need to make good on them.

[Chuck H]

I expect that Voodoo is coming from the standpoint that since all the tubes blew at the same time it must be something other than the tubes themselves. OK, so the tube warranty no longer applies since the failure, most likely, was not the fault of the tubes. So how about the workmanship??? If Voodoo installed the tubes, biased the amp and delivered it to you as serviced and ready to use, what did cause the failure and is it Voodoo's fault??? That Voodoo says there is nothing wrong with the amp is suspect. Unfortunately this suspicion can't be pinned to on Voodoo. With this in mind I expect that Voodoo is claiming "user error" as the reason all four tubes blew at the same time. You've already taken the amp back to Voodoo after the failure so any problem that may have been Voodoo's fault may have been corrected and not disclosed. This is unscupulous but would save Voodoo a pretty good chunk of change. So...

At this point what you have is an amp that was taken to a shop for new tubes and a bias. The amp was delivered back to you in good working order according to Voodoo. Then in your care the amp blew all four power tubes. When you returned the amp to Voodoo they found nothing wrong with the amp. So the tube failure is presumed user error since there is nothing wrong with the amp when used properly that could cause the failure and therefore the tubes aren't covered by the tube warranty and the workmanship has been rendered faultless. Any second party tech looking at the amp now would then have to support that conclusion. So...

All speculation as to what caused the problem or who SHOULD step up and be accountable aside, in the real world you might be screwed.

[Gtr_tech]

Its possible it was a victim of the infamous conductive pc board material. AFAIK, that started with the 2000 series, but they may have cheaped out on the later issue 900s. If this was the case, the bias would look OK when on the bench and won't drift until it heats up with prolonged use. Really....a bias failure is the only thing that can take out all output tubes at once assuming there wasn't a problem with the tubes themselves. And if teh latter was the case, it shoulda been evident to the tech working on it....

[Chuck H]

I agree that a bias falure is most likely. What about a dead short for a speaker load? If Stoma was using an attenuator that was broken or hooked up incorrectly so as to be a virtual short with the amp cranked up full tilt??? Probably not, but maybe. Of course I would expect that to happen during use and not idle. Just wondering out loud what Voodoo's premise for not honering any warranty on parts or labor could possibly be!?!

[km6xz]

There is just too much not known, and likely to never be known about this case to prove conclusively the cause and where the fault lays. For example if the amp was idling or whether a signal was being used for burn-in. If no signal, that crosses the load problem as being the cause. If on the other hand, a normal burn in with a compressed CD cranked for an extended time, the load could have been the cause.
If there was shipping involved, rough handling in transit could have vibrated or jarred something loose but initial playing would have revealed that problem if there was no bias.
The tubes themselves should have a warranty, even if the deal who sold them is apparently not interested in covering them.
Contact the tube company to see if they will honor the warranty. Since they are not being covered by the deal, most states require old parts to be returned to the customer. Those tubes would probably be requested returned to the tube maker for an inspection. They could tell quickly if there was a bias problem.
Try to get a detailed write up on the work originally performed and measurement values for bias, B+, mains current at idle if he logged any of that information.

[Amp Kat]

There is just too much not known, and likely to never be known about this case to prove conclusively the cause and where the fault lays. For example if the amp was idling or whether a signal was being used for burn-in. If no signal, that crosses the load problem as being the cause. If on the other hand, a normal burn in with a compressed CD cranked for an extended time, the load could have been the cause.
If there was shipping involved, rough handling in transit could have vibrated or jarred something loose but initial playing would have revealed that problem if there was no bias.
The tubes themselves should have a warranty, even if the deal who sold them is apparently not interested in covering them.
Contact the tube company to see if they will honor the warranty. Since they are not being covered by the deal, most states require old parts to be returned to the customer. Those tubes would probably be requested returned to the tube maker for an inspection. They could tell quickly if there was a bias problem.
Try to get a detailed write up on the work originally performed and measurement values for bias, B+, mains current at idle if he logged any of that information.

Not trying to be subjective here but most tube manufacturors would spend very little time if any evaluating these tubes because they simply don't have the time to do it. They don't even have time to test their own tubes much less anyone elses. They would also come to the quick conclusion that the possibilities of 4 tubes failing out of box at the same time or almost slim and none and would make the quick assumption that it was a total bias failure of the amp that caused them to fry and would put it back on the amp modder which is obviously what happened here. You can spin it any way you want to but the amp was working fine before he sent it to Voodoo and he got it back and now it's not. I like Voodoo and Trace and I are friends (I hope) and I absolutely love his amps he builds and want to buy one but you have to bite the bullet here and take one for the team. just my.002

[Jazz P Bass]

I keep thinking on this (4 tube failure).
Maybe one of the tubes puked B+ into the grid.
That in itself would toast the other three.

[Prairie Dawg]

Use Occam's Razor analysis-the likely answer is the simplest one. I saw something like this on a JCM2000 that had had a master volume pot fail-result no bias, all power tubes cooked.

Basic troubleshooting theory tells you that 95 per coent of problems were caused by the last person who "worked" on the amp, and sticking you with the tab for their screwup is pathetic.

They should stand behind their work. That's what's called being a professional is.

[Chuck H]

They should stand behind their work. That's what's called being a professional is.

OOOooo... That depends entirely on what your definition is. When someone makes money and doesn't pay out much they are considered successful. Someone who loses money is considered unsuccessful. It's common business to give as little as you have to for your returns. Those who succeed at his practice drive nice cars and live in big houses that they own. People look at them and think "They are successful. I'll patronize them instead of that guy in the '74 woodside station wagon with one hubcap and an orange door." Even though the guy in the station wagon has spent his life doing the "right thing" and being emminently fair to his fellow man. Thus he has no money and looks unprofessional. It absolutely ceases to matter if success was gained by preserving profits at any opportunity even when it's as base as not paying out when you don't have to. Even when you should. The goal is to get money. Not to help people. Helping people is a product. A successful business plan is to get money and offer a product, in that order, with priority.

There is a line to be drawn though. If you offer a grossley bad product and people know about it you won't sell anything. This has proven to be a very gray area in things like construction, appliance repair, etc. Amp service and repair has many parallels. If no one here patronizes Voodoo that's not much of a loss for Voodoo. We fix our own amps. And who is stoma going to complain to? Probably not a potential client of Voodoo. So the priority becomes preserving money. Which is a very professional business practice.

EDIT: Please understand that I am playing devils advocate and do not subscribe to this morality (or rather, lack of).

[km6xz]

The main product a company has of value it not the stock on the shelf but perceived reputation or perceived value by the pool of potential clients. Seeking to reduce risk on the short term by denying warranty on a repair that went awry is a poor business decision and can damage reputations, rightly or wrongly, that has a much greater long term impact than the whole sale cost of a quad of tubes. He saves $50 but potentially looses hundred or thousands in the long term in reduction in the size of the potential customer pool.
I dare say my shops handled a great many more repairs yet never remember a case where a returned unit was not simply repaired even if we were confident that it was not our fault. The internal operations of a unit are unknown to the user and if a defect occurs after a repair, within the warranty repair, his impression and those of his friends he talks with, is that it IS our fault. Arguing or being stubborn, and right, can be very expensive in the long term. It just is not worth the fight to "win" an argument that in the final judgement is entirely up to the customer's perception.
The tube brand probably takes the same view with established accounts, and, if they intend to keep a good client shop, will usually accept the word of the shop owner in parts failures. Loosing a few dollars on a set of tubes will not be the best outcome if is also means loosing a reliable customer for thousands of dollars in tubes each year.
That applies to companies who value their customer relationships and intend to grow. It is one of the reasons many small businesses do not grow in this field, winning the minor battles at the expense of losing the war.
In this case, I have no idea of who Voodoo is but I would need more positive feedback before recommending his services than what this one customer's situation suggests. Casual readers, or those searching on Google for referrals will also be influenced.