Burnt power tube

[Deftone]

So on Saturday 5/11 I sold one of my JCM800 2204's on FB Marketplace. I tested it out on Friday night and it sounded great. Everything worked fine, I cycled all the pots to make sure they weren't scratchy and everything was good. Should be fine, because I had it serviced back on 5/2021, and that included a new full tube set+ biasing, new caps, etc... Played it plenty of times since then as well.

I met the buyer in a parking lot and I exchanged the amp for cash and called it good.

Later on that evening I get a text and a pic from the buyer that says one of the PT's isn't working. The pic shows the silver stuff in the top of the tube is all gone...burnt away?



Anyway, we are texting back and forth about what to do, he says it plays fine it just has a burnt/dead tube. Maybe it's been dead all along? Bad tube from the factory? Maybe something jostled loose during transportation? I'm thinking I will at the very least pay for half or all of a visit to the Amp tech, maybe even just give him his money back.

I start searching for answers and of course I find an old Marshall Forum thread and I see someone suggested mis matched impedance. Could that be the cause? He says he has nothing but Mesa amps and I'm thinking my Mesa is 8ohms out, and my Mesa cab had four 8ohm 12" speakers for 8ohm in. What if he didn't switch the amp to 8ohms? Or even worse what if has a 2x12 that's 4 ohms? Could that be the cause?

So I shoot him a text and ask him if he matched the impedance of the amp to the cab and I haven't heard back since. Been almost 24 hours since I've heard from him now. I'm wondering if that was it.

 

[Jon Snell]

If the oxide turns white, the valve has air in it, usually from rough handling or just one of those things.
The 'getter' is the circle or other shape of metal that is attached to one of the anodes and is heated up with an induction heater for a few seconds, that allows the Barium to evaporate and then condense on the inside of the glass. When gas is in the vacuum, the barium vapour absorbes it.
Incorrect output impedance is not the cause.
A crack in the glass envelope and air entering the valve is the cause.

A new matched pair of output valves is the cure.

 

[Deftone]

If the oxide turns white, the valve has air in it, usually from rough handling or just one of those things.
The 'getter' is the circle or other shape of metal that is attached to one of the anodes and is heated up with an induction heater for a few seconds, that allows the Barium to evaporate and then condense on the inside of the glass. When gas is in the vacuum, the barium vapour absorbes it.
Incorrect output impedance is not the cause.
A crack in the glass envelope and air entering the valve is the cause.

A new matched pair of output valves is the cure.

Thank you sir. I will pass along this information to the buyer.

You can tell this by looking at the pics of the tube?

 

[Pete Farrington]

Yes, it's not a vacuum tube anymore, just a useless tube.
I suppose extreme overheating might provoke a physical effect that results in the vacuum failing.

So I can envisage that pushing the amp into half or quarter of the intended load impedance might result in that.

But yes, normally vacuum failure is due to rough handling.

 

[V-man]

Got a nice discount on my 2203 JMP. Original owner set it up and I demoed it loud. POOF!

It had been so long he forgot about/how to set impedance and had a mismatch. Blew a tube and a fuse. We got a new fuse at the hardware store and I pulled 2 to ensure the iron survived.

Playing a mismatch loud can blow tubes. Happened to me that time and one other in life on another amp (my fault- stupid fucking 1936 cab).

 

[Jon Snell]

If it was an overload issue, I would expect overheating to occur and that makes the colour of the JJ logo change to a slightly different tone, usally faded brown.
Valves flash over inside and get damaged if there is say a 16 Ohm load placed on the 4 Ohm tapping and played for a long time at full power but under normal use the back EMF from the output transformer is absorbed enough to limit the extra high voltages present on the anodes.

Just as a matter of interest, the 807 valve is a 6L6 with the anode going to the top cap, avoiding a flashover between the anode pin 3 to pins 2 or 4. This flashover used to occur in AM modulators back in the older transmitter days and the 807 is happy with 800volts on its anode without cause for concern.
In class AB2 with an anode voltage of 800volts and a load of 12.8K, 112 Watts of Audio is acheivable with just one pair of 807s in push pull. 18% THD ... but hey, think of the power output! Lovely valve.

 

[william vogel]

The most likely cause of this tube failure is just defect vs misuse of any kind. The real question is was the tube defective when you still possessed the amp or did it just occur after you handed it off? The tubes aren’t new and who’s responsible for the replacement? The crappy part is that I don’t think it made it 24-48 hours apon changing hands but it’s sold as is and a tube is perishable. It’s kinda like a car tire. I’d hate to be the one that got to play ignorant but I don’t think you should be responsible for replacing the tubes.