12AX7 failure

Maybe a break in the grid wire near its winding start? Does the filament glow on that side? Is the cathode shorted to the filament?

AS with $5 vacuum tubes, $150 audio vacuum tubes sometimes fail at an early age.........One culls them out and often very old tubes just keep on going fine. 12AX7's have become "fools gold" in this inflated acoustic psycho-babble golden ear but tin brain market...........

And this answers the question how?

Jeff

If we told you, We'd have to kill you. Ken

It could be an indication of grid emission.

zeroing

grid

No C/H leakage either triode. Can only see heater glow with this make of tube, badged GT if relevant, by viewing underside of envelope. Both heaters equally bright.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Many years ago ! I have seen O/c anodes on these. Look for a weld fracture near or in the glass where the pin is welded to the anode support.

Likely, the other way around.

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There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you're crazy.

I also have tubes that have no emission but no visible defects; I ran across a 25AX4 like that the other day. I suspect one of the welds is broken but the gap is too small to see.

The 12AX7 is a miniature tube. I have miniature tubes with pins that have broken inside the glass base. Usually I notice it when the broken piece falls out. If it didn't fall out, I would have a defective tube with no visible problem. (This usually happens outside the seal area so that air doesn't enter the tube.)

In any event, your tube doesn't work. You're going to have to replace it no matter what happened to it. You probably didn't cause it. It's just one of those things.

N_Cook wrote:

I dunno. My cat had a fine litter of kittens, except the dead one. Looked perfect, but ...

Stuff happens, do not get too depressed ;)

Happy Ears! Al

ing

e.graffiti.net/diverse-

You will almost never see the weld crack since it usually only shifts a thou. or two. I tested a tube for failed cathode ribbon connection by connecting a 'good' tube filament - cathode [cold] across an RF inductor on a Q meter and tuning for resonance, then connecting the 'bad' tube in it's place. The capacitance, tiny as it normally is, was much less as shown be a significantly higher resonant frequency.

Neil S.

zeroing

Everyone who's ever died...

amazingly was alive right before their demise.

Probably just a coincidence though

Which raises another question- why would one side of a 12AX7 (or any other similar twin triode) test markedly different from the other side? I've seen big mismatches in 12AX7 tubes. I can see where used tubes that have different "roles" in the circuit could "wear" unevenly depending on how their used (driver, phase splitter, etc).

Poor assembly tolerances?

Life, the universe and everything. Was it a Chinese valve ?

I got a Fender Twin Reverb to fix once. It was initially a puzzler. All the valves lit up brightly and you could trace signal to the grids of the output valves but after that ? ZILCH. O/P transformer was fine too.

Turns out ALL four Z&I (Zaerix brand) Russian 6L6GCs had gone zero emission but looked visually perfect.

I had to sub some EL34s temporarily to get it going for him for a gig. I never held any 6L6s. Only time I needed any in fact.

Graham

Amen to that ! They were the 2N2222/2N3904 or BC108/BC184 of their day.

Graham

Chuck it in the bin and charge the customer for a new one plus mark-up and time of course. That's how Valve repairs are done.

Graham

Who cares ? It's broken. Replace it !

Graham

I hereby second you to the tube manufacturer to check them out.

Graham

Ooops I said tube !