12AX7 failure

[N_Cook]

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any zeroing
on the tester even setting current down to 0.5 mA or less and for any grid
voltage setting. Can't see any obvious internal weld failure.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse-

 

[Meat Plow]

On Sat, 22 Nov 2008 16:36:08 -0000, "N_Cook" <diverse@tcp.co.uk>wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any zeroing
on the tester even setting current down to 0.5 mA or less and for any grid
voltage setting. Can't see any obvious internal weld 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?

 

[Peter Elem]

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...........

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:gg9cbf$hm$1@news.motzarella.org...

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any
zeroing
on the tester even setting current down to 0.5 mA or less and for any grid
voltage setting. Can't see any obvious internal weld failure.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse-

 

[Jeffrey D Angus]

Peter Elem wrote:

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

 

[Ken]

Jeffrey D Angus wrote:

Peter Elem wrote:
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

 

[Sofa Slug]

Meat Plow wrote:

On Sat, 22 Nov 2008 16:36:08 -0000, "N_Cook" <diverse@tcp.co.uk>wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any zeroing
on the tester even setting current down to 0.5 mA or less and for any grid
voltage setting. Can't see any obvious internal weld 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?

It could be an indication of grid emission.

 

[N_Cook]

Meat Plow <meat@petitmorte.net> wrote in message
news:2c649h.r74.17.1@news.alt.net...

On Sat, 22 Nov 2008 16:36:08 -0000, "N_Cook" <diverse@tcp.co.uk>wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any
zeroing
on the tester even setting current down to 0.5 mA or less and for any
grid
voltage setting. Can't see any obvious internal weld 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?

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
http://home.graffiti.net/diverse:graffiti.net/

 

[Baron]

Meat Plow wrote:

On Sat, 22 Nov 2008 16:36:08 -0000, "N_Cook" <diverse@tcp.co.uk>wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A
very careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any
zeroing on the tester even setting current down to 0.5 mA or less and
for any grid voltage setting. Can't see any obvious internal weld
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?

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.

--
Best Regards:
Baron.

 

[Michael A. Terrell]

Ken wrote:

Jeffrey D Angus wrote:
Peter Elem wrote:
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

Likely, the other way around.


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


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.

 

[Jim Mueller]

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:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any zeroing
on the tester even setting current down to 0.5 mA or less and for any grid
voltage setting. Can't see any obvious internal weld failure.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse-

--
Jim Mueller wrongname@nospam.com

To get my real email address, replace wrongname with dadoheadman.
Then replace nospam with fastmail. Lastly, replace com with us.

 

[Staring at old audio chas]

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

 

[nesesu]

On Nov 22, 8:36 am, "N_Cook" <dive...@tcp.co.uk> wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any zeroing
on the tester even setting current down to 0.5 mA or less and for any grid
voltage setting. Can't see any obvious internal weld failure.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list onhttp://home.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.

 

[philo]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:gg9cbf$hm$1@news.motzarella.org...

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any
zeroing
on the tester even setting current down to 0.5 mA or less and for any grid
voltage setting. Can't see any obvious internal weld failure.

12AX7's are a lot like human beings.


Everyone who's ever died...

amazingly was alive right before their demise.

Probably just a coincidence though <G>

 

[TwoMuttHeads]

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
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).

 

[John Byrns]

In article <d9efcc43-7959-4314-bf00-99208d1b7a78@w22g2000yqd.googlegroups.com>,
TwoMuttHeads <menwagoh@msn.com> wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
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?

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

 

[Eeyore]

N_Cook wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.

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

 

[Eeyore]

Peter Elem wrote:

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...........

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

Graham

 

[Eeyore]

Jim Mueller wrote:

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.

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

 

[Eeyore]

TwoMuttHeads wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
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).

Who cares ? It's broken. Replace it !

Graham

 

[Eeyore]

John Byrns wrote:

TwoMuttHeads <menwagoh@msn.com> wrote:

What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
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?

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

Graham

Ooops I said tube !