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12AX7 failure

P

Peter Elem

Jan 1, 1970
0
The 12AX7 owner said he was a careful owner, but maybe not careful enough.
Maybe if he had ordered the Snake-Oil brand tube cozies......and vibration
smugglers and wrapped his 12AX7's in those that tube would be alive and
transfering the emotion of true music today..........
 
P

Peter Wieck

Jan 1, 1970
0
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.

Hmmm... you did not differentiate whether the tube is 4-years-in-use
NOS, or 4 years from manufacture. Nor did you specify country-of-
origin if New.

If this is a Chinese 12AX7 four years from manufacture - that is a
VERY long time for some of these tubes. The owner should count himself
lucky for even that much life.

Peter Wieck
Melrose Park, PA
 
A

AB9GO

Jan 1, 1970
0
Hmmm... you did not differentiate whether the tube is 4-years-in-use
NOS, or 4 years from manufacture. Nor did you specify country-of-
origin if New.

If this is a Chinese 12AX7 four years from manufacture - that is a
VERY long time for some of these tubes. The owner should count himself
lucky for even that much life.

Peter Wieck
Melrose Park, PA

I have some Groove Tubes here and they are all "Made in China". They
are marketed to the Guitar amp user. Many are sold through Sam Ashe,
Guitar Center and other music shops. Even the box they come in is
styled like a Fender amp. They have an odd distortion marking scheme
and biasing advice. The tubes are rated as to distortion: Early
Distortion 1-3, Normal Distortion 4-7, and Latest Distortion 8-10. "
The bias should always be adjusted when changing power tubes, UNLESS
YOU ARE REPLACING GROOVE TUBES OF THE SAME TYPE AND RAITING, THEN NO
ADJUSTMENT IS NEEDED. " Bad advice IMHO.

Find an old timer repair guy and they will tell you that getting only
4 years life on a tube is not unusual. Some go bad in minutes and
some run forever or so it seems.

Randy AB9GO

p.s. Here is an example of running forever: http://www.centennialbulb.org


Age: 107 years and counting (as of 2008)
Installed: First installed at the fire department hose cart house on L
Street in 1901. Shortly after it moved to the main firehouse on
Second. In 1903 it was moved to the new Station 1 on First and McLeod,
and survived the renovation of the Firehouse in 1937, when it was off
for about a week.
Proof of Longevity: From local newspaper records; also GE engineers
researched it. Was donated to the Fire Department in 1901 by Dennis
Bernal who owned the Livermore Power and Light Co.


And there is another one in Texas:

Happy 100th B-Day Palace Bulb!!!

On September 21, 2008 the Palace Bulb of Fort Worth, Texas will be the
2nd bulb in history to have burned 100 years! Our best wishes for a
wonderful celebration to all the folks at the Stockyards Museum, and
for another 100 years!
 
A

AB9GO

Jan 1, 1970
0

That's funny! I buy all of the old 60/40 solder I can find because if
the tin dendrite issue that will be comming in the future. Tin
dendrites have been known to grown to over 10mm in length. That is
why NASA and critical biomed devices (pacemakers and ICD's etc) will
continue to use lead in their solder, and so will I for as long as I
have a supply.

Randy AB9GO

Current carrying capacity of dendrites
Bjorndahl, W.D.; Lau, J.C.
Aerospace Applications Conference, 1994. Proceedings., 1994 IEEE
Volume , Issue , 5-12 Feb 1994 Page(s):399 - 405
Digital Object Identifier 10.1109/AERO.1994.291179
Summary:Dendrites are sometimes found on electronic hardware during
failure analysis or trouble shooting activities. Problems due to
dendrite formation can vary from catastrophic failure to occasional
electronic glitches. If a dendrite is thought of as a fuse in a
circuit, then it is apparent that it can have, depending on size, a
wide range of current carrying capability. Mathematical modeling and
experimental analyses were conducted to determine the reasons for the
wide range of observed behavior. Mathematical modeling indicates that
the resistivity of the dendrite and the thermophysical characteristics
of the substrate on which it is grown determine its current carrying
capacity. More specifically, the modeling indicates that maximum
dendrite size and time to failure are determined by the power loss
within the dendrite and the rate at which heat can be transferred away
from the dendrite. Small dendrites are stable and can grow until they
reach a size at which internal heat generation cannot be accommodated
by the heat loss due to conduction through the substrate. Dendrites in
low voltage applications on substrates with high thermal diffusivity
are more likely to lead to failures with high power loss, whereas
dendrites in high voltage applications on low thermal diffusivity
substrates (e.g. tin on glass/epoxy circuit board material) are more
likely to lead to high resistance shorts. Experimental results and
failure analysis observations agree qualitatively and semi-
quantitatively with the results of the idealized dendrite model
 
E

Eeyore

Jan 1, 1970
0
AB9GO said:
That's funny! I buy all of the old 60/40 solder I can find because if
the tin dendrite issue that will be comming in the future. Tin
dendrites have been known to grown to over 10mm in length. That is
why NASA and critical biomed devices (pacemakers and ICD's etc) will
continue to use lead in their solder, and so will I for as long as I
have a supply.

Likewise.

You can still get it legally even in the EU for repair of pre PB-free equipment.

Randy AB9GO

Current carrying capacity of dendrites
Bjorndahl, W.D.; Lau, J.C.
Aerospace Applications Conference, 1994. Proceedings., 1994 IEEE
Volume , Issue , 5-12 Feb 1994 Page(s):399 - 405
Digital Object Identifier 10.1109/AERO.1994.291179
Summary:Dendrites are sometimes found on electronic hardware during
failure analysis or trouble shooting activities. Problems due to
dendrite formation can vary from catastrophic failure to occasional
electronic glitches. If a dendrite is thought of as a fuse in a
circuit, then it is apparent that it can have, depending on size, a
wide range of current carrying capability. Mathematical modeling and
experimental analyses were conducted to determine the reasons for the
wide range of observed behavior. Mathematical modeling indicates that
the resistivity of the dendrite and the thermophysical characteristics
of the substrate on which it is grown determine its current carrying
capacity. More specifically, the modeling indicates that maximum
dendrite size and time to failure are determined by the power loss
within the dendrite and the rate at which heat can be transferred away
from the dendrite. Small dendrites are stable and can grow until they
reach a size at which internal heat generation cannot be accommodated
by the heat loss due to conduction through the substrate. Dendrites in
low voltage applications on substrates with high thermal diffusivity
are more likely to lead to failures with high power loss, whereas
dendrites in high voltage applications on low thermal diffusivity
substrates (e.g. tin on glass/epoxy circuit board material) are more
likely to lead to high resistance shorts. Experimental results and
failure analysis observations agree qualitatively and semi-
quantitatively with the results of the idealized dendrite model

Do you have a link for that ? I heard somewhere a typical tin whisker can support 30mA.

Graham
 
H

Heinz Schmitz

Jan 1, 1970
0
Eeyore said:
Why do you fret over nothing ? Valves 'GO'. It's their nature.

Depending upon their use, they may go quickly or slowly.

Regards,
H.
 
P

PTurney

Jan 1, 1970
0
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.

Hi,

Given that it is dead, if you really wish to find out you could always
crack & remove the glass and then check the internal connections.
Nothing to lose!
 
A

AB9GO

Jan 1, 1970
0
Likewise.

You can still get it legally even in the EU for repair of pre PB-free equipment.

Yes, but the price is getting higher and it is no longer commonly
stocked.
Do you have a link for that ? I heard somewhere a typical tin whisker cansupport 30mA.

Graham-

ieeexplore.ieee.org/iel2/1110/7210/00291179.pdf

Is where I got the abstract. I am not a member so I cannot access the
entire document. Sorry.

Randy AB9GO
 
E

Eeyore

Jan 1, 1970
0
AB9GO said:
ieeexplore.ieee.org/iel2/1110/7210/00291179.pdf

Is where I got the abstract. I am not a member so I cannot access the
entire document. Sorry.

Likewise.

Graham
 
H

Heinz Schmitz

Jan 1, 1970
0
Omer said:
Why is it so important to debunk a failing tube that gave four years of
satisfactory performance?

Dude, just $#!^can the offending tube, and move on!

Its not *four years*, but only the ON-time in four years.

You living on some island where tubes grow on trees?

Regards,
H.
 
Its not *four years*, but only the ON-time in four years.

You living on some island where tubes grow on trees?

Regards,
H.

Yikes!!

a) If the tube was Chinese, 6 weeks of "on-time" would be a lot in
some cases.
b) Current-production tubes are readily available. Not for free, and
one may vastly over-pay, but it is not as if it were a 19T8 or some-
such.
c) After determining that the equipment itself was not at fault, then
yes s&*t-can the tube and move on.

Peter Wieck
Melrose Park, PA
 
H

Heinz Schmitz

Jan 1, 1970
0
Michael said:
There are a lot of things that are involved in MTBF.

Yes, indeed.
But I guess that we are interested in those which we can influence.
Like e.g. (?)
- heating before applying anode voltage,
- avoiding long periods without anode current,
- ??

Regards,
H.
 
H

Heinz Schmitz

Jan 1, 1970
0
b) Current-production tubes are readily available. Not for free, and
one may vastly over-pay, but it is not as if it were a 19T8 or some-
such.

I hear that presently there are only russian and chinese productions
going on. Hard to get at - if you'd agree that an eBay-buy from
Ukraina or China is somewhat "hard" :).

I think that most of the post-war radio tubes are available only as
NOS (ECH81, EF4x, EF8x,ELxx ...). Somebody should make some
fivehundred EM34/35 and earn himself a good meal.

Generally I found that to buy tubes is a lottery except if you have a
known and trustworthy dealer. Many deaf nuts being handed around.

Regards,
H.
 
E

exray

Jan 1, 1970
0
nobody said:
Same problem here.

I googled vaccuumtubefans and all I got was pointers back to this thread.

The fellow who parroted and corrected another guy's link has a
reputation for not having verified his sources of info. Nuff said.
 
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