D
David Hopper
- 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
Peter said: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...........
Do their brains suffer from tin pest ?http://en.wikipedia.org/wiki/Tin_pesthttp://www.indium.com/blogs/Dr-Lasky-Blog/entry.php?id=310
Graham
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.
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
Summaryendrites 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
Eeyore said:Why do you fret over nothing ? Valves 'GO'. It's their nature.
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.
Likewise.
You can still get it legally even in the EU for repair of pre PB-free equipment.
Do you have a link for that ? I heard somewhere a typical tin whisker cansupport 30mA.
Graham-
AB9GO said:Yes, but the price is getting higher and it is no longer commonly
stocked.
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.
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.
Omer S said:An interesting link, for further analysis of what could bring the demise of
12AX7 vacuum tubes,
Http://www.vaccuumtubefans.org/forums/Topic_12AX7
Michael said:There are a lot of things that are involved in MTBF.
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.
Omer said:
Omer said:
nobody said:Same problem here.
I googled vaccuumtubefans and all I got was pointers back to this thread.