Good afternoon gentlemen.
I refer to the above subject posting, and thought that the resolution of the
problem might be of interest to the group. For those who did not see the
original posting. Briefly, the problem was the apparent failure, after five
years use, of a pair of Amperex 3-500Z tubes, installed in a commercial
amateur amplifier, which belongs to an old friend, who lives in another
country. During a visit, and on inspecting the amplifier, I discovered that
Pin 1, on each 3-500Z tube, had become overheated, the solder between the
pin sleeve and the tungsten wire from the tube envelope, had melted /
carbonised, and the 3-500Z filaments were showing virtually open circuit on
continuity test. I was told that this was possibly the same failure, that
had required the original OEM supplied tubes to be replaced. The amplifier
manufacturer had recommended, change the 3-500Z tubes and the tube bases. In
my view, replacing the tubes would be premature, for we had no way of
knowing the emmision state of the thoriated tungsten filaments, and it was
possible, that one would be throwing away serviceable tubes. It would also
be a very costly cure of the symptoms, but would not, in my view, not cure
the problem.
In October 2005, whilst visiting my friend, I thoroughly cleaned and
resoldered the defective pin 1, on each 3-500Z tube, the tube filament
continuity returned to the measured, and expected, low ohmic state. I then
returned home, to the UK. I consisered the problem, and then fabricated a
small metal plate, on which I fitted two small, but powerful axial fans, to
provide an additional cooling airflow. The fans are 60mm X 60mm X 15mm, and
each have a rated air flow of 20cfm. This metal plate is an exact
replacement, overall size, fixing holes etc. for the metal inspection
cover, fitted at the bottom of the amplifier cabinet, by the amplifier
manufacturer, and this cover is directly beneath the tube bases of the
3-500Z tubes. The replacement cover plate / fans, places a fan directly
beneath each of the Johnson ceramic tube bases. The air flow from each fan,
would then flood the underside of the ceramic bases and sockets / pins with
a cooling air flow. I sent this assembly to
my distant friend, who then fitted the replacement cover / fan plate to his
amplifier, to my written instructions. The results, the amplifier has been
in daily use for two months, at a power output of around 500 watts. Tests
show that the repaired Amperex 3-500Z tubes do provide 1 Kw output, into a
50ohm dummy load, and the tube filament pin heating problem appears to have
been resolved.
Now, I have recently discovered that I have "reinvented the wheel" and that
the late Bill Orr, W6SAI, who had been an Eimac employee, had covered this
3-500Z filament pin heating problem, in an article published in CQ, for May,
1991, page 78 ( recommended, download from the CQ archives) and this CQ
article was also covered in the RSGB " Radcom" Technical Topics, shortly
after.
For those who may require them, the details of the axial fans I used, are as
follows:-
BiSonic Model BP601512M02
Manufactured by BiSonc TechnologyCorp.
12v DC @ 0.16amp
Air flow 20cfm
Expected lifespan 50000 hours
Dimensions 60mm X 60mm X 15mm
Source, in the UK. Farnell In One.
Farnell order code 971-4901. Cost approx $6 per fan.
(www.farnellinone.co.uk)
The cost of resolving this problem was considerably less than the expected
cost of components and labour in replacing, the now obviously serviceable
3-500Z tubes, and tube bases. As the late Bill Orr says, in his article,
"you cannot apply too much cooling air, the limit is reached, only when the
applied air pressure, blows the tubes from the sockets !!"
..........regards, John. G3JVC.
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