>
> 1: Does this sound like a symptom of parasitic oscillations?
> I thought the 3-500Z was pretty rugged with respect to faults.
Parasitics, even assuming they would occur, can't cause tube failures unless
you see some solid evidence of excessive current on meters.
I don't know what time period you are talking about, but I've had several
Amperex 3-500Z's fail in my own equipment. Amperex, like all foreign tube
companies, is a bit difficult to work with. My last conversations with
Amperex were only through Richardsons Electronics(who owns Amperex), and
involved 3-500Z grid-to-filament shorts.
I had four 3-500Z tubes in a test fixture cycling only the filament
off-and-on once every minute, and within a day two tubes failed. In a week
they were all bad. All of them failed because the center of the filament
helice moved over and touched the grid.
My conclusion is the repeated thermal cycling of the filament, and a
material or assembly flaw in the tube, caused the helice to move the few
thousands of an inch required to cause a short.
> 2: Does anyone know of any problems with recently built Amperex bottles?
Absolutely there are.
> 3: I'll probably help him install the parasitic supressor kit somethime
this week. Are there any other things to look for or suggested corrections
to this thing that would make it less prone to eating another tube?
Put a dead-chicken leg over the cabinet, and say a prayer instead. It will
be more effective.
Seriously, ask him how much often he cycles the amp on and off. Also be sure
voltages are set correctly on the power transformer taps, and that there is
no error in filament voltage. Ameritron's transformer vendor, Schumacher,
moved to Mexico about a year or so ago and transformer quality was suddenly
horrible. Bad connections were the least of the problems (it looked like
they soldered transformers with a hot rock), there were also some build
problems in transformers.
I'd assume Ameritron measured filament voltage at the tube, but they might
not have measured it correctly or with a good meter. The quality of
technical people is dropping like a rock as time passes. I'd re-measure the
filament voltage, especially in light of the nature of tube failures.
Make sure your friend isn't needlessly cycling the filament off-and-on,
since the failures are on-and-off cycle related in tests I have done. I knew
a guy who ate 3CX1200D7's in a Henry like pop-corn because he cycled the
filament off and on about ten times a day. He'd get about two months on a
tube.
Remember you are dealing with a white-hot filament that has one common
mechanical support point *at the very bottom*. The filament is about 2-3
inches long when wound in a tight helice, and is thousands of an inch from
the grid. Even if the tube is built properly, it isn't a good idea to
heat-cycle the filament needlessly.
Ask him how often he turns it on and off BEFORE you tell him anything else,
and see if it is abnormal. Also be sure he knows to keep operating grid
current below 125-150mA on carrier.
73 Tom
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