>
>>
>> 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.
>
A jump to 250mA of ZSAC is not a clue?
>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.
>
It takes more movement than this to short a 3-500Z.
>> 2: Does anyone know of any problems with recently built Amperex bottles?
>
>Absolutely there are.
>
But Tom Rauch is apparently the only person on this planet who is
convinced of this.
>> 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.
>
The recognized amplifier expert plays the Ad Hominem Card.
>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.
Say what ?
>
>I'd assume Ameritron measured filament voltage at the tube, but they might
>not have measured it correctly or with a good meter.
So is Tom Rauch still employed by Martin F. Jue ?
> 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.
>
Grid-fil shorts from too much filament V ?
>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.
>
The 3cx1200D7 has no spec on fil inrush current.
>Remember you are dealing with a white-hot filament
1830 degrees K is hardly white hot.
>that has one common
>mechanical support point *at the very bottom*.
There's a ceramic support at the top of the filament.
>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.
>
Eimac has no such advice in the specs.
>73 Tom
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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