richard w. ehrhorn wrote:
>
> From our experience over 25 years and many thousands of tubes, it's very
> clear that in the vast majority of cases, after one or two BANGs early in
> life, the tube continues on for a normal lifetime of normal performance,
> just as if nothing had happened. If the cause wasn't gas or some other
> physical anomaly that is basically eliminated by the arc - if in fact it
> was parasitic in nature - what killed the parasite during the BANG?
> Especially if parasitic suppressor R increased as a result?
>
> Also, it's much more common in our experience for a new tube to BANG when
> in standby with full cutoff bias applied rather than when running RF,
> keyed or key-down. How to explain that? Seems like most these are most
> unfavorable conditions for spontaneous start of a VHF parasitic - or any
> other sort of oscillation.
I normally just read and learn from this reflector, and certainly am not
qualified to argue technicalities with most on this list; however, I built
an amplifier with several 3-500Z tubes about 10 years ago, and Dick's
comments above really rang a bell.
After I'd had the amp running for a couple of weeks I was sitting admiring
my work :-) without any cover on the amp. With my face not 18" from one
of the tubes it suddenly "banged" like you shot a .22 caliber rifle! I
almost wet my pants, but nothing was damaged.
The interesting part to me is that I SWEAR the arc looked like it followed
the glass of the tube! It's still emblazoned in my mind what the blue arc
looked like-- I can't say whether the arc was inside or outside the tube,
but the amp had been on for a couple of hours and was just sitting with the
tubes at cutoff.
Plate voltage was 3700 volts, and I was using a string of 1N4001 diodes in
the cathode return for bias; 10 diodes for operating, 30 for cutoff. A
subsequent "bang" a couple of weeks later did take out the cathode bias
cutoff string (and my grids were directly grounded to the chassis), so this
has remained a mystery for these last years. At that time I had BIG filter
capacitors and no series resistor in the B+ to limit current.
The amp probably "banged" 4 times total during its early life with new
tubes, and then basically settled in and operated calmly for several years
afterward.
Just my observation, for what it's worth.
73,
Jon
--
Jon A. Barclay N5JA (ex-AA5BL)
N5JA@contesting.com
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