>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,... ...
... ...
>... ... 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.
Sound travels through air. If there was an arc inside a container that
is virtually devoid of air, does it make sense that it would sound like a
0.22 rifle shot to an observer outside the container?
>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.
An anode-to-grid arc does not send a current pulse through the cathode
bias diodes.
cheers, Jon
Rich...
R. L. Measures, 805-386-3734, AG6K
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