>At 03:57 PM 8/22/02 -0400, WA2BPE wrote:
>>...Had this happen to me with an old SB220 destroying one tube; luckily (??),
>>they were about to be replaced anyways. Later investigation showed that the
>>resistors in the parasitic chokes had increased between 5 & 10x their
original
>>value. I was "fortunate" - only the tube arced with no other damage.
>
>Which "this" is that? A parasitic, or a random grid/filament short,
** In my opinion, there is no such thing as a random grid/filament short
unless there is too much filament inrush current or the tube is operated
horizontally for more than several months.
>or
>arcing due to a mismatched load, or a Rocky Point arc?
** RCA's Rocky Point Investigation was somewhat specious since there is
no such thing as disappearing gas and vanishing "barnacles". . Later,
General Electric's G. W. Fyler seems to have gotten to the root cause. [
Fyler, G. W. ''Parasites in Transmitters'', Institute of Radio Engineers
journal. Sept. 1935 ] :
- Fyler's Conclusion:
"... If necessary, the plate or grid parasitic circuits should be damped
with resistance. ... . ..."
>There are lots of
>things that can happen with a manually tuned amp, and I'd be more inclined
>to look at other things to blame besides a workhorse tube that is probably
>in more amateur amps than any other...
>
** Agreed. The fault lies not in the 3-500Z. As Eimac's W. B. Foote
told me: the fault of parasites is not with the tube manufacturer, it is
with the amplifier's engineer.
>
cheerz, Pete
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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