I have been waiting patiently for someone to mention Rocky Point effect
in connection with "big bangs" in linear amplifiers. Tom finally brought
up the subject with a very interesting post. I first heard of it in one
of Bill Orr's columns in Radio magazine. About 1 or 2 years ago it was a
topic of discussion on Amps. I keep files on about 20 topics of interest
which are composed of extracts from postings which are cut and pasted to
the files. One of my files is on Rocky Point effect and I thought there
might be interest in 2 notes from that file. Unfortunately, I do not
always note the author but I am almost certain that the first note was
posted by Dick Ehrhorn of Alpha fame and the second came from a posting
by a British ham. Here they are.73, Dan, N5AR
"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? Would seem that gain at the parasitic freq would be higher, not
lower....?
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."
-------------------
"G6JP, who spent his whole working life in tube manufacture, told me
that
this was known at one time as the 'Rocky Point effect', so called
because it was first noticed at the Rocky Point, LI, station of, I
believe, ITT ( but it could have been RCA) on the 150KW long wave SSB
transmitter.
Classical 'Rocky Point' effects are seen with the tubes in cut off. It
was beleived that the phenomena was caused by ionisation at sharp points
on the electrode structure, presumably by ion attraction as in the gas
ion pump, but with enough ionisation to cause a flash over. The cure was
apparently to run the tube in cut off with a very high plate voltage to
allow the arcs to batter the points off! (Current limited, though)."
--
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