Doug says:
>The hole in the tube is remarkable. I've been haming for 38 years and I've
>seen cracked glass before but not a hole with flowing glass!
I've been hamming for about the same length of time, as well as doing radio
professionally - I'm old enough to have designed tube transmitters for
production - and I've seen this effect twice. Once was in a 6JS6 used in an
old FL100 tx, and the other in a home brew CW rig using a 5B/254M (that's a
UK manufactured 807 in a smaller envelope on a loctal base). Both cases,
the hole was on the side by the RF Choke, towards the top (hot end) of the
choke, and there was very little clearance (1/4 inch from memory) between
the choke and the glass envelope.
So I guess the evidence is that it's not that common a phenomena. My
instinct suggests VHF parasitics such that there's a high RF displacement
current between the choke winding and the plate, leading to localised over
heating. I don't look on parasitics quite as the electronic equivalent of
Pandora's Box, but in this case..........
73
Peter G3RZP
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