One of the worst tubes I've run into for being gassy are the old
PL-172/8295A.
These were, to me, a great tube. They are an external anode metal tube with
a very coarse set of fins in the radiator so they do not require much in
the way of back pressure. Just a plain old fan blowing air up around the the
tube and through the fins makes for suficient cooling. Unfortunately
instead of ceramic the tubes use a glass to metal seal and although there
seem to be quite a few floating around, many are gassy. The only ham amp
that used them was the Hallicrafters HT-32A and B series AFAIK.
Other than just letting them cook with out the high voltage on, it there any
thing that can be done for these outside of turning them into expensive
conversation pieces?
BTW the straight 8295A was ceramic/metal and they do not seem to show up
gassy, or at least I've not seen any. OTOH a NOS 8295A is worth a small
fortune if you are the one doing the purchasing.
Yes, I do have an HT-32B I'm just finishing up on the restoration. The
PL-172 in it is good.
73
Roger (K8RI)
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