[AMRadio] Ever seen this?


Todd, KA1KAQ ka1kaq at gmail.com
Wed Oct 5 10:31:09 EDT 2005


Geoff -

I've read a little bit about transmitting tubes that sit for long
periods releasing molecules that can form gas inside of the tube.
Someone came up with a procedure for de-gassing them with a home built
jig of some sort, to run them for a period of time getting them hot
enough for the getter to absorb the gasses and other crap that had
been released while dormant. It involves little or no voltage beyond
the heater itself.

Can't comment on your particular situation or the mechanism at work
with the low voltages, but it sure sounds like one of those tubes that
was otherwise good, but went gassy from sitting idle. Hopefully
someone with more knowledge on the subject will comment. I have a
number of old transmitting tubes in storage and worry about this very
thing.

~ Todd/'Boomer'  KA1KAQ

On 10/5/05, W5OMR/Geoff <w5omr at satx.rr.com> wrote:
> some of you older tube-type guys might know what this is, but it's the
> first time I've ever seen this.
>
> In my mid-50's built final, using a pair of 250TH's, the sockets are
> those that have the flat spring-steel tab that sticks out towards the
> middle of the 4-pin jumbo (2N) socket.  AFter all these years, there's
> been some corrosion build-up and carbonization of the spring-steel tab,
> which leads to resistance which leads to heat.  After a number years,
> enough heat will, if not outright melt, at least soften the soldered
> connection for filaments.  I had this happen last year, and changed that
> socket and I guess it's time for the other one to go out, which it is.
>
> The 'quick-fix' was to stick another 250TH in there, and that's what I
> did this morning.  Filaments are nice and bright.
>
> then things got 'interesting'.
>
> After a few minutes of warm-up time, I hit the plate switch on the
> exciter which then closes a relay and applies primary voltage to the
> plate transformer which, in turn applies DC to the plates of the tubes.
> The primary voltage is controlled by a variac, which was sitting around
> 60vac.  A guesstimation would be that there was around 800v on the
> plates of the finals.  There was a short.  I quickly shut off the
> exciter, relays dropped out and the tube is still lit, nice and bright.
> I crank the VariAC down to 0V, apply excitation again, and  "HOLY CRAP!
> WHAT'S THAT?"  The recently installed 250TH, with just 35 or so watts of
> grid drive looked like an 866 rectifier, under full load!  A translucent
> 'blue glow' appears all around the inside of the glass envelope, with
> grid-drive applied.
>
> What causes that?  Is the tube good?
>
> This 250TH hasn't seen air in about 5 years (since I got it) and appears
> to be brand-new.  Can't tell that it was ever inserted into a socket,
> before today.
>
> Thoughts?
>
> --
> 73 = Best Regards,
> -Geoff/W5OMR
>
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