[AMRadio] Ever seen this?

W5OMR/Geoff w5omr at satx.rr.com
Wed Oct 5 10:22:02 EDT 2005

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.


73 = Best Regards,

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