[AMRadio] Bad tubes?

Geoff/W5OMR ars.w5omr at gmail.com
Tue Jan 9 21:15:18 EST 2007

Brett gazdzinski wrote:
>  John has let me know that some of the
> tubes I sent out may be bad.
> Do tubes leak air over time?

I think so.  To kinda follow up on my previous post about Big 
Transmitting tubes... I've got a few 250TH's, and I'll admit - some are 
ebay specials, and others have been lingering around here for a while.  
I stuck one of those in the final, and let the filaments burn at 5.5v 
for about 10 hours.  (mind you, this was before I re-read the info about 
tube filaments and burn-in time).  All seemed fine.  Usually, the 
'gassy' tubes I've seen, turned blue like a mercury vapor rectifier with 
just grid drive applied.  That didn't happen this time, but when I 
applied the B+, all was ok, for about 5 seconds, then the blue cloud 
appeared near the plate, and started growing exponentially.  I hastily 
pulled the B+ off and turned the tubes off and let 'em cool off for a 
bit, then pulled that tube out and put a known good one back in it's place.

I have a transformer with one of those huge white ceramic insulators on 
one end... I've seen 'em in use as 872A transformers (5v @ 15amp).  the 
plan is to stick 250TH's in this setup and let 'em burn in for 200 
hours.  After which, I'll test 'em in the final again, and -THEN- if it 
turns blue/gassy I'll take it in the back yard, toss the damn thing in 
the air, and ping it with a pellet gun!  ;-)

No, no... I'll probably make a lamp out of it with an extra socket.

Here's my question (Bacon?  Don?  John?)

If the tube has been used before (obvious by some dark spaces on the 
glass and discolored places on the plate) is it worth the time to try to 
're-burn-in' the tube in question?


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