[R-390] Tube Failures

[Todd Bigelow - PS]

Gord Hayward wrote:

> The heater to cathode short is a good one to know about.  I had a 
> humongous amount of hum when I
> turned the limiter on, but not when it was off.  I eventually found 
> the problem to be the tube.  It tested
> OK (previous posting about testers with forked toungues is true!) but 
> when I replaced it the problem
> vanished.  With the high impedance circuit, even miniscule AC leakage 
> into the cathodes was enough.
>
> 73 de Gord, VE3EOS. 

Surprised it didn't show a short on your tester, Gord. But this is an 
excellent example of why a tube tester is a great tool for getting you 
into the ballpark, but also how/why swapping tubes is still the only way 
to be certain. Even a good tube tester can't give you the critical 
results that the hands-on approach gives.

Someone on the Collins list was just mentioning how a complete re-tubing 
of their KWM-2 was not the great idea they thought it was. Apparently 
two new Phillips 6AZ8s were too hot for the rig and caused oscillations. 
His revised view is to replace one tube at a time, remember where each 
tube came from, and have some known-good replacements to use as 
substitutes. Replacing the Phillips with RCAs cured the problem.

Good stuff....

de Todd/'Boomer'  KA1KAQ