[AMPS] TL922 troubleshooting....

[KA2YKC@aol.com]

I am troubleshooting a TL922 for a friend. Here's the story:

Amp was working fine on 20m. Took amp offline, removed covers to remove dust.
Tubes were removed from sockets, then replaced after cleaning, in their
original positions (L/R). 

Upon firing up the amp after re-assembly, he heard what he described as a
"soft sizzling sound, around a 800ma indication on the plate current meter,
then the AC fuses blew. All with the amp in stby mode, CW position.

That's when I got involved.

Upon initial inspection, the 470uH choke for the left tube was melted/pulled
together. I performed a simple ohm-meter test on each tube, no apparent
shorts. The 7.5v bias zener and associated components test good (as I would
expect since this happened in stby mode, bias cutoff). 

Question: Would a gassy tube break down and short only when HV is applied ? 

               Would this cause the grid choke to fry ?

I am guessing that the handling of the tubes during removal/insertion may have
introduced a leak of the tube's vaccuum. Is this possible?  Both tubes look
identical, no visible differences that I noticed.

Since I think the amp will need new tubes, let me ask now. What is the best
500Z to go with since Eimac 3-500's are no longer to be had (easily) ? I would
prefer a drop-in replacement, with no bias mods or other changes required. 

What else can I check/test ?

Thoughts/comnments/suggestions ?

Thanks & 73
Eric KK2ED (ex ka2ykc)

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