[AMPS] Blown TL922A... What to do?

[Tom Rauch]

> So, on to questions, part two.
> 
> I looked under the amp and did not see an explosive
> destruction on the resistor/choke assembly like
> I did on tube number one. Whew!
> 
> But... the arcing seemed to occur in the same place
> under the chassis on the right side.
> 
> Am I going to have to invest in new tubes to further troubleshoot
> this thing? How can I test the tubes? I'd like to narrow it down a
> bit more, although it seems like I'm on the right track, thanks to Pete!

The worse thing in the world for 3-500Z's is to have them used in an 
amplifier, especially one with marginal cooling, and then store them 
away for months.

The tubes getter themselves (remove gas) by heating the anode to 
a variety of temperatures. Heat activates the gray zirconium 
coating sprayed on the tube anodes. Zirconium needs to be about 
1000 deg C to work fully, with areas of lower temperature. It 
absorbs different gasses at different temperatures.

All large vacuum tubes have some gas ingress through seal 
leakage, as well as outgassing from materials in the tube. When 
you let a tube sit cold for months and months it gasses. 
 
> Now what? Is there a possibility that all this banging and arcing has
> caused additional damage?

Maybe. 

> So... back to the testing. How can I test the tubes/amp to see which
> is the culprit? It appears that the tubes are causing a problem, because
> the HV is on when they are missing. Would it buy me anything to test them
> plugged into the socket, but with the plate not connected? Especially
> remembering that I really don't wish to have a melt-down here...HI

You really need to hypot the tubes. (You also need to add 
protection to the 922, like a HV surge limiting resistance and a 
diode protection system for shunts and meters.)

You might be able to recover the tubes by letting them "arc" with a 
current limited arc (an arc will getter some gasses) and/or by 
"cooking" the tube at low anode voltage with slightly positive grid 
bias. If you applied about just enough voltage to the tube to allow it 
to be forward biased to 300- 400 watts of anode dissipation (be 
sure the grid isn't over-dissipated) and let them cook with good 
airflow (the 922 does not have proper airflow so the tubes will 
overheat) you might recover the tubes.

Running the filaments just won't do the job,  because the anode 
won't get hot enough.


73, Tom W8JI
w8ji@contesting.com

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