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Powering up amp after long hibernation?




 
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March 21, 2019, 04:52:41 PM *
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WB3JOK
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« on: March 17, 2019, 12:39:35 PM »

My homebrewed 4x4-125A amp has been sitting for a couple of years. Will check the big electrolytic filter (a 58 uf surplus defibrillator cap) for re-forming, and dust off the tube envelopes as well. I can hook up a variac to the plate transformer primary to bring it up slow. Max is 4.2 kvdc unkeyed, 3.8 kv idling.

I seem to recall a recommendation to run the 4-125A filaments (only) for some period of time, before bringing up the HV?  Thanks. Huh
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KK4YY
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Gol na vydumku khitra


« Reply #1 on: March 17, 2019, 01:41:59 PM »

I believe the 4-125A uses a zirconium getter applied to the anode which only becomes active, as a getter, at high temperatures. I don't know if filament power alone will get you there - maybe. It couldn't hurt to try, as long as you don't exceed the seal temperature ratings. When you do apply HV, use a series resistor to limit current in case of a flash-over. 4.2KV is 140% of the data sheet rating of 3KV so you're prudent to be cautious.


Don
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Fate does not protect its worshipers any more than its deniers.
WB3JOK
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« Reply #2 on: March 17, 2019, 05:15:20 PM »

It's not quite that bad - I remembered wrong. The defibrillator cap is rated at 4.2 kv. The actual voltages are something like 3.5/3.2 kv. But you're right, it's a bit over the Eimac datasheet rating.

I've used this amp before and not had any problems with flashover (but have a "glitch resistor" in the circuit anyway - 4-125A's are cheap but not that cheap).  Wink
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