Re: 572B

That's prolly the answer. I put my friends amp back together so I cannot test it now plus it obviously puts stress on everything so I'll mark it bad and test it at a later date.

Your theory makes sense.

Thanks.

Bob W4JFA?

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On Fri, Sep 8, 2023, 11:07 AM Chuck Neal <cdoneal@...> wrote:

The tube is likely gassy and will only conduct when HV is applied.? It usually takes a few hundred volts for a gassy tube to conduct so the low voltage from a test meter will not reveal a gassy tube.
?
Hint:? Watch the tube upon applying HV.? You might see a quick bluish flash before the fuse goes.
?
-Chuck K1KW
?
From: [email protected] [mailto:[email protected]] On Behalf Of Bob
Sent: Friday, September 08, 2023 10:21 AM
To: Ham Amplifiers Ham Amplifiers
Subject: [ham-amplifiers] 572B
?
Hi all,
?
I'll condense this, you'lll get the idea.
?
A friend has an 811H with 3 year old 572B's. One shorted?and causes the fuse to blow immediately. Tube replaced and all is well.
?
Measuring with a DVOM shows no shorts at all. I do know some tubes will short after they get warm/hot but this one blows the fuse immediately (no time to get hot) but does not measure any short circuit.
?
I've now talked to two other hams on the air that say the exact same thing happened to them.
?
What causes this??
?
73, Bob? W4JFA

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Re: 572B