I now know of two Chinese 3CX800 "low grid current" tubes that have recently
failed in the same way.
These, sold by RF Parts and Alpha Amps:
In the two cases, it was different amps, both with excessive grid current
In my case, everything was working fine, until the next time I turned on the
amp. Initially, there was no short. About 2 minutes later, the short occurred.
Completely repeatable by letting the tube cool down.
For Alpha 76's, what other tube substitutions are reasonably possible, besides
8874's and 3CX800's ?
From: Amps [mailto:firstname.lastname@example.org] On Behalf Of Bob Gibson via
Sent: Saturday, 30 March 2019 7:08 AM
Subject: [Amps] QRO 2500DX MARK lll 4CX800
After sending all three tubes down to be checked at D&C Electronics all
three tubes came back bad..the test showed..that once the tube comes up to
operating temperature there is a resistive cathode to grid short present
possibly indicating a warped grid or screen.. My question is why would all
three come back the same..I knew one tube was bad but to have all tubes test
the same was a little odd..Anyone have any ideas..
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