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Re: [Amps] QRO 2500DX MARK lll 4CX800

Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800
From: Steve London <>
Date: Sun, 31 Mar 2019 09:31:13 -0600
List-post: <>
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 protection.

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 ?

Steve, N2IC

-----Original Message-----
From: Amps [] 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..

Bob W5RG

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