[Amps] QRO 2500DX MARK lll 4CX800

[Steve London]

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:
https://www.rfparts.com/3cx800a7-tay-lgc.html

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 ?

73,
Steve, N2IC


> 
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Bob Gibson via
> Amps
> Sent: Saturday, 30 March 2019 7:08 AM
> To: amps at contesting.com
> 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
>