[Amps] QRO 2500DX MARK lll 4CX800

[Leigh Turner]

My commiserations over having all three tubes fail like this Bob.

Other than failed or ineffective protection circuits that might have caused
grossly excessive input over-drive to the grids, or an absence of Plate HT
causing a consequential large rise in screen dissipation (and associated
electrode warping and shorting), this scenario is most surprising.

I would very carefully check the efficacy of these important tube protection
mechanisms as the 4CX800A / GU74B tube is a notably robust and rugged one!

Also check the continuity of the 50 Ohm input swamping resistor R607.

Leigh
VK5KLT

-----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

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