Yes, these grid and screen dissipation ratings / limits are indeed the
reason why efficacious protection mechanisms in the amplifier design that
respect and conservatively accommodate these dissipation limitations is
important.
Unfortunately one does not have many sourcing options for these
GU74B/4CX800A tetrode tubes nowadays and one has to work with the still
available tubes.
Out of interest my now 20+ year old single GU74B Emtron DX-1 amplifier still
has its original factory shipped tube in it and still meets full
specs.....so these tubes demonstrably have enduring emission and longevity
when operated within their ratings.
Leigh
VK5KLT
-----Original Message-----
From: Carl [mailto:km1h@jeremy.qozzy.com]
Sent: Monday, 1 April 2019 2:46 AM
To: Leigh Turner; 'Bob Gibson'
Cc: amps@contesting.com
Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800
No more robust than any other wimpy tube with a 2W G1 and a 15W G2 rating
and very easy to destroy.
Since the actual rated PD is 600W the later 800W rating is a pure fraud by
Svetlana USA that leads to hams pushing them too hard with a rapid decrease
in emission.
Unlike Eimac, Philips and similar manufactures who design tubes for CCS
service along with plenty of reserve emission to compensate for aging. that
tube and most amateur service Chinese are run at the edge. And cheap hams
cry constantly.
Carl
----- Original Message -----
From: "Leigh Turner" <invertech@frontierisp.net.au>
To: "'Bob Gibson'" <w5rg@yahoo.com>
Cc: <amps@contesting.com>
Sent: Sunday, March 31, 2019 2:08 AM
Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800
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@contesting.com] On Behalf Of Bob Gibson via
Amps
Sent: Saturday, 30 March 2019 7:08 AM
To: amps@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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