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

To: "'Carl'" <>, "'Bob Gibson'" <>
Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800
From: "Leigh Turner" <>
Date: Tue, 2 Apr 2019 12:04:45 +1030
List-post: <>
Precisely Carl.  Agree with your comments about Eimac tubes being designed
and built to CCS specifications and their associated excellent longevity.

I am of the conservative school of belief that one takes a good hard look at
a transmitting tube's Data Sheet, or a linear amplifier employing any given
tube, then make a decision to de-rate the maximum specified operating
conditions / power output by circa 3 dB.

Doing so will preserve emission such that the damn tube will virtually last
forever in amateur service!  Especially if the tube's filament/heater has a
slow controlled soft-start mechanism to avert inrush current and thermal
shock at initial switch-on. In SSB linear amplifier service such power
output back-off significantly improves the linearity and achievable IMD

My venerable Kenwood TL-922 amp still has its original factory fitted 3-500Z
tubes....and still makes specified PEP!  

Now anybody perusing my pages will notice I also happen to be an
aficionado of the often and unfairly disparaged GU84B is an
admirable performer that also exhibits great longevity when properly used!

It is misguided folly to push any transmitting tube too hard for like you
say no perceivable benefit at the Rx end.



-----Original Message-----
From: Carl [] 
Sent: Tuesday, 2 April 2019 1:38 AM
To: Leigh Turner; 'Bob Gibson'
Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800

Since your Australian power limit is 400W, and I would "assume" you always 
obey that, then emission life willl be extended. I am still running the 
original 1986 Eimac 3-500Z's in my LK-500ZC at 1200W as an example of REAL 
longevity at hard use in contesting and on AM. The RCA 8122's in my 1964 
NCL-2000 prototype I converted to 6M never failed but very gradually lost 
emission and I replaced them at the 700W point about 10 years ago.

The original Eimac 8877 in my early 80's Dentron DTR-2000L still produces 
1200W out at about 70W drive. This amp dates back to when 1000W CW and 2000W
SSB PEP INPUT were FCC rules so the HV is very conservative.

Other amps in use here use Eimac 8873, 8874, 8875's with the same long life 

I DO NOT like having to replace tubes by running them hard for no measurable

improvement at the receiving end.

OTOH the discussion is about a USA amp which is where the 4CX800A 
nomenclature (make believe and never registered) was intended to lull owners

into believing they could be pushed as could Eimac and other quality brands.

We already see where the GU-84B/4CX1600B amps wound up in the trash heap (or

rebuilt with alternatives) and caused manufactures grief and financial 
losses. This renaming took place by the same fraudlent Svetlana USA  group 
and at the same time.


----- Original Message ----- 
From: "Leigh Turner" <>
To: "'Carl'" <>; "'Bob Gibson'" <>
Cc: <>
Sent: Sunday, March 31, 2019 8:06 PM
Subject: RE: [Amps] QRO 2500DX MARK lll 4CX800

> 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
> these tubes demonstrably have enduring emission and longevity
> when operated within their ratings.
> Leigh
> -----Original Message-----
> From: Carl []
> Sent: Monday, 1 April 2019 2:46 AM
> To: Leigh Turner; 'Bob Gibson'
> Cc:
> 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" <>
> To: "'Bob Gibson'" <>
> Cc: <>
> 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
> -----Original Message-----
> From: Amps [] On Behalf Of Bob Gibson 
> via
> Amps
> Sent: Saturday, 30 March 2019 7:08 AM
> To:
> 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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