You're right Jim about the former invention and clever marketing promotion
of the so-called "4CX800" tetrode tube by a bunch of entrepreneurs!
Nevertheless some commercial amplifier vendors notably Acom, Emtron, Alpha
have been able to design very capable and well regarded grid-driven AB1
linear amplifiers around the characteristics of the GU74B tetrode tube, as
have savvy ham homebrewers. The Acom family of amps from Bulgaria in
particular have been tremendously successful in the global market with their
Svetlana 4CX800A / GU74B based linear amplifier product range. By clever
design and RF NFB they achieve a good -40 dB IM3.
Same with Slovakia based OM Power who produce an excellent range of QRO HF
linear amplifiers based on the larger GU84B tetrode and / or the Chinese
Of course any of these amps when married up with a modern SDR exciter
running Pure Signal IMD cancellation feedback produce truly state-of-the-art
One must not be too hasty in casting unwarranted aspersions against the
venerable GU74B and GU84B tubes!
If one can't extract excellent performance from these tubes then it's more
of a reflection on their RF amplifier design skills than the tubes per
In the Tx tetrode tube space today there are very few choices to be had.....
Date: Mon, 1 Apr 2019 13:34:48 -0600
From: Steve London <firstname.lastname@example.org>
To: email@example.com, firstname.lastname@example.org
Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800
< So, Carl, where would you suggest us cheap, constantly crying, hams get
< at least new-old-stock Eimac 4CX800's and 3CX800's ? Lotsa luck.
< Steve, N2IC
## Eimac NEVER produced a ... 4CX-800 !! Its not in any of their
catalogs, never was, still isnt.
The tube in question is a SURPLUS Russian tube called a ... GU-74B. US
hams are not gonna buy an amp with a ...GU-74B in it. So they re-labeled
the tube, and called it a ... 4CX-800..... and also came up with bs new
## Of course hams thought that the...Russian 4CX-800 was a clone of an
Eimac tube. In actuality its a surplus russian tube..with 600 w of anode
diss, and lousy imd. To get half decent imd from those tubes, they have
increased the idle current through the roof. 225 ma PER tube is crazy.
Thats 450 ma for a 2 holer..and 675 ma for the 3 holer.
## Emtrons .... fix for the sky high idle current was to use
EBS..electronic bias...which sorta works between words, but not between
syllables. Emtron has since gone tu.
## 3CX-800A7s are used in single tube and also 2 holer configs in FM
broadcast. Eimac came out with a warning for FM broadcasters to use the
3CPX-800A7 instead. The 3CPX-800A7 is the pulse rated version, and will
handle a lot higher B+...like 3600 vdc. The issue was, in some foreign
countries, the incoming line voltage was all over the map..that plus
spikes, transients, surges etc, would flash over between anode to grnded
grid. The pulse rated version solves that issue, however, the grid
current is typ 10 ma higher, non issue.
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