[Amps] QRO 2500DX MARK lll 4CX800

[Jim Thomson]

Date: Mon, 1 Apr 2019 13:34:48 -0600
From: Steve London <n2icarrl at gmail.com>
To: amps at contesting.com, km1h at jeremy.qozzy.com
Subject: Re: [Amps] QRO 2500DX MARK lll 4CX800

<So, Carl, where would you suggest us cheap, constantly crying, hams get new, or 
<at least new-old-stock Eimac 4CX800's and 3CX800's ? Lotsa luck.

<73,
<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 specs.

##  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. 

Jim   VE7RF