[Amps] The ongoing 4CX250B verbage!

Ian White GM3SEK gm3sek at ifwtech.co.uk
Tue Apr 4 08:58:24 EDT 2006

Steve Thompson wrote:
>Jerry Muller wrote:
>> Although you couldn't tell the difference between a 4CX250 and an 8877,
>> anyone operating near you in frequency could. The triodes are MUCH cleaner
>> in terms of IMD than the tetrode. The 3rd order IMD performance of the
>> 4CX250 is not much better than -20dB where the 8877 is about -38dB.

>Crikey! The pair I ran on 2m came close to the figures STC give in their
>specs. - -30dBc 3rd order and -50dBc 5th order, at 550-600W pep from the

That seems quite reasonable, because the tubes are only being driven to 
275-300W output per tube.

Turning to 4CX250Rs, GW4FRX measured 3rd-order IMD of -30dB below either 
tone  for a pair of 250Rs driven to 250W output per tube (though this 
requires good screen voltage stabilization - more details at:
http://www.ifwtech.co.uk/g3sek/cleansig/imd1.htm )

By changing to the common US convention of "dB below PEP", that figure 
instantly "improves" to -36dB  :-)

But then we have the Eimac data sheet for the 4CX250R, which includes a 
set of operating conditions to squeeze 400W PEP out of a single tube. 
Not surprisingly, the 3rd-order IMD then increases to only -23dB 
"referred to signal level" (which I take to mean "below PEP"; if so, it 
equates to only 17dB below either tone). In other words, Eimac were 
recommending operating conditions that produce totally unacceptable 
levels of IMD.

This data sheet has had several bad consequences:

1. It led many users to be too greedy for output from 4CX250Rs.

2. It gave some users to claim that -23dB 3rd-order is OK "because Eimac 
recommend it".

3. It gave the entire 4CX250 range a bad reputation for "-20dB IMD".

73 from Ian GM3SEK

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