Rich Measures wrote:
>I run a pair of 4cx250Rs at the max. ratings on 2m SSB. The anode supply
>is around 2400v, and the screen is regulated by a 400V zener string.
>With 10w of drive the gain is 20db. There is zero grid current.
>- How do you run 4CX250Rs, Ian?
Not the way that EIMAC recommend, with 400V on the screen and -80V on
the grid. The EIMAC recommendations have earned 4CX250Rs a poor
reputation for IMD - basically because they are too greedy for power
output. The datasheet figure claims 940W from two tubes, but with only
-23dB 3rd order IMD and -27dB 5th order.
This is not acceptable to other weak-signal DXers on a crowded band.
EIMAC's recommendations would probably get you disqualified from any VHF
contest in Europe.
For much better IMD, try 360V on the screen (well stabilized) and grid
bias to give ZSAC of 100mA per tube. This will be about -50V grid bias.
Adjust drive to give zero grid current (or maybe a touch negative), and
load to give a screen current of +2mA per tube. At this loading, with
2000V on top, the anode current will be about 450mA (two tubes) and the
RF output should be about 500W. With your 2400V you should see more
power with similarly good IMD.
Obviously the power output is much lower than you get at full blast, but
that's the maximum achievable output with good linearity.
These conditions were worked out by GW4FRX using a spectrum analyser, to
determine the maximum linear output with good IMDs. They happen to suit
the UK 400W PEP output limit very well, too.
Sometime when FRX and I can get all the equipment together, we plan to
repeat the IMD measurements with a range of operating conditions,
including EIMAC's recommendations.
I hope that explains why I don't always agree with EIMAC :-)
> I have
>had no tube failures.
You wouldn't expect failures if you follow EIMAC's recommended operating
conditions. Nobody knows as much as the tube manufacturer about
failures, and I certainly wouldn't argue with EIMAC about that.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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