Message text written by INTERNET:km1h@juno.com
>I wish to build a 50MHz amp with either a 4CX250B or 4CX250R, I have both
new in the box. The questions are what are the ideal plate and screen
voltages and idling current in order to obtain the best IMD at 150-180W PEP
output? I have 15W of drive available but efficiency is not
important, I could run close to Class A if it would result in an extremely
clean signal. I am striving for 3rd order IMD of -35dB or better if that is
possible with these tubes. <
Somewhere in the region of 2000 on the anode (plate) and 350V on the screen
with 100mA of standing current should give you around -32dB third-order
with optimal loading. Better than that will require quite a lot more
standing current. In my experience there's little to choose between 250B
and 250R in terms of IMD.
I'm a little curious as to the reason for wanting good close-in IMD, since
an "extremely clean signal" implies good high-order IMD performance rather
than low-order -- the third-order products fall within the audio passband,
and also in the place where the suppressed sideband would otherwise be. For
really good high-order IMD performance (and indeed better performance than
Eimac says you can obtain) you need a good-quality screen supply which can
source and sink plenty of current whilst displaying good voltage stability.
You also need to take care to get the loading right.
I think Ian, G3SEK, has some material about these issues on his Web page.
73 John
GW4FRX
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