In message <19980517.172852.6511.3.km1h@juno.com>, km1h@juno.com writes
>This is primarily directed to Ian but I would appreciate comments from
>anyone with experience.
>
>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.
>
>Now comes the real question. Am I wasting my time with a tetrode? Would I
>be better off just converting an SB-230 with the 8873 and wind up with
>-40dB or better IMD at those drive/output levels? Eimac specs indicate
>that I can but is this realistic?
>I have several 8873/8874/8875's here and can also homebrew around any one
>of them.
>Again the #1 goal is the best IMD that I can obtain.
>
True class A will always deliver the best linearity, but you get low
output power.
With bipolar transistors you typically get about 25% of dc dissipation
as output power for -40dBc IMD on two tone. It's normal to run with dc
dissipation of about 50% of the max. power dissipation rating. I guess
that similar rules of thumb might apply to vlaves, although you can
probably push the actual power dissipation closer to the max. limit
safely.
Personally I'd try the 8875 - single supply and no messing around with
blower pressure.
Steve
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