Hi Tom,
In answer to a couple notes:
20 feet of coax should pose no problem as long as both ends are 50 ohms.
There may be some modern devices that perform better than those ancient TRW
things I suggested. I'm not really current with those parts. Texas
Instruments has some great Op-Amps which, at around 100mW are not quite as
powerful as the old hybrids, sport really fine distortion numbers at 4Mhz.
Take a look at the OPA695.
I don't know much about the insides of the FT-1K. A look with a spectrum
analyzer would be better than my speculation. :-)
Running the tubes "lighter" improves the numbers at a faster rate than
negative feedback. NF nets about a dB distortion reduction for each dB of NF.
Reducing the stage power 3dB reduces the 3rd order products (theoretically) 9dB
-- the cube! The higher order products diminish at progressively higher rates.
Since you running at a relatively low frequency, you might be able to
implement a feedback loop around three stages. The phase shift is the limiting
factor.
To answer the questions about the 10KW Hughes amp:
That drawing is from more of a marketing than engineering document. It
does contain a few errors & some (obvious) omissions. I added the ground
symbol to the final autotuning sense ckt. myself before scanning.
Some numbers:
Grid bias:
4CX250R: -62V
4CX1000D: -340V (adj for 500mA resting plate current)
A resistive feed to the bottom of the grid tuning coils, as Karl-Arne
noted, is valid to feed the bias.
Signal levels:
Input: 100mW / 2.25V RMS
4CX250R grid: 22V RMS (this illustrates how light the 6146 is being
run!)
4CX1000D grid: 210V RMS (this illustrates how light the 4CX250R is
being run!)
Power Out:
10KW avg. 14 KW peak
better than -40db IMD @ peak power
Negative feedback: 12dB
A resistive bias feed to the bottom of the grid tuning coils, as Karl-Arne
noted, is valid to feed the bias.
73 & Good morning,
Marv WC6W
P.S. -- Steve T. posited in another post that there was little intrinsic
difference between a vanilla 4CX350A and the 4CX350FJ. The data sheet claims
10dB lower distortion at equal power. That's a lot. I wonder if Eimac uses a
striped cathode aligned grid scheme, as in the 4CX600J & 4CX1500B, in the
'350FJ?
*
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