Thank you Marv. You da man!
That answers a lot. I'll ask you just a few more, please, and then let you
be... :-)
I've got a few of 4-400A's, sockets and chimneys here. I was hoping to
substitute one for the 4CX-350 driver. I think the IMD figures are not as
good, but running it in deep class A with just voltage needed to drive the
4CX-3000 might work OK? -62V on the grid, just like the 4CX-350, appears to
idle a 4-400A into class A too.
I will look at the OPA695 RF op-amp you suggested. A friend also suggested
I could take the 10 mw out of the FT-1000D and build a Norton push pull
amplifier with some 2N3375's he has. A pair would do about a watt class A.
Maybe that would be a good driver for the 6146, since only 100Mw is needed.
[keeping with the light duty rule]
I'm glad you supplied the specs for bias, drive, etc. I can see the
advantage of running everything light in the drivers. Then let the final
make the IMD and try to control it best with NFB. I would imagine that a NFB
cathode resistor in each early voltage stage would help if no loop is used
back there.
So, bottom line is I plan to have one watt, class A, at -55db 3rd order,
available to drive the 6146, which drives the 4-400, then the 4CX-3000. I
would like to go for more like 20-25db NFB. [Trying for -50 to -55db 3rd IMD
total] Do you think a second 6146 would be required to generate enough gain
to do this, or am I just asking for trouble with stage stability from too
much gain and NFB phase problems?
73,
Tom, K1JJ
--------------------
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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