[HomeBrew] An RF Pre-amp design question

[Philip Atchley]

Hi,

I have a Superior Instruments (??) Genometer (signal generator) model 
TV-50A that I've repaired and which works, but in reality is a rather 
poor test instrument for general radio work.  Not overly stable and 
the tuning knob being directly on a variable capacitor shaft with no 
vernier makes it nearly impossible to set "accurately" enough. 
Probably ok at IF and BCB frequencies, but that's about all.

HOWEVER!  It DOES have a NICE RF deck containing a bandswitch and 
tunes 100KC to 60MC in 5 ranges (the 45-180MC range is really a third 
harmonic of range 5).  The coils each have a tap (for Hartley Osc) 
with the cold end grounded directly and the bandswitch selecting both 
coil and tap (naturally).  It uses a 12AT7 Oscillator tube.

I've been wanting to build a NICE tunable RF preamp for the shortwave 
range (I have already built one for 17-530 KC) but have been putting 
it off due to not having all the parts and cabinet on hand (and a 
tight budget).

THIS THING IS CRYING TO BECOME AN RF PRE-AMP 8^) Here are some 
thoughts I've had on the matter. In all cases the RF deck, tuning 
capacitor and cabinet would basically be used intact, removing the 
oscillator circuit and subbing a RF amplifier.

1.  First thought was a TUBE Pre-amp, perhaps a cascode one using 
something like a 6BQ7 tube that would fit in the present tube socket 
(but which I'd have to BUY).

2.  I've given up that idea and am considering a J310 High Impedance 
FET input stage followed by either a bipolar transistor emitter 
follower OR J-310 FET input stage followed by a MAR-6 mmic (of which 
I've been given a few).  In the latter case the FET would probably be 
wired as a "Source Follower" and provide the impedance matching to the 
relatively low input Z of the MAR (which has about 20dB gain and 3dB 
noise figure).

3.  The present dial calibration should remain sufficiently accurate 
(especially as the coils are adjustable) and the large cabinet with 
it's "bonus" holes would be useful for providing things like a 
receiver select switch, Attenuators, 100KC Xtal calibrator etc. With 
LOTS of room top spare I shouldn't have any problem providing 
sufficient internal shielding for stability etc.

An additional advantage of the solid state version would be 12 Volt 
operation off the station battery bank instead of 110VAC.

What are the "thoughts" of folks out there concerning this "project"? 
I'm sure it'll work electrically, but maybe some folks have better 
circuit suggestions.

73 de Phil,  KO6BB
http://users.mercednet.com/beaconeer/
Merced, Central California, 37.3N  120.48W  CM97sh