[HomeBrew] An RF Pre-amp design question

[Chris Trask]

    You could easily turn that into a tube preamp as follows:

    1.    Remove the feedback path from the oscillator cathode to
          the coil tap(s).

    2.    The coil tap(s) then become the RF input by way of the
          switch wiper.

    3.    Connect a 0.01uF capacitor from the oscillator cathode
          to ground.

    4.    Use the second half of the 12AT7 as a grounded-grid
          follower.  This makes a cascode amplifier, which has
          very good reverse isolation and which is very stable.

The 6BQ7 (391 ohms ENR) is only slightly quieter than the 12AT7 (417 ohms
ENR), but it is much more linear.  Better yet is the 6BK7, which is slightly
less linear than the 6BQ7 but far quieter (261 ohms ENR), and it has the
same pin-out.  The 6ES8 has even better noise (200 ohms ENR) but is not very
linear.

Chris

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----- Original Message -----
From: "Philip Atchley" <beaconeer at mercednet.com>
To: <homebrew at mailman.qth.net>
Sent: Tuesday, December 07, 2004 1:55 PM
Subject: [HomeBrew] An RF Pre-amp design question


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>
> 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
>
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