[HBR] Mixers

[donald haworth]

Thanks for all the info....about the only mixer I have not tried is the
push-push dual triode.  I built a one tube superhet using a 6me8 in a
double reflex circuit originated by Bob Weaver.  The results were
amazing...200+ stations logged --the beam deflection tube(s) really are
something!  Bob's radio can be seen on his web site.  The 6me8 was used in
a self-oscillating configuration.  My version was rock-stable-less than 500
cycles of drift from turn on----and this using a toroidal inductor in the
tank....and with rather high voltages 50+ for the deflector plates......on
an FT 140-61 core....never did try it on anything other than the BCB...
I mention the above because that was my first superhet-tons of regens in
the past 50 odd years-but that was my first foray into the world of
superhets.  And that, of course led to The HBR....
My LONG SLOW project is based on the Collins r392.  It is now in it's 3rd
iteration--the final one-I hope!
In the first and second versions I used a conventional heptode converter-no
problems, so I will probably use it in the final version......but Walt's
dual triode looks like something I would like to try.
-----maybe a 12u7 ?  I think Walt said plates and cathodes were in parallel
with LO to one grid and signal to the other....
---I am using 26 volt tubes (R392), so I would have only 26 volts on the
plates.
--So far the greatest challenge has been the coils.  All my coils are
toroidal.  I am able to wind them to within +/- 1 uh.  'Pruning' the coils
is done by spreading/compressing the windings and measuring with an LCR
bridge. The big problem has been designing a way to make the toroids
'pluggable',  so far that has taken more time than the rest of the radio!
--Just keep repeating....its only a hobby.....only a hobby.......only a
hobby....................
Don (wb0rai) Nashville

On Sat, Nov 12, 2011 at 7:47 PM, Walt Hutchens <waltah at earthlink.net> wrote:

> Donald asked:
>
> > What is your favorite mixer, and why?
>
> Anything that is non-linear is a mixer if you put in two inputs.   There
> are
> a VAST number of such circuits and each has advantages.   Picking the best
> one depends on your application.
>
> Low noise mixers have a big payoff at 10M and up, perhaps sometimes as low
> as 20M.  The Pullen circuit would be an example.  Below 20M, atmospheric
> noise will overwhelm tube noise.  The basic paper describing the circuit
> can
> be found with a Google for 'pullen triode mixer.'
>
> High gain mixers can be very useful in simple designs.   Pentodes are a
> common example and the W6TC designs all use them, as does the HR-10 and
> many
> other (mostly lower end) ham receivers.  They are, however noisier than
> triodes and easier to overload.
>
> Balanced (and 'somewhat balanced') mixers have the virtue that some inputs
> and certain mixing products are cancelled at the output.   These shine in
> receivers for strong signal or very crowded band conditions, also for some
> transmitter designs where spurious outputs are an issue.
>
> Favorites?   In simple/low cost receivers, any of the common pentode
> circuits.   Pentodes usually have both signals applied on the same grid so
> oscillator isolation is an issue -- it's a good idea to use a buffer stage
> or an ECO.
>
> Even the lowly pentagrid coverters -- 6SA7, 6BE6, 6BA7 -- have a place for
> extreme simplicity when desired signals are reasonably strong and some
> performance compromise is tolerable.
>
> Triode mixers can be configured for low noise -- the Pullen configuration
> --
> or good signal handling ability.   They also work at higher frequencies
> than
> multigrid tubes.
>
> In the usual HF ham receiver I prefer a push-push dual triode mixer -- say
> a
> 12AT7, 6J6. or 6DJ8 with the plates and cathodes in parallel, the received
> signal on one grid, and the LO on the other.  This is 'somewhat doubly
> balanced' -- that is, both input signals tend to cancel in the output --
> and
> it handles large signals well.  This is a very simple circuit -- always an
> advantage.  Mixing gain is relatively low and for sensitivity/low noise
> operation it needs an appropriate RF stage.
>
> The beam deflection mixers -- 6JH8, 6ME8, 6AR8, and 7360 -- combine
> sensitivity with singly balanced operation with respect to the signal on
> the
> control grid AND good large signal handling capability.   Output is
> push-pull -- a slight complication -- and a relatively high signal level on
> the deflection plates (usually the LO signal) is needed to get good
> sensitivity.   Some of these tubes connect a signal electrode to one end of
> the filament so they can only be used in a configuration where that pin can
> be grounded.
>
> For 'the most ham receiver with the fewest tubes' I'd start with a beam
> mixer and no RF stage.   A 12AT7 as a push-pull LO.  You might even get by
> with a single IF stage by using one of the frame grid pentodes such as the
> 6KT6.  I believe you can use AGC on the beam tube control grid so that
> would
> give you two controlled stages.
>
> Solid state diode mixers have a place: They've been used in microwave work
> since WW II because vacuum tubes fade away rapidly above 1000 Mcs or so.
> The modern solid state mixer circuits are certainly excellent and easy to
> use.
>
> The choice comes down to what best fits what you're trying to do -- and of
> course, what interests you.
>
> Walt
> KJ4KV
>
>
>
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