[HBR] Partially balanced mixer

[Walt Hutchens]

Ian Wilson posted:

> This is a slight modification of a circuit in the Collins SSB book - a 12AT7
> differential amplifier: cathodes connected together with a 470 ohm tail
> resistor; plates connected together through a parallel LC tank to +250V; grids
> each have 100k to ground. You get some suppression of the inputs because each
> tube sees more or less the negative of what the other tube sees. The
> suppression isn't great because the tail resistor isn't very large compared
> with 1/gm of the tubes.
> 
> Results using a 6J6 with 2.7k tail resistor are quite good.

This is the circuit I use in most all mixer situations.   It's not widely
used or appreciated -- I think because it doesn't have the gain of other
circuits -- but in the majority of HF applications it is a better match to
the system requirements than more conventional circuits.

To be repetitious: There are few applications for a low noise/high gain
mixer at HF.   In nearly all situations the abilities to handle large
signals well and partially suppress the two inputs are more useful.

(Most receiver designs will use an RF stage because (a) it isolates the two
tuned circuits, and (b) it allows for AGC ahead of the mixer.   Once you
have an RF stage the gain and noise figure of the mixer aren't very
important.   A low gain mixer may require that the total receiver gain
requirement be made up with either an additional IF stage, higher gain tubes
in the IF, or a detector having significant gain.   However this is past the
'knothole' -- the main part of the selectivity -- so only the wanted signal
is an issue.)     

The 6J6 is a fine tube in this (partially balanced mixer) circuit and with
the added virtue of being the only twin-triode in a 7-pin format --
sometimes helpful for retrofits.   You might also try the 6DJ8/6922: I have
never done a head-to-head but was impressed with the latter when I tried it
separately.    

I believe the value of the tail resistor is a trade off between gain (which
will be highest at around 470 ohms for the 6J6) and suppression of inputs
(improved for higher values).

Beam tube mixers offer a different combination of good traits:  High mixing
gain and very high rejection of the control grid signal at the output, and
very good linearity on the control grid path.   The price is a much more
complex circuit compared to the partially balanced circuit described above
and when you start simplifying, you give up much of the benefit.

Walt Hutchens
KJ4KV