[HBR] dual triode mixer

[Walt Hutchens]

Donald asked:

> On the dual triode mixer, as I understand it, you have the cathodes tied
> togeter and the plates tied together.  The LO is fed on grid one and the
> signal comes in on grid two.  What cathode biasing is used(if any)?  Do you
> take the IF output from the (two) plates via a small cap-50pf-or so? What is
> the load circuit for the plate?

You have the grids right -- one each to signal and LO.   I generally use
grid resistors of 100k or so but that's not critical.

The cathode goes to ground through 470 ohms or so;  you can experiment with
other values but this has given about the maximum conversion gain with the
tubes I've used -- 12AT7 and 6J6.   The tube must be biased more than for
use as an amplifier.

The plate goes to the usual plate tank circuit -- IF transformer or
whatever.  High Q is good since the nearer to a short circuit it looks like
to other frequencies the better the rejection of the (unmixed) input
signals.  

This is NOT a fussy circuit;   hook it up and it will work.   It isn't very
high gain but in many applications that's not what you want anyway and it
does deal well with strong signals.

(In fact, in the usual HF receiver design you really want to put as much as
possible of the gain in the IF section, AFTER the sharp filter.   This is an
advantage of putting a crystal filter in the mixer plate: You can put most
of the gain in the two IFs.)

Just as with other mixer circuits, this one can be used as a product
detector.   I've never seen a handbook circuit for that but use a 12AX7 with
a 3000 ohm cathode resistor and a plate circuit as follows:  From B+ 10k to
a filter cap of 47 mfd or more, thence to 100k: take audio here via .001 to
.005 mfd to your volume control and bypass this point with 100 mmf to 470
mmf to reduce RF getting into your audio, thence 10k from there to the plate
which is bypassed with another 100 mmf to 470 mmf.

With very low plate voltages you might want to use an RF choke instead of
the 10k resistor at the plate end but I've had good results at 140V or so
with the resistor. 

This circuit needs VERY pure DC to avoid hum -- that's the reason for the
extra filtering.

If you disable the BFO this circuit acts as a plate detector to deliver
audio from an AM signal.   The volume will be considerably less than from
the same signal with the BFO on, however that can be made up in the audio
section, either by manually increasing the volume control setting or by
changing the gain according to whether the BFO is on or off.

(In the HR-10 to HBR project I'm including the gain change in the BFO
switch.)   

An added advantage to this circuit is that you can pick off the LO or BFO
signal at the cathode (of the mixer) and it is well buffered at that point
-- ideal for connecting to a frequency counter or meter.   It may be
necessary to disable the signal input (unplug the antenna from the
receiver!) to avoid contaminating the count.   I'm now sticking a cathode
test point on the chassis to simplify tracking and drift testing.

Yet another advantage is that the BFO is isolated from the signal path,
making it simpler to to keep it out of the AGC.   However other product
detector circuits also give you this.

> --I was thinking of using a 12U7....not a 12AU7,

Ah yes.   I've never tried to use those 12VDC plate voltage tubes -- the
last gasp for vacuum tube auto radios -- so the number was a 'never seen
that.'   Thanks!

Walt
KJ4KV