[HBR] dual triode mixer

[donald haworth]

Thanks Walt.  As I understand it-both cathodes go to ground via a 470 ohm
resistor, and the plates are tied together and go to B+ via the tank ckt.
Unlike the Pullen which has different plate voltage on the two plates.  Is
that correct?
Thanks Don

On Mon, Nov 14, 2011 at 11:16 AM, Walt Hutchens <waltah at earthlink.net>wrote:

> 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
>
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