[ARC5] More on the Pullen Mixer circuit

[Ian Wilson]

Nice collection of the pertinent data, Ken.

Just for completeness, note that (2) is only correct if you are using two
triodes of the same type. In this case, it is necessary to set the operating
conditions so that the triodes have different gm. The Pullen article
describes
this in some detail.

If you are using triodes with different gm, then the plate voltages are
whatever
the device characteristics say.

I have had good luck with using a Pullen as a product detector, with one
low-gm triode and one high-gm triode from a triple-triode Compactron. I
forget
which tube that is (probably 6U10), but I seem to recall that the triodes
are similar
to (one half of) a 12AU7 and (one half of) a 12AX7. The low plate current
of the
high-mu/low-gm triode lets you use a very large plate resistor, which gets
you
lots of conversion (voltage) gain.

73, ian K3IMW


On Tue, Jan 3, 2017 at 1:08 PM, Kenneth G. Gordon <kgordon2006 at frontier.com>
wrote:

> Those contemplating using the Pullen Mixer circuit in any receiver MUST be
> aware of some
> issues which are not adequately covered in any article I have re-published
> on my website or
> have seen elsewhere.
>
> These are:
>
> 1) The COMPLETE circuit requires TWO DUAL-TRIODES, not one as shown in most
> articles on this mixer. One is used as the cathode-coupled mixer itself
> and the second one
> is used as the HFO followed by a cathode-follower which then couples the
> HFO to the
> mixer-proper. This use of the second dual-triode eliminates or at least
> significantly reduces
> pulling. The cathode-follower following the HFO is key.
>
> 2) The key to the operation of this circuit is the reduced plate voltage
> on the mixer-half of
> the dual-triode. This MUST be done, and MUST be in the correct proportion
> when compared
> to the plate voltage of the input cathode-follower stage.
>
> 3) The Pullen Mixer circuit MUST have an RF amplifier ahead of it. It will
> NOT work properly
> without one, i.e. it should never be used in a "mixer input" type of
> receiving system where
> the antenna input is directly connected to the input cathode-follower
> stage.
>
> I hope this helps.
>
> Reading Keats' original article on this mixer is essentially a necessity
> to really understand it.
> Here is a link to that article.
>
> http://www.w7ekb.com/glowbugs/rx/Pullen/PullenMixer.pdf
>
> I fully intend to build direct plug-in octal-based replacements for the
> 12K8 using a pair of
> 7963 subminature dual-triodes, transconductance 13K, and then will test
> these thoroughly. I
> expect good results.
>
> Ken W7EKB
>
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