[ARC5] More on the Pullen Mixer circuit

[Kenneth G. Gordon]

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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