[HBR] Pullen mixer

[donald haworth]

Some time ago I posted some questions about a dual triode mixer, as
described by Walt.  A very simple circuit, indeed, with the plates tied
together, a common cathode to ground via a 470 ohm resistor, and the signal
and LO fed on the grids.
-Long story, short--I built it and it worked, as  advertised.  Noise was
about same as the pentagrid convertor, however, and gain was much
lower-that was no surprise-but I did expect the noise to be a bit better..
I must add that I am running only 26 volts on the plate.(26d6).  The twin
triode I was using is a 6gm8.  I then tried it in a Pullen configuration.
One plate with 20 volts and second plate (with LO to grid)  2.2 volts--this
via a 50k pot so I could adjust the voltage.....WOW!!  What a difference!
All kinds on gain, NO noise---sounds like the antenna has been disconnected
-then you tune across a signal and it pops out.....!  Really impressive.
And the best part----no spurs --no birdies---I am using a N3ZI DDS for the
LO.  Injection is 250mv.  Antenna running into the other grid-no
tuning-nothing-and it blows away every mixer circuit I have tried, to
date.  I have found the best way to test a mixer circuit is to input a
signal to the (signal) grid--no rf amp-no tuning...this will show up a bad
/ good design in a hurry.  A good RF front end will cover a multitude of
mixers problems, so eliminating it will give you a good idea of how well
the mixer preforms.....
--I am now a believer.  All the things they say about the Pullen are
true---and all it took was three resistors added to Walt's dual triode
circuit!  I must say that the Plate voltage ratio is critical--it will make
the difference between no signal and Grins and Giggles-but a pot in a
voltage divider circuit to feed the second grid, and just tune for the best
output......for the 6gm8, I have 20volts on plate 1 and only 2.2 on plate
2............
----Give it a try!
---and a Happy Thanksgiving to all
Don (wb0rai) in Nashville