[HBR] Pullen mixer

[Paul Dulaff]

Don

Could you post details on the mixer circuit such as the plate load resistor values, cathode resistor value, etc.

Some literature states that the 6ES8 and the 6DJ8 can be substituted.

Best 73's

Paul - WB2NMI

----- Original Message -----
From: donald haworth <donmhaworth at gmail.com>
To: HBR Receiver List <hbr at mailman.qth.net>
Sent: Wed, 23 Nov 2011 20:06:44 -0500 (EST)
Subject: [HBR] Pullen mixer

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
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