[HBR] Pullen mixer

Fri Nov 25 03:07:03 EST 2011

Paul
The tube I am using is a 6gm8 dual triode which was designed to run with 30
on the plate(s).  IIRC, the transconductance is around 7800 @ 30 volts.  My
radio is based on the Collins r392, so my B+ runs at 25-26 volts.  Same for
the heaters.
As this was somewhat uncharted ground (at least for me), I was not sure
what The 6gm8 plates should run at.  The 6es8 calls for 150 on the 'signal'
triode and 50 volts on the "LO" triode.  I tried to keep the same 3:1 ratio
on the 'gm8.  I ended up with a 50k pot-wiper to the second plate.  Top of
pot to 25volt B+.  Bottom of pot to ground.  The 'signal' plate B+ was fed
via a 1.8k resistor, 20 volts on that plate, and the plate is at RF ground,
through a .01 cap.
The grid of the 'signal' triode has a 120k resistor to ground.  The 'LO'
triode grid has a 120k resistor to the CATHODES-not to ground-.  The
cathodes are tied together and go to ground via a 300 0hm resistor.
I forgot to mention-the output is taken from an IF transformer.  The
primary is between the plate (LO triode) and the B+ supplied from the wiper
of the 50k pot.  Secondary to IF stage. There is a .01 cap to ground on the
B+ side of the IF transformer.

With an antenna connected to the grid of the 'signal' triode (via a .01
cap), and a signal from a signal generator set to 5455 kc--assuming your
using a 455kc IF-tune the pot until you hear WWV!
As I mentioned, I am using a tube designed for low voltage B+.  I see no
reason why the pot voltage 'tuning' would not work with other tubes.  I did
find that .5 volt made all the difference in the world, so the pot was left
in the circuit.  The adjustment is rather critical-at least with the
6gm8-but once you find the 'sweet spot' it is very stable, however..........
TRY IT!!  I have yet to feed it with a tuned rf stage--RF stages can hide a
bunch of mixer problems--so if it works well-and it does! w /o any
tuning....it should really shine coupled with a good front end.
--let me know how yours works, and if I can answer any other questions.
Hope the above makes some sense....am full of turkey!
73's
de wb0rai Don

On Fri, Nov 25, 2011 at 12:33 AM, Paul Dulaff <pdulaff at embarqmail.com>wrote:

> 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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