[HBR] Pullen mixer

Fri Nov 25 20:26:08 EST 2011

Paul, Yes the 'gm8 is north of 20 bucks...might just as well be putting a
down-payment on a 7360 beam deflection tube.  I must have miss-read the
Gm--I thought the transconductance was 2500 with 6.3v on the
plate....increasing to about 8k or so at the max plate voltage (30
volts)...will have to go back and look again.
--The 6es8 is an interesting tube in its' own right, and I think I'll give
one a try-at 30 volts- Another tube to try is the 6gk5--two of them-- I
made a regen with one once that was hotter than anything I've ever come
across-and that with plate voltage of 18 volts....two nine volt
batteries...... I think (two) 6cw4's might be interesting.....
At any rate, build it with the pot so you can adjust for best operation.
It makes a HUGH difference between a good mixer and an outstanding one.
-The 6es8 was used by Eddystone in one of their receivers, as a cascode RF
amp in the front end.  It was also used as the front end in a lot of TV
sets.  Very good tube,  Frame Grid tubes are a cut above, no doubt about it.
best 73
Don

On Fri, Nov 25, 2011 at 4:29 PM, Paul Dulaff <pdulaff at embarqmail.com> wrote:

> Don
>
> Interesting stuff !
>
> I was checking on pricing for the 6GM8. The audio folks have taken this
> tube since they can build tube preamps using low voltage and not hurt
> themselves. These folks are also seeing good signal to noise with these
> tubes.
>
> Bad news is the audio folks have driven the price of these tubes very high.
>
> The data I have so far has the 6GM8 at 2500 micromho's for
> transconductance. The 6ES8/6DJ8 has 12,500 micromho's. But, I think the key
> to low tube noise is low plate resistance.
>
> I plan to get back into my HBR and my 75A-4 using the Pullen Mixer, but
> haven't gotten the time just yet.
>
> I see that the 6ES6 is the low voltage pentode used for RF/If amplifier in
> the same car radio applications. May be a good mate for the RF amp to your
> Pullen Mixer.
>
> Best 73's
>
> Paul - WB2NMI
>
>
>
> ----- Original Message -----
> From: donald haworth <donmhaworth at gmail.com>
> To: HBR Receiver List <hbr at mailman.qth.net>
> Sent: Fri, 25 Nov 2011 03:07:03 -0500 (EST)
> Subject: Re: [HBR] Pullen mixer
>
> 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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