[HBR] Pullen mixer

Fri Nov 25 20:59:22 EST 2011

Walt
Good reading, as always.  I have always wanted  a SS-1r......the stuff
dreams are made of-to quote a famous movie line.....had both the R390 and
390a at one time, as well as a WJ HF1000.  Sold my HRO500 about 5 years
ago.....so now I have only the HQ180 to compare my homebuilt stuff to.
As to strong signal handling ability, the 6gm8 (Pullen) is able to hear a
signal on 630-with only minor noise from WSM on 650.  Same is true on the
other side of 650.  670 in Chicago comes in like a local with no shot noise
from WSM.  This with a long wire end fed at about 40 feet high, input on
directly on the grid.  WSM, with my tuned-loop antenna, measures 0.9 volts
p/p  with the scope hooked to the antenna-lots of signal!  So anything I
can hear  either side of 650, well, I know I am on the right track.....
I will try to get some 'real' numbers asap....
-Hope you had a good Thanksgiving
73 Don
On Fri, Nov 25, 2011 at 7:26 PM, Walt Hutchens <waltah at earthlink.net> wrote:

> Donald said:
>
> > RF stages can hide a bunch of mixer problems
>
> Mixer characteristics -- which may or may not be problems in a particular
> application -- include internal noise, linearity on the signal path, and
> gain.   An RF stage can make up for low gain (though usually that is better
> done in the IF section), and hide the mixer noise.
>
> An RF stage generally has three functions:
>
> 1.  Raising the signal level above mixer noise.
>
> 2. Isolating two signal frequency tuned circuits, thus simplifying their
> design; and,
>
> 3. Acting as a controlled attenuator for signals much stronger than needed
> to overcome mixer noise that would otherwise drive the mixer into the
> non-linear range.   Generally this attenuator function needs both automatic
> control (AGC) and manual (AGC threshold or RF/IF gain) control to cope with
> the various situations.
>
> Examples of ways this can play out are:
>
> 1.  The many 1940's designs with extremely noisy pentagrid and hexode
> mixers
> that were nonetheless tolerable sets because of a high gain RF stage;
>
> 2. The G2DAF receiver with a 6ES8 RF stage for isolation and AGC purposes
> operating at less than unity gain ahead of a very quiet 6DJ8 push-pull
> triode mixer; and,
>
> 3. The Squires-Saunders SS-1R set with a beam deflection mixer (very quiet
> AND able to handle very large signals) and no RF stage.
>
> When you break new ground -- I'd guess that a Pullen circuit with ~2V on
> the
> plate of the mixer scores as 'new ground' -- you get to figure out how the
> rules apply to your creation.   I gather that the noise is low and the gain
> ample but I don't see how it could handle very large signals while
> remaining
> linear.  Maybe NO RF stage?   Donno ...
>
> You can do the standard interference free dynamic range (IFDR) test with
> two
> signal generators 20 kcs apart to get a number for this.   Or just set it
> up
> and listen to 80M SSB in the evening traffic peak: If everything sounds
> just
> as good then as it does at 2 AM local time, you don't have a dynamic range
> problem.
>
> (IFDR test: What is the signal strength that produces a chosen S/N ratio --
> say 10 dB at 3800 kcs?  What is the signal from two (equal output)
> generators when they are tuned to 3820 and 3840 that produces the same S/N
> ratio at 3800 kcs?   The ratio of the two signals is the IFDR.
>
> (There is some fussiness about connecting the two signal generators so they
> don't see each other and cross-modulate in their own circuits.   There is a
> fairly simple circuit to prevent this; the details are in the more recent
> Handbooks.)
>
> IFDR testing generally puts greatest stress on the last stage before the
> really sharp selectivity.   That's because once you go through the
> 'knothole' the signals that are 20 and 40 kcs away from the desired
> frequency are so much attenuated that they can no longer cause
> non-linearity.
>
> In a set like the HR-10, the stress would be on the mixer because the
> crystal filter is in that plate circuit.   In a 'true' HBR it could be in
> the first or second mixer or even in the first LF IF because 20/40 kcs
> isn't
> that far off for these stages and there is gain at each stage, as well.
>
> Poorly designed RF stages -- too much gain, particularly if the tube itself
> doesn't handle large signals well -- can also be the limiting factor in
> IFDR.  The antenna coil Q is often limited by the load of the antenna so it
> gives little help.   Basically an RF stage should have just enough gain to
> cover the mixer noise and the mixer should have just enough to cover the
> noise of the 1st IF.   If the 'knothole' is in the mixer plate circuit
> you're home free after that and as much additional gain as needed can be
> supplied in the IFs, detector, and even audio stages.
>
> One question would be how the mixer job was done in the auto radios that
> were designed around the space charge tubes.  Auto radios MUST handle large
> signals so that ability should be a given with that tube/circuit.   And
> most
> auto radios are reasonably sensitive as well.
>
> The reason you found the push-push triode circuit to be high noise is that
> it's not a good choice for use without an RF stage.  It does, however, have
> a high dynamic range, meaning that if you boost the signal above the tube
> noise you still have room before you drive the tube into non-linearity.
>
> Somewhere I was hanging out a decade or so ago somebody was joking that an
> 833A might be an ideal receiver RF stage or mixer.  It wouldn't, really,
> but
> that guy did understand that the ability to handle some power is a
> requirement for HF receiver front ends.
>
> My nominee for the least-well exploited mixer circuit is the beam
> deflection
> tubes -- 6AR8, 6JH8, 6ME8, and 7360.   While there are some design hurdles
> to get over they're not beyond anyone who could build a W6TC set from
> scratch and you will have a radio with very few peers -- it should equal or
> even beat the R390s for IFDR.   Unfortunately no ham design that I know of
> made good use of these tubes.
>
> The Miser's Dream and related sets (the Advanced Six Tube Amateur Receiver
> in the '69 handbook) are the only ones I know of and the designer of the
> Advanced Six ... really didn't get it: He turned out a single band receiver
> (80M) using a 7360 mixer with no RF stage -- okay so far -- but used a 455
> kcs IF, presumably so that he could use a Collins mechanical filter for
> selectivity.   Great on 80, but images will be a problem if the front end
> is
> tuned for higher bands.  Well, he knew that, so he put a crystal controlled
> converter in front of it.   Image problem solved but the 1st mixer is now
> in
> the converter and is perfectly conventional in design.
>
> It's not bad as such things go but a showcase for the possibilities with a
> beam mixer it isn't.   The SS-1R design is the one to follow:  If stripped
> of all the complexity it will do the job and the circuit is out there.
>
> General information on the SS-1R is here:
> http://www.w1vd.com/SS-1R.html
>
> Look at those performance numbers!   Yeah, the 0.5 uV sensitivity isn't
> remarkable but it's all you need on HF and the numbers you do need, beat
> everything including the R-390.
>
> (My hat is off to anyone with the tools to MEASURE an IFDR of 94 dB!)
>
> The schematics can be found here:
> http://bama.edebris.com/manuals/squires/ss1r
>
> In the five files with 'sch' in the file name.
>
> There's probably some sort of beam tube HBR in the KJ4KV future.   Maybe on
> the second HR-10 chassis?   Maybe three years from now?
>
> Walt
> KJ4KV
>
>
>
>
>
>
>
>
>
>
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