[HBR] A Homebrew design that is rarely mentioned

[Shoppa, Tim]

7360 Mixer beam deflection voltage is limited by simple capacitive coupling from the Hetrodyne mixer. The 10pF series cap has the virtue that it is probably a good and simple way to keep LO level constant as frequency goes up. It's just that the level is always too low by your or my standards :-).

Running enough power through the heterodyne mixer to give 10V at the deflection plates of the 7360 would probably make even more spurs - not made in the 7360 mixer, but from the heterodyne mixer. The heterodyne mixer only has a LC tank to tame the spurs.

Applying HBR-XX style plug in coil per band (would take more than a 5 pin base though) might be a good way to get balance and more drive voltage to the 7360 deflection plates.

In terms of plug in coil sets if we went this hypothetical way... we would get rid of RF amp. Have one plug in coil/cap set (double tuned? I'd aim for that) for front end coupling mixer to antenna; another one for heterodyne oscillator (make it singly balanced to tame spurs even more); a third for coupling heterodyne mixer to 7360. That just might work.

Would certainly be a different approach to all-band coverage, than say Squire-Saunders. That has its elegance too (and optimized the design for the band where you needed the mixer not to crunch - 40M. 40M is tame today compared to how it was when I was a kid! Oh man, SW BC and the woodpecker all over the place.)

Tim N3QE

-----Original Message-----
From: hbr-bounces at mailman.qth.net [mailto:hbr-bounces at mailman.qth.net] On Behalf Of Walt Hutchens
Sent: Tuesday, November 29, 2011 9:00 PM
To: HBR
Subject: [HBR] A Homebrew design that is rarely mentioned

Namely, the W5OMX communications receiver, described in QST for January 1968 and in the Single Sideband handbook, 5th edition, 1970.

In concept this could be the best performing of the various home brewable
ham band only sets.   The main features are:

1.  Single conversion using a premixed LO.  The LO tunes 5-5.5 Mcs for all bands and is mixed with a different crystal for each band -- for example,
7.5 Mcs for the range 3.5-4.0 Mcs.   The IF is 9 Mcs.   The sum of the LO
and band crystal is 9 Mcs above the channel frequency.

2. 7360 beam deflection mixer.

3. Crystal filter in the mixer plate circuit.

4. Bandswitched for 80-40-20-15-11-10M.

5. Pentode RF stage with relatively high-Q (Miniductor) coils.

6. All the other usual HBR features: Product detector for SSB/CW and infinite impedance detector for AM,

7.  VERY nice panel layout, a Millen two-speed dial, generally good
construction work.   This is a pretty receiver.

Author Dave Curtis (Colonel, USAF) developed the set himself from multiple sources; he credits W6TC for some of the features.

Except for ease of construction (where the W6TC designs trump all for home-buildable station receivers) this design should deliver pretty much the best of all worlds:  The excellent strong signal performance that comes from having a single mixer ahead of a fixed IF, the stability you get with running the tunable LO at a lower frequency,  and the low noise of the beam deflection mixer.  

As built, however, the set has a few faults:

1.  Low mixer gain due to inadequate deflection plate drive.   The author
doesn't say but I'd guess these signals were closer to 1V p-p than the 10V that would give high gain.

Basically the conversion gain of this tube is directly proportional to the
deflection voltage.   Cut the voltage in half, drop 6 dB of gain.

2.  Due to the low mixer gain he used a fairly high gain RF stage -- and chose a sharp cutoff pentode for the job.  He also had to boost the IF stages by adding coupling capacitors across the IFTs.  Doing so makes the IFTs overcoupled, though at 9 Mcs their contribution to selectivity wouldn't be important anyway.

He still has a very quiet and sensitive front end and a receiver that is as stable as the best of them but with these two errors he probably almost erased the outstanding large signal handling of which the beam mixers are capable. 

If an RF stage was to be used, it needed to be very low gain -- no more than slightly above unity -- and use a tube with remote cutoff or even variable
mu characteristics.   The G2DAF set runs the 6ES8 RF stage at LESS than
unity gain -- it is strictly to provide isolation for the two tuned circuits
and allow AGC control of the signal ahead of the mixer.   And the SS-1R uses
no RF stage at all.

3. The chassis is crowded.   The premixed type front end is prone to
'birdies' and thus demands fanatical care in layout and shielding.   That's
hard to do that if you are short of 'real estate.'

4. The single ended crystal/tunable LO setup passed up the opportunity to
use balanced mixing to further reduce birdie troubles.   For example a
single ended tunable LO feeding the control grid of a beam tube premixer would be rejected in the (balanced) plate circuit that could drive the
signal mixer beam tube.   With decent shielding and bypassing that's one
less signal floating around the set, and the crystal frequencies are in the tens of Mcs so they're not going to be a big problem.

In addition he fed LO signal to only one deflection plate of the mixer.
That eliminates the balancing of leakage to the control grid where LO spurious responses are most likely to enter.

The mixer output is taken from just one of the beam tube plates so the mixer cannot help reject 9 Mcs signals from the antenna side..

Basically what you've got here is a very quiet high gain pentode mixer: the beam features aren't used at all, except as they contribute to those characteristics.  

These simplifications made the set a little easier to build but introduced
the second of the two noticeable flaws.   The writer does identify the
birdies as a failing but says that a more felicitous choice of mixing
frequencies might be the solution.   Certainly correct choice of frequencies
is important -- you wouldn't want the premixing image (i.e., xtal freq.
MINUS LO freq. or any simple combination) to be in the corresponding ham
band.   But chucking the chance to attenuate what you do have by 20-30 dB
(through balanced mixer technique) is a self-inflicted wound.

Nonetheless this was a very ambitious project and its goals were largely
realized.   Very few ham receivers of the late 1960's would have come even
close to the performance.  And the glitches could be avoided in a re-do.

I tried to find out more about Colonel Dave Curtis USAF, W5OMX.  I got nothing from the FCC database: It doesn't even know the call was issued.
There are some 600 Curtis-es currently licensed and without knowing an
initial or anything else, that's it.   There's no info about him either in
the QST article or the SSB Handbook reprint.   There are various on-line
references to a Dave Curtis in the USAF -- an F-80 pilot in 1954-5 and others -- but nothing that can definitely be tied to W5OMX.

Cold trail, I'm afraid.   That's a shame because it would be nice to give
more meaningful credit where due.

Shame on the ARRL for not picking up this project for the Handbook.  Yes, it's more challenging than a three or six tube set, yes this one might require a bit of re-engineering in the ARRL lab to meet their standards, but it offers superlative performance and isn't at all beyond someone who has been successful building one of the HBR-series radios.

Once you've done the five or six basic circuits in a receiver, battered them into proper operation, AND gotten them all to work together, more complex sets built from established designs are just more drilling and wiring.

Anyhow, this design would be interesting for future work.   So many radio
projects, so little time.

Walt
KJ4KV   


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