Pratical Wireless receiver design, Ae straight to 7360 mixer

[Gi4CZW Cliff]

Not certain I've got the correct place, Mod will correct I'm sure.
From another thread on virtues of HRO/AR88 and dynamic range, I remember a receiver project from Practical Wireless, where the aerial was direct to a double balanced mixer 7360. As there was no RF stage and the 7360 could take large signals the dynamic range should have been good. Does any one remember this and was it as good as hoped. Cliff.

[Radio Wrangler]

Try any of the RSGB blue handbooks from the end of the 60's through the 70's and you'll fing the G3PDM receiver. A genuine high dynamic range job. Antenna into tuned circuits into 7360 and a decently linear IF. Was it as good as hoped. Yes.

There was also the Squires SS-1R in the US with similar architecture

David

[G8HQP Dave]

Bear in mind that the 7360 and other beam deflection mixers are not particularly low noise so they rely on tight antenna coupling to get a reasonable noise figure. This is OK for 50 ohm feeders but not for random wires unless an ATU is used.

The good dynamic range arises from the absence of an RF stage. The mixer itself is no more linear than an ordinary pentode.

[Gi4CZW Cliff]

Just saw the cover of a Sept. 1974 PW with a "EPSOM general coverage sw receiver " by Eric Dowdeswell G4AR. Could this be the one I was thinking off ? I remember being asked to check it out and the bifilar winding wasn't correct. Cliff.

[KeithsTV]

I've found the article. Do you want a scan of it?

Keith

[Radio Wrangler]

Given the difficulty of finding good 7360s in the UK, might I suggest biting the bullet and considering an H-mode mixer for a receiver sans RF amp? It'll have appreciably more dynamic range than the 7360, and as Dave said, the 7360 is still a relatively noisy pentode, it's just that the electrons later get switched between two anodes. And that doesn't affect the basic pentode performance.

David

[G8HQP Dave]

If you want to stick with valves there are other beam deflection mixers which may be cheaper than 7360, and possibly slightly better.

[Gi4CZW Cliff]

Hello All, many thanks to you. took a look at " H-mode mixers ". It must be hard to keep up with so much development. No sooner is there an advance in filters than mixer design takes a jump forward. Gosh no, my home construct days are over, thought I can still burn a finger on the soldering iron. Keith, yes love a look at the article photo/scan do nicely, sent you an email.

[KeithsTV]

Here's the first part of the article with the circuit diagram.

Keith

[Gi4CZW Cliff]

Hello Keith, many thanks for the scan, great to see curly coils and zig-zag resistors. So I got it wrong, not 7360 but 6JH8, noticed the design close to G3PDM's. The one I had a look, at seemed quite deaf, compared to the KW2000B that I ( had at the time ) to compare it with. I did wonder if others had met with this receiver. Cliff.

[G6Tanuki]

That PW design is only half-balanced - the L.O. signal's fed to only one of the beam-deflection plates which is distinctly sub-optimal. A proper design feeds L.O. to both plates in anti-phase.

Beam-deflection-mixers can sometimes sound 'deaf' compared to traditional-style radios: that's because in a proper B.D. mixer the internally-generated noise is rather lower than traditional multigrid mixers like 6K8s and ECH35s.

My traditional test is to stick a non-inductive resistor of the same value as the receiver's quoted input-impedance between antenna and earth terminals - then fiddle with the antenna-trim control. If you can get a peak of 'sharsh' from the speaker under these circumstances [where you're essentially listening to the thermal-noise generated by the resistor] then the receiver has adequate sensitivity for use with a real-world antenna.

[Anthony Thomas]

I remember seeing this project all those years ago and obtaining the valve from Langrex, I don't know if I have one or two of them in amongst my bits and pieces. The other valves etc were mostly in the spares box at the time, I even found a tuning scale not disimilar to the one used in the G4AR prototype.

Tony

[G8HQP Dave]

Much of the noise you hear when you peak the antenna trim is actually thermal noise from the dynamic impedance of the first tuned circuit. If you can't detect much peak the receiver may still be fine for HF with a reasonable antenna. If the peak is really obvious then it is too sensitive and may need an attenuator!

[jonvk2fsw]

Hi KEITH I read the post about the 7360/6js8 mixer receiver in pw part 1 would it be possible to get a copy of part 2 as it looks to be an interesting design .The coils as used are of course unobtainable in AUS ,and never were but substit. utes could be wound up .73s JON

[KeithsTV]

Here's the second part.

Keith

[jonvk2fsw]

Hi KEITH thanks very much for your very fast reply .I have several 7360 valves which I obtained for spares in an FT 200 ,long gone now . Also I have used 6BU8 in the past for product detectors . All the best JON

[Bazz4CQJ]

Firstly, thanks to Keith for uploading the article.

I don't have the expertise to comment on the merits or otherwise of receivers with direct aerial input to a mixer like this, but a couple of aspects are of interest.

This design is rather like a TRF; you're going to be doing 2-handed separate tuning of the "front end" and the oscillator. In Part 2, the author comments that the tuning is sharp and it's easy to miss a signal; hardly a virtue. It may be old fashioned but I do love my single-handed, 4-gang tuning harnessed to National's wonderful PWD. He's got the oscillator tuning covering quite a range and the design of the oscillator looks way too simple; not a single component for temperature compensation.

My guess is that this would probably be a disappointment; the time and effort could be better spent

[Radio Wrangler]

It's certainly an odd beastie. 7360 meets 2N2926 and LM380.

Back in the day, the Eddystone 898 dial was a very expensive bit of kit and only appeared on serious full-capability radio gear like G2DAF transmitters and receivers, or that RSGB handbook receiver with the Electroniques front end.

That LO is going to be rather drifty.

I looked at the design back in its day and pretty much dismissed it as a probable disappointment. If I was going to go to the trouble of some of those components and especially that dial/drive, I'd be shopping for the crystals for the G2DAF.

David

[G8HQP Dave]

Is there a missing decoupling cap on pin 2 of the mixer? The arrangements for the deflection plates seem to be designed to introduce AC imbalance.

[turretslug]

I don't like to be too damning of something that someone had obviously genuinely intended to be a useful, straightforward, value project to many amateurs but there's an element of "must get in a beam-deflection mixer"- the "magic bullet" syndrome that we saw with lots of "must have" devices such as GaAs FETs. The need to keep cost and complexity down meant unbalanced drive and an uncomfortably wide ranging, free-running 1st VFO- not good compromises for sensitive amateur SSB use. An AR88 would probably be more stable! (And by the way, that's not a condemnation, simply an indication that anyone building this set and expecting noticeable improvement over the comms receivers of yesteryear might have been underwhelmed for the effort).

To me, BFO apart, and considering the coverage, it might have been better pitched as "Performance BC SWL receiver with beam-deflection mixer"- where ultimate stability and sensitivity would be less crucial and good signal-handling in the days of "shout loudest" HF broadcasting would have been useful.