Most Minimal Superhet

[Neil Purling]

I was wondering what is the most minimal superhet that one could construct & it would work?

I used to have a radio in a 'Little Maestro' type of case which had a X109 frequency changer, 10P13 as output and a U107 rectifier. The detector was regenerative, being adjustable by a trimmer. Turn it up to get R4 on LW, or RTE 1 at the top of MW and it used to howl on the local stations.


Then I found an Italian circuit employing a UCH81 and a ECL80.
I couldn't work out the way it got the heater supply, but the mains transformer primary had multiple taps. The 160V one provided the HT supply
The radio may have been a commercial product, so I won't name it.
Detection was by a OA72 diode to feed the triode of the ECL80.
I have an amplifier and I have found ECL80's to be twitchy devices in the past.

Has anyone made anything usable with a single conventional IF transformer and frequency-changer valve with detection by a diode?
I know that the beggars do tend to produce images when the circuit is pruned back so drastically. My amplifier only needs 150mV input, so it is perfectly capable of amplifying the detected audio.

[Hartley118]

Well, a wee bit of positive feedback can do wonders for both gain and tuned circuit Q, but does tend to be tricky for the user.

Was your Little-Maestro- cased radio one of the many and various commercially available kits from the early 1950s?

Martin

[G6Tanuki]

I built one once using an ECH81 frequency-changer, one half of a 12AT7 as an untuned IF-amp and the other half of the 12AT7 as a regen detector, which fed-back to the primary of the IF transformer through a variable capacitor (from detector anode to ECH81 anode) to control the regeneration.

It used a 1.6MHz IF and only drove headphones. Worked surprisingly well and was stable enough to resolve 3.5MHz SSB with the regenerative stage gently oscillating.

[Neil Purling]

It was probably one of the first L.M. cased radios I bought & I paid way over the odds for it.
If it was a kit, then it must have been one of the last ever. The chassis was only punched with the small holes & for the one IF can (which I ought to have kept). I don't think it was a kit, unless you know better: Page 77, centre of Tony Thompson's Vintage Radios.

I thought the ECH81 & diode would give enough AF to run to my Mullard 3-3.
The ECH81 is supposed to make less noise than the multiple examples of ECH35 and 6K8's I also have.

[David G4EBT]

Just to clarify, Tony has written several books on vintage radios.

The book that Neil refers to is entitled 'Crowood Collectors Series - Vintage Radios'. Published in 2007, now out of print, sought after, and commanding high prices. EG:

https://www.amazon.co.uk/Collecting-.../dp/1861269498

It says much about the book that of the six used ones on offer, two are from the USA, and two from Spain (in 'good used condition', £208 each!!). Two new ones from the USA at £102 each.

Two of Tony's other books, still in print can be seen at the same link, 'Vintage Radios - Collecting, Servicing Restoring' is on it's third edition, updated and enlarged in 2015.

(For the avoidance of doubt, I'm not Tony's literary agent - I just happen to own his excellent books!)

[Argus25]

Originally Posted by Neil Purling

I was wondering what is the most minimal superhet that one could construct & it would work?

If you were doing this with transistors you could have just one mixer oscillator transistor (driven by the usual ferrite rod antenna) which feeds the first IF transformer, then one transistor IF amplifier and another IF transformer following that, then the detector diode. The level of the audio can be got up to enough for a small speaker with just two transistors, the output stage being class A , So that is 4 transistors & a diode.

With valves you can use one mixer/osc tube such as a pentagrid converter (6A8) or a triode-hexode type (6k8) or similar and then one IF tube, if that tube is reflexed (the audio reflexed through the IF stage) then it saves an an audio pre driver, a 6B7 or similar works. Also helps if that reflexed IF stage is made from a valve with a diode in it, like a 6B7. So then you would just need an audio output stage. So that is 3 valves, plus the rectifier valve (unless you use silicon rectifiers to cut back on one valve)

So, the answer is 4 transistors or 4 valves approximately. You could possibly do it with one less of each, if only low level audio is needed for headphones rather than speaker.

Of course most commercial superhets have at least one extra valve or two or three extra transistors for optimal performance.

[paulsherwin]

Short superhets without an IF amp were relatively common in Australia and New Zealand, presumably reflecting the reception conditions in urban areas there. I can't think of a single British or European one offhand - short superhets here dispensed with the audio preamp rather than the IF amp.

[Neil Purling]

Is it permisable to name the Italian circuit & give a link Mr Moderator?
It was that which made me remember the minimal superhet radio I had long ago
I am not entirely sure whether it was an amateur design or an actual manufactured product. There are some very odd aspects of the drawing that make me wonder if the circuit has errors.

[paulsherwin]

Of course.

[ms660]

If you happen to come by a 6M11 valve...a one tube recipe:

http://onetuberadio.com/2015/11/13/1...tube-superhet/

Another one tube job:

http://electronbunker.ca/eb/OneTubeSuper.html

Quite a few on the web.

Lawrence.

[Neil Purling]

The radio model was 'Ninnolo' a.k.a 'Peba': https://www.radiomuseum.org/r/peba_s...o_unknown.html

I will get the link to a large diagram soon.

[paulsherwin]

That is a very peculiar design.

[Argus25]

Originally Posted by paulsherwin

Short superhets without an IF amp were relatively common in Australia and New Zealand, presumably reflecting the reception conditions in urban areas there. I can't think of a single British or European one offhand - short superhets here dispensed with the audio preamp rather than the IF amp.

I can't recall seeing a single set without an IF amp. All sets had two IF transformers, one in the anode of the converter and one in the anode of the IF amp. But what they did was reflex the audio signal through the IF amp, so the IF amp amplified audio and the intermediate frequency at the same time, this gave the extra gain to dispense with the audio driver stage. So there was just the typical 42 or 6F6 output stage. The IF amp tube chosen typically contained the diode too.

One disadvantage of this system was the volume control, it could never quite get the volume to zero.

[Sideband]

The Italian circuit does work....I built one for fun but the selectivity is not good because the diode damps the tuned circuit too much. Better to use an anode bend detector which has a very high input impedance. Just for fun, a few years ago I built a simple superhet using an ECH81 frequency changer, EF80 anode bend detector and an ECC82 as AGC and audio output. One triode was strapped as a diode for the AGC and the other triode was used to drive a small output transformer....surprisingly loud for a single triode output. The only reason I used an ECC82 in this way was because of very limited power available from the only mains transformer I had so a high degree of economy was required.

Most commercial designs seemed to use a standard frequency changer and either a regenerative IF stage or a leaky grid detector followed by an output pentode.

[Argus25]

I've attached a typical Aussie circuit that was once very popular from Radio & Hobbies 1950. A number of commercial sets used it here. The audio is reflexed through the IF stage, saving a valve. Notice the unusual volume control.

[Gridiron]

Cossor made sets in the 30's, such as the 484, with no I.F. amplifier valve, the mixer valve fed a regenerative detector via a I.F. filter circuit.
Mike.

[G6Tanuki]

You could do an entirely-valve AC mains powered 'full' superhet in 3 valves!

ECH81 as frequency-changer, ECF82 as IF amp and anode-bend detector, then one of the not-often-seen-this-side-of-the-pond combined output-beam-tetrode-and-half-wave-power-rectifiers like the 117L7/117M7GT drives the speaker and rectifies the supply for you.

Maybe even possible to get an AGC voltage by splitting the grid-leak for the detector into two parts and putting it between the bottom end of the IFT and earth (top end wired direct to grid)?

And if you're into battery-valves, the 1B8GT gives you a diode-triode-output-pentode in one convenient, easy-to-use Octal-based bottle; there's also a small battery-powered double-pentode whose number I forget, that could be made to work as a self-oscillating frequency-changer and IF amp.

[Neil Purling]

Take a look at this thread: http://www.forum.radios-tv.co.uk/vie...php?f=3&t=6469 The circuit is attached to post #1.

[Sideband]

Originally Posted by Argus25

The audio is reflexed through the IF stage, saving a valve. Notice the unusual volume control.

Unless I'm missing something, are they using the screen grid of the reflex stage as a 'virtual anode' and taking the audio from there?

[Sideband]

Originally Posted by Neil Purling

Take a look at this thread: http://www.forum.radios-tv.co.uk/vie...php?f=3&t=6469 The circuit is attached to post #1.

If you follow that archived thread you will see how we developed the minimal superhet including the version I mentioned above.