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Old 17th Nov 2017, 5:30 pm   #21
newlite4
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Default Re: Most Minimal Superhet

How about the Italian Pulgarcito superhet (three valves) or the Spanish Gnomo TRF equivalent with only two valves. These are very minimalist sets and are hard to find:

http://www.antiqueradios.com/forums/...c.php?p=926996

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Old 17th Nov 2017, 6:23 pm   #22
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Default Re: Most Minimal Superhet

Pye in the UK produced the P117 AM/FM superhet with only 3 valves + rectifier. The ECC85 was used as a reflexed RF/IF amplifier + mixer/oscillator on FM and as an oscillator/mixer on AM. IF amplification was furnished by an EBF89, the 2 diodes being used for AM detection and AGC. The FM detector was a couple of Ge diodes and the output stage an ECL82.

It worked passably well, being slightly short of gain on FM and a bit prone to drift. The AM performance was surprisingly good. The ECL82 was not an outstanding performer, the triode being prone to picking up hum. The later ECL86 had much better internal screening.

This design was never repeated. With a multitude of "hot" signal switching, it must have been a nightmare to produce and align. I have one as a curiosity (only). I quite like, for a cheap set, the concept of utilising 6.3V valves with a heater transformer with mains derived HT. The sets run cool without heater-cathode insulation problems or induced hum.

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Old 17th Nov 2017, 7:17 pm   #23
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Default Re: Most Minimal Superhet

I am doing a deal on a Champion 784 chassis & I have one of those kit TRF's. The kit radio is case-less & anonymous
I will have to see which would do best. The kit radio has a 6J7 as detector & 6V6 o/p. At present I would need to see about mounting a IF can. I wonder if the aerial coil of the TRF will serve with a specific oscillator coil? I do have a MW aerial coil. Re-wire the base of V1 to use a ECH35. If it works, OK. If not plate over the hole to fit an B9a base & ECH81.
I do remember that when I tried a Superhet before I had a variable capacitor with 2 equal gangs & it took a 1000pf trimmer across the osc coil to get the necessary off-set. Not sure whether there's the room to mount a coilpack under the chassis.
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Old 17th Nov 2017, 10:52 pm   #24
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Default Re: Most Minimal Superhet

Quote:
Originally Posted by Neil Purling View Post
I do remember that when I tried a Superhet before I had a variable capacitor with 2 equal gangs & it took a 1000pf trimmer across the osc coil to get the necessary off-set. Not sure whether there's the room to mount a coilpack under the chassis.
This is one of the most important parts of a superhet design but often gets glossed over and some just pick a padder capacitor value from another set when making a homebrew superhet.

Normally, if the two gangs on the V/C are equal capacity, the gang for the oscillator has a capacitor called a padder, placed in series with that gang, to lower the overall capacitance, but it has to be the correct value.

Normally the local oscillator runs above the received frequency by an amount equal to the IF frequency.

For correct tracking the oscillator minus the IF frequency, for any mechanical position of the V/C's shaft, equals the received frequency peaked by the antenna tuned circuit on the other gang on the V/C, but this can only happen in three exact places, near the beginning of the band, in the middle and near the end. Between those places are tracking errors.

Therefore, in other places on the dial the tracking is always off by a little (tracking errors) with an error below and above the mid-band received frequency. In these places the gang tuning the antenna coil is peaking the resonance of that at a frequency a little above or below the frequency of the oscillator minus the IF (which is the received frequency), so its "mis-tracking" in these zones . When the padder capacitor value is right, the tracking errors on each side of center are equal and minimal.

The best way I have found over the years to get the padder value right was initially with the equations in Terman, but it uses a side of A4 and wears down a pencil, then a number of software utilities appeared on the net to do it, making the whole process so much easier.

In transistor radios you will see often the gang on the V/C for the oscillator was a smaller capacity anyway and this dispensed with the need for the padder capacitor and the V/C itself is manufactured for the correct tracking.

In a lot of valve radios, the padder capacitor itself was also used as a tool for setting/trimming the low end of the band oscillator frequency when there was no slug or no adjustable slug in the oscillator coil.

I'm sure there are a lot of folks here very familiar with the tracking and padder issue and can make remarks or corrections to the above.

Last edited by Argus25; 17th Nov 2017 at 11:00 pm.
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Old 17th Nov 2017, 11:45 pm   #25
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Default Re: Most Minimal Superhet

Quote:
Originally Posted by Sideband View Post
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?
Yes, lovely isn't it. I've always been very fond of this electron coupled notion, also used in the electron coupled oscillator and TV sync sep circuits, and others, it makes great use of the utility value of the screen grid. Getting more functionality out of the one part.

I have attached another circuit of a reflexed set with a more conventional volume control and another image helping to explain the stage, in this case audio extracted from anode in the other image.

I would make a general remark though about the whole idea of of building a minimal superhet...it could go to far. Others may disagree:

Obviously, after a lot of research here in AU, these reflexed superhets were commercialized and worked pretty well. They are really about as minimal as you can go simplifying a superhet without flushing the radio's performance down the toilet.

I don't think cutting back to less than three active stages: (converter/IF(reflexed)/Audio) for a superhet is a good idea at all and that is getting double use of one stage.

Of course you can use valves with triodes & pentodes etc in the one envelope to "cut back the valve count" and convince yourself it is simplified but it doesn't really change the fundamental gain-bandwidth requirement. Unless you want to make a "deaf superhet". Likewise adding in regeneration to acquire more gain, is a difficult thing to control.

In my view there is not a lot of point of putting in the construction work to make a badly performing radio. If you make one at all, it is nice that it would be a peppy performer, at least that is what I think.

On the other hand with other radio designs with regenerative detectors etc, very simple low valve count radios can be made, but they are not superhets and they have their quirks.
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Last edited by Argus25; 18th Nov 2017 at 12:12 am. Reason: typos
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Old 18th Nov 2017, 8:13 am   #26
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Default Re: Most Minimal Superhet

I was going to see what I can do with the kit radio, turning the RF stage into a mixer/osc & leaving everything else as normal. Thus keeping the anode bend detector that won't damp the previous stage. I have a squad of ECH35's & 6K8's that are unused.
If I should need a smaller 1st IF transformer I will ask in the appropriate place.
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Old 18th Nov 2017, 11:21 am   #27
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Default Re: Most Minimal Superhet

Hello.
Unfortunately the link a few posts back to another forum has many posts removed due to a member leaving. This member is a friend and his minimal superhet rocks. It is three valves, ECH81, EF91 as an anode bend detector and an EL95 output. He has a YouTube channel but I don't know how to link it to here. The radio in question can be seen on a clip.
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Old 18th Nov 2017, 11:32 am   #28
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Default Re: Most Minimal Superhet

That's got to be Trev, he's on GVR forum, video link for min superhet:

https://www.youtube.com/watch?v=aXLy...SlauZ6irWMCogA

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Old 19th Nov 2017, 9:20 pm   #29
Neil Purling
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Default Re: Most Minimal Superhet

I have made several attempts at making a regenerative TRF. RF gain seems like the big problem & them how would you keep the RF & detector stage tracking perfectly?.
We are fortunate that we have valves with great gain now, but even so.....
My best effort was adding a untuned RF stage to the electron coupled detector, then feeding a high slope output stage. This could be a EF91 or Mazda SP61 They have an apparent gain advantage over the EL84. Even so you don't have a wall shaker & you still need a long wire aerial. You don't need a powerful audio stage anyway.

So, the writing was on the wall for my TRF.... The first image is the TRF. You can see the tacked on EF91 RF stage.
I stripped out everything apart from the output stage and power supply.
The second image shows the coil pack I intend to use temporarily positioned.
The red, brown and black wires are from the IF transformer which perfectly fit the holes drilled on the chassis. I can convert this to a full superhet if necessary.
The only thing I don't seem to have is a tuning capacitor with unequal sections. I found the 1000pf trimmer I used in a previous project.
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Last edited by Neil Purling; 19th Nov 2017 at 9:44 pm.
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Old 21st Nov 2017, 10:55 am   #30
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Default Re: Most Minimal Superhet

Has anyone done any tests of the sort of signal level you can expect to see at the secondary of the IFT for a RF level that would equal that of a off-air signal?
As the chassis was punched for Octal bases I am staying with those for now.
If the ECH81 is a better frequency changer I can plate the hole over for the new base.
The same would go for the other two valves.
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Old 22nd Nov 2017, 12:26 pm   #31
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Default Re: Most Minimal Superhet

Hello.
The ECH81 is reckoned to be the best FC that was made. On a conversation a couple of nights ago I was told that some of the octal based and equivalent valves don't make the best Anode Bend Detectors, allegedly the EF80 & EF91 work very well indeed. As far as I have been told an Anode Bend Detector has a zero point that needs to be above before any audio is produced, it is its downfall. With a BFO it works well as the carrier from the BFO lifts the detector above the zero point, it works well on my mate's home made set.
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Old 22nd Nov 2017, 1:43 pm   #32
Neil Purling
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Default Re: Most Minimal Superhet

I can always fit an International Octal base in place of the Mazda one for the detector if needed.
It has been my finding that the type of valve in the anode-bend detector does not seem to be ultra critical.
I bought the SP61's by mistake when I had a genuine Premier kit TRF. That had a VR116 as detector, equivalent to a 6.F.32. Both sorts seemed to work OK. Usually you see a 6K7/EF39 or 6J7.
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Old 22nd Nov 2017, 2:12 pm   #33
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Default Re: Most Minimal Superhet

Aside from the ingenious reflexed versions of short superhets I posted earlier (which didn't attract many remarks) good performing short superhets have been designed & built in Australia since the 1940's, without reflexing.

The classic was "The Little General" In these radios they did not reflex the audio through the IF and used a 6V6 which helped. See attached image.

The Little general was again introduced over the years in the 1950's & early 60's in a number of variations with other valves and a 6BM8 as the output valve and ferrite antenna added. They were intended for home construction.
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Old 22nd Nov 2017, 2:28 pm   #34
Neil Purling
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Default Re: Most Minimal Superhet

I have not been able to find any reference date for the 6G8G valve. The 1961 version uses a EBF80 equivalent.
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Old 22nd Nov 2017, 6:45 pm   #35
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Default Re: Most Minimal Superhet

http://frank.yueksel.org/sheets/084/6/6G8G.pdf
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Old 22nd Nov 2017, 7:17 pm   #36
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Default Re: Most Minimal Superhet

Does the ECH81 give more output for the same level of RF input. Just wondering what makes it such an improvement on the ECH35, or 6K8.

As for that earlier version of the Little General the circuit does not even show the diodes.
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Old 22nd Nov 2017, 7:22 pm   #37
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Default Re: Most Minimal Superhet

Quote:
Originally Posted by Neil Purling View Post
Does the ECH81 give more output for the same level of RF input. Just wondering what makes it such an improvement on the ECH35, or 6K8.
It has a lower Equivalent Noise Resistance [from memory the 6K8 is around 290K whereas the ECH81 is about 60K]

Meaning that the importance of having a RF-amplifier ahead of the mixer (particularly for shortwave use) to overcome internally generated mixer-noise is reduced - though for shortwave use you'll probably still want that RF amp to reduce the image-response.
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Old 22nd Nov 2017, 7:49 pm   #38
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Default Re: Most Minimal Superhet

I will give the SP61 a try. It worked in Premier I had, though I don't have the particular component values involved there.

So the ECH81 just makes less noise in the conversion process. And this is going to be noticeable to the naked ear?
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Old 22nd Nov 2017, 8:38 pm   #39
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Default Re: Most Minimal Superhet

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So the ECH81 just makes less noise in the conversion process. And this is going to be noticeable to the naked ear?
For a simple broadcast-band radio no, but on the shortwave bands yes.

The "Conversion conductance" of the ECH81 is also better than the earlier Octal valves [think of 'conversion conductance' as essentially the gain of the mixer, though it's really rather difficult to quantify gain in a mixer]
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Old 22nd Nov 2017, 9:03 pm   #40
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Default Re: Most Minimal Superhet

Apparently, one of the strictures behind the design of the ECH81 was to act as an effective 1st IF amp in VHF/FM receivers with simple, minimal-budget and low-gain VHF tuners- that would have emphasised good noise performance and it's likely that all the tricks of 20 years of multi-grid mixer development would have been deployed, such as grid alignment for minimal screen-grid current and hence noise contribution. The compact and precise construction also helps to minimise self-inductance and transit-time, helping with extending its frequency capability. The US 6BA7 was also a last-fling, deploy-all-the-lessons multi-grid frequency changer but using the US-favoured topology of the self-oscillating heptode (as opposed to externally-fed oscillator grid). Don't overlook the common-place ECH42- it's also a highly-developed device and doesn't give much away to the ECH81, the latter was an incremental evolution of what came before, rather than a revolution.
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