[Sticky] Project: VRAT Minimalist Superhet Radio.

Hi Rich.

I did a single valve super regen FM radio with a Rod Pentode 1ZH29B that actually worked rather well, would receive all the locals with no aerial and the weaker ones with just a short wire, HT is 60v LT 1. 2v the main disadvantage was slope detection.

Nathaniel take a look on my channel and see some of the crazy things I do. 

https://youtube.com/channel/UCms-IAU6iSlauZ6irWMCogA

Posted by: @sideband

I suppose I could try a reflex FM radio but these can be tricky to set up with their slope detection principal......

It worked well enough for Schaub with the Pirol 56GWU. From memory a UCC85 followed by a UEL71 and a solid-state rectifier.

An excellent write-up here: https://www.radiomuseum.org/r/schaub_pirol_56gwu.html

It would be interesting to re-create that with maybe a UCL82 instead of the UEL71, diode rectifier, 2 valve FM set anyone?

@boater-sam I think someone here or on Paul's site did an FM reflex using a PFL200 effectively making it a one valve FM radio.....

Posted by: @boater-sam

It would be interesting to re-create that with maybe a UCL82 instead of the UEL71, diode rectifier, 2 valve FM set anyone?

Only problem is the UEL71 is a tetrode/pentode.

My mind is now turning to the E/PFL200, but where to get the bases?

(Edit: post crossed with Sideband's)

Apropos the Schaub - it really works much, much better than it has any right to (I have one).

Grundig produced something not dissimilar - the 840W. That however has three valves.

https://www.radiomuseum.org/r/grundig_840w.html

Posted by: @cathovisor

Posted by: @boater-sam

It would be interesting to re-create that with maybe a UCL82 instead of the UEL71, diode rectifier, 2 valve FM set anyone?

Only problem is the UEL71 is a tetrode/pentode.

My mind is now turning to the E/PFL200, but where to get the bases?

(Edit: post crossed with Sideband's)

Apropos the Schaub - it really works much, much better than it has any right to (I have one).

Grundig produced something not dissimilar - the 840W. That however has three valves.

https://www.radiomuseum.org/r/grundig_840w.html

 

Bases here:-XXXXXXXXXXXXXXXXXXXXXX

Moderator Note:

eBay link removed please remember rule #6

Your timing is excellent!

A couple of us on the Aussie forum have done just that, but better. I had the idea a week or so back, Fred Lever has built and debugged it.

How about a 1 valve superhet with no compromise performance?

• 10uV sensitivity
• 2.4 watts audio output
• AGC
• Negative feedback on the audio.
• No mains dropper resistors or live chassis so it's safe for newcomers.

Yes it actually works! It uses a 6Y9, better known to you guys as an EFL200 or it's series heater version, PFL200.

It seems I can't post a PDF of the as-built circuit so I'll take a screen dump and put it here - very soon!

Didn't work, I'll try again!

That's better!

Fred is still working on this to make it work even better!

Not sure if you've ever seen a pentode used as a self-oscillating mixer or a power output stage reflexed to handle the IF.

The mixer stage appeared in late 20's - early 30s radios in Australia before Pentagrids appeared. Reflexing was used by Oz setmakers as a means of saving a valve.

Fred and I were surprised that we could AGC the front end. I had designed and simulated an inter-stage diode attenuator but we didn't need to use it. The neutralisation was needed, putting that much gain in one small bottle you'd expect trouble and we found it!

The 6Y9 is ideal here, both pentodes are very high gain. The Philips DECAL series represents the aegis of vacuum toob technology, trying very hard to push back against transistors!

Posted by: @irob2345

Not sure if you've ever seen a pentode used as a self-oscillating mixer

Yes, common practice here before heptodes &c. Murphy used an audio output valve (the AC/PEN) as a frequency changer in the model A4. I have a number of sets with a screen-grid valve doing the same, the oscillator is coupled between anode and cathode in some. An MS4B in the Marconi 262 did this job.

Sorry to breach rule 6.  B10A bases are available on an auction site commonly used by enthusiasts.

An interesting development, what is the frequency coverage?

You might have better luck searching for B10B 😉 

http://www.r-type.org/static/baseb10b.htm

I might have to have a go at this, I’m sure I have some new PFL200’s in the valve cupboard!

 Regards 

Lloyd 

Looks like a bit of fun. PFL200's are a bit modern for me!! I have a couple of good ones though so I just need to find a B10B base from somewhere.

Hi. 

That's a very interesting circuit and one I'll try in due course. I may even see if it's possible to use the filamentry Rod Pentodes in this circuit with two of them and see if its possible to make a pocket radio. 

I'll answer a few questions:

Standard AM Medium Wave. About 500kHz to about 1700kHz.

I might take it further and make a two valve FM superhet!

I did think of a battery reflex design but I doubt a 3V4 output bottle has the gain we need at 455kHz to make it viable. The 6Y9's power pentode is a video amplifier and it has very high gm in both pentodes.

For battery valves, certainly a 4 valve with 2 IF stages, the 2nd one reflexed, would be a hot performer.

Or, a two valve using the 1D8GT. Some reaction around the triode running as IF amp might make this a goer.

By the way, the negative audio feedback from the speaker to the cold end of the volume control is not just a luxury. Reflex designs without it suffer from a disconcerting "minimum volume effect" which the negative feedback cures.

Not only but-----B10B valve bases available on the internet auction   ( I get there in the end )

Was any more ever done with this?

A friend of mine and I collaborated on a 1 valve superhet using a 6Y9 (aka EFL200) that actually works - yes driving a speaker.

See it here:

https://vintage-radio.com.au/default.asp?f=12&th=130&offset=1

Sadly I still haven’t got round to it! I’ll have a play when I get back from RetroTech 🙂 

Some more work has been done on this.

The Autodyne frequency changer was always a cranky beast. This one had a tendency to drop out of oscillation at some frequencies. Then I remembered the 1930s radios that used the 57 in that stage. They all had 175kHz IFs.

It seems a 455kHz IF is not ideal unless the primary is damped, otherwise you lose osc activity in the middle of the band.

A tapped-down winding might be a better way to go, as when this circuit was widely and successfully implemented with transistors.

Interesting to read of "no IF amp" superhets. We never saw them in Oz. Probably because of low powered commercial radio stations (limited until 1950s by regulation!) and distance. No-one would buy a radio that could only pick up the ABC! Half the radios made in the 1930s had RF stages too, for that reason. Trams had large RFI-filter inductors on their roofs alongside the pickup pole base.