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Old 21st Jul 2010, 9:39 pm   #1
JHGibson
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Default One Tube Superhet

An interesting project for homebrewers is this one tube superhet. The article appeared in 1956 Popular Electronics entitled "Build the Minidyne"
It uses the pentode half of a 6U8 as a reflexed IF amp/audio output. the mixer is a diode. A second diode is used as the detector. The triode of the 6U8 is the local oscillator.
The article can be found at http://home.gwi.net/~jdebell/pe/1956/
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Old 21st Jul 2010, 9:44 pm   #2
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Default Re: One Tube Superhet

Gives real meaning to 'short superhet'...!


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Old 22nd Jul 2010, 2:41 am   #3
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Default Re: One Tube Superhet

I see an error on the schematic, the bottom of the volume control should be earthed.

John.

Sorry, my error, the volume control is earthed through the secondary of the output transformer.
John.

Last edited by JHGibson; 22nd Jul 2010 at 2:43 am. Reason: My error
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Old 22nd Jul 2010, 12:48 pm   #4
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Default Re: One Tube Superhet

Compared with this 1-valve reflex receiver, I see the following:

Minidyne has 3 tuned circuits giving selectivity (1 at RF, 2 at IF); the PW circuit has 2 (both at RF).

Minidyne has 2 stages of amplification (pentode at IF, pentode at AF); the PW circuit has 3 (Pentode at RF; triode leaky-grid detector giving AF gain; pentode at AF).

Minidyne has 2 diodes in signal path (cheating!); the PW circuit has none.

My gut feeling is that the PW circuit should have the edge on sensitivity, plus lower noise and no oscillator radiation. But, interesting all the same!
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Old 22nd Jul 2010, 6:00 pm   #5
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Default Re: One Tube Superhet

Here's a one-valve double-reflex superhet circuit, using a 6ME8 gated-beam valve to do, well, everything:

http://www.antiqueradios.com/forums/...ic.php?t=96539
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Old 24th Jul 2010, 10:10 pm   #6
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Default Re: One Tube Superhet

After looking at the "One Tube Superhet" article, if you go to the parent directory, you will find a wealth of similar circuits.
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Old 24th Jul 2010, 10:34 pm   #7
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Default Re: One Tube Superhet

Quote:
Originally Posted by JHGibson View Post
It uses the pentode half of a 6U8 as a reflexed IF amp/audio output. the mixer is a diode. A second diode is used as the detector. The triode of the 6U8 is the local oscillator.
Could have a bit of fun building this for something to do. It uses an ECF82 which I dont have. I wonder if I could get a PCF80 to work.....got plenty of those.


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Old 24th Jul 2010, 11:02 pm   #8
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Default Re: One Tube Superhet

I have several spare ECF82s if you'd like one, free for the cost of postage - drop me a PM.
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Old 5th Jul 2013, 5:41 pm   #9
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Default Re: One Tube Superhet

Quote:
Originally Posted by JHGibson View Post
An interesting project for homebrewers is this one tube superhet. The article appeared in 1956 Popular Electronics entitled "Build the Minidyne"
It uses the pentode half of a 6U8 as a reflexed IF amp/audio output. the mixer is a diode. A second diode is used as the detector. The triode of the 6U8 is the local oscillator.
The article can be found at http://home.gwi.net/~jdebell/pe/1956/
John.
My thought about this circuit is this, I like the reflexed IF/AF idea, but the simple germanium diode front end leaves much to be desired.

It turns out that the 6U8 tube makes a nice mixer/oscillator tube for small superhets, as seen in the 'SimpleX' circuit that was featured in QST and the Radio Amateur's Handbook in the 1960's:

http://www.bignick.net/simple_x/images/SimpleX.pdf

....and here, subbing a 6EA8 for the 6U8:

http://home.comcast.net/~btse1/vintrad/hb/1960s.htm

The SimpleX is a nice circuit, but I'm thinking about just improving the original Minidyne.

Then I looked in Phil's Valve Radio site:

http://philsvalveradiosite.co.uk/phi...eceivers_1.htm

....and saw this:

http://philsvalveradiosite.co.uk/phi...nverterone.jpg

ECF82 = 6U8

I believe that Phil's circuit combined with the reflexed IF/AF stage using, perhaps, a 6AU6 in place of the 6U8's pentode section would be just the ticket to a much improved 'Minidyne' circuit. Perhaps call it a 'Minidyne 2' because it will then have 2 tubes.

6AU6 = EF94

Any thoughts about this idea?

Last edited by Norm_N_Tam; 5th Jul 2013 at 6:09 pm.
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Old 5th Jul 2013, 9:26 pm   #10
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Default Re: One Tube Superhet

I received the following reply from Phil, via email, regarding the above posts:

"Hi my friend. I have this moment had a look at the circuit and although the oscillator circuit is the same the mixer works slightly different which I will briefly explain. It is designed to reflex audio as well as RF which is the purpose of the diode mixer to give it that extra kick in gain and isolate the oscillator from audio so there for the pentode is acting as an AF and RF amplifier. As a result of this my circuit would be unsuitable. If you have not yet built the receiver then I would take my simple advice regarding reflex circuits. I built a 1 transistor reflex in the 70s and found it could only receive BBC Radio 4 at usable earphone volume and all the other stations were very faint, No better then a crystal set. Reflex receivers are also very critical in their layout and can be very temperamental. There are some very good circuits for regenerative receivers around and are much advised as a starter receiver and when experience is gained and funds allow you could also then move on to a superhet receiver. The choice is yours but I would stick to the original circuit and talking of the 250V HT line this set needs I don't think it would have stand a chance of giving speaker reception at lower voltages like some of the receivers of mine that you may come across due to only one active valve. The reason I mentioned this later statement is because 250V DC is a very lethal voltage to be dealing with if you have never worked with mains valved receivers before. I Wish you all the success in radio building and I hope you do get this receiver to work but take my advice, There are much better circuits around that will work at much safer voltages. Bye for now."

Therefore, beginners beware of high HT circuits like this!

Last edited by Norm_N_Tam; 5th Jul 2013 at 9:49 pm.
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