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Author Topic: 2 tube shortwave superhet with EF97 fed by 12 VDC  (Read 2591 times)
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« on: November 11, 2018, 11:37:57 AM »

12VDC EF97 superhet receiver

A while ago someone asked if I could design a small receiver with valves but not using high voltages, so his small son could play with it and familiarize with valves. I used EF97 valves which are very happy with anode voltages of 12 VDC.
The filaments are connected in series to be fed by 12 VDC. The second grid of the tubes is connected to the filament interconnection. (6 VDC)
The result is really not completely with valves, for the sake of simplicity I made the LO and the AF output with a FET, but that is also possible with EF97 if desired.
If  one likes to use a loudspeaker it should have an impedance of a few hundred ohms and  the 3K9 resistor in the AF output should be replaced by a lower value, the volume is sufficient. Than it is advisable to use a variable resistor for the source resistor in order to adjust the volume.
In order to avoid the search of IF transformers, I used small 4700 uH chokes a few mm apart, each with its 120 pF tuning capacitor. That gives an IF around 200 kHz. If the coils are the same (no different models or brands) tuning of the IF is not required and the IF band is quite surprisingly small and usable. The coils do couple magnetically when separated approx 8 mm (between centre of coils).
The input tube is also the mixer. The mixer signal is fed to the third grid in order to avoid coupling between oscillator and input coil what absolutely should result in frequency pulling.
I did tune the input and oscillator coils to the shortwave band. Using a ferrite core in the coils and an extra trimmer C at the input coil, the receiver covers 6 – 20 MHz when the range of the frequency is extended with the ferrite cores.
For shortwave use, it advised to use a tuning capacitor for the oscillator smaller that 50 pF, otherwise the tuning becomes very sharp and one needs a reduction drive or band spread capacitor to be able to tune at a station. The bandwidth is quite small.
Off course it is possible to use the receiver at MW or any frequency from Shortwave to Longwave, just a matter of coils and capacitors.
The sensitivity of the receiver is good, a 1 uV AM signal can be read.
Do not take the input coupling coil too large in order not to load the input coil too much. The input should have a high Q in order to avoid mirror frequencies to be received. I used a ratio of 1 : 10 with the coupling coil wound around the cold end of the tuning coil.
I enjoyed making this simple receiver, which has been reproduced already quite a few times.
More pics in the following pages
Regards
Nico


* EF97 12VDC shortwave superhet schema.jpg (55.42 KB, 1141x800 - viewed 445 times.)

* EF97 superhet receiver 001.jpg (39.62 KB, 780x585 - viewed 339 times.)

* EF97 superhet receiver 003.jpg (84.41 KB, 780x585 - viewed 357 times.)
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #1 on: November 11, 2018, 11:41:34 AM »

A change to the diagram. Please use 0.1 uF ceramic caps for the decoupling of cathodes, second grid and filaments / power instead of 10 nF


* EF97 superhet 002.jpg (90.8 KB, 800x600 - viewed 289 times.)

* EF97 superhet receiver 002.jpg (36.23 KB, 800x600 - viewed 255 times.)

* EF97 superhet receiver 004.jpg (84.89 KB, 585x780 - viewed 321 times.)
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #2 on: November 11, 2018, 11:46:29 AM »

As seen in the pics, the IF transformers are mounted at small vertical  pieces of PCB.
The sensitivity is sufficient to receive many stations at just a few meters of wire. Long antennes will tend to overload the receiver.


* EF97 superhet receiver 004.jpg (84.89 KB, 585x780 - viewed 282 times.)

* EF97 superhet receiver 005.jpg (63.52 KB, 780x585 - viewed 275 times.)

* EF97 superhet receiver 006.jpg (79.99 KB, 800x600 - viewed 285 times.)
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #3 on: November 11, 2018, 11:55:38 AM »

I did make a small modification at a later state. The AM detector had some distorsion due to the AVC voltage giving it a threshold. So it is better to use a separate high impedance detector for the AVC to avoid this problem and make the decoupling capacitor of the AVC not too big.
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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #4 on: November 11, 2018, 04:32:43 PM »

Nico,

Very compact and sensitive receiver!  They sure are a lot of fun to build.  Safe at 12VDC.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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