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 Post subject: 12 Volt EF97 shortwacve superhet
PostPosted: Oct Thu 13, 2016 7:02 pm 
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Joined: Sep Tue 27, 2016 5:53 pm
Posts: 90
Location: Alhaurin de la Torre, Spain
A while ago I could purchase a number of EF97 tubes, that are designed for a anode voltage of 12V. That was (of course...) a reason to build something . So I made a simple shortwave superhet with two EF97 valves and a FET LO.
The IF transformers I made of small 4700 uH chokes at a distance of approx 8 mm centre to centre and tuned with 120 pF. That resulted in a BW of approx 20 kHz and an acceptable curve without any tuning.
The filaments are in series to get 12 Volt supply. The second grids are tied to the filament of the first valve, 6 volts, to keep it as simple as possible. Again a few resistors less.
In order to avoid LO pulling when tuning the antenna circuit, I choose to mix in the third grid of the first valve, which indeed gives a very nice isolation. The second valve is an IF amp.
In the schematic diagram, the cathode decoupling is 10 000 pF, that is an error, should be 100000 pF (0.1 uF) due to the quite low IF. Otherwise oscillation may occur.
Due to the low IF, the mirror suppression is quite poor, but when the input circuit has a high as possible Q (low antenna coupling) it is acceptable to have fun and one can choose two frequency bands by tuning above or below the LO.
The detector output has a very high impedance and should be loaded with a FET. That gives directly sufficient volume for a headphone.
I did tune the receiver at various places in the shortwave and the receiver has quite an amazing sensitivity. A tuning reduction at the oscillator capacitor is advised or mount a big knob. The tuning is quite sharp. lately I added a small AF amp from Aliexpress and a volume control to have LS volume.
At a few meters of wire, very many shortwave stations can be heard and the antenna noise is evident. Nice for kids as well to start, tubes and only 12 Volts... And when they hear stations from all over the world, it may be an iniciative to start with the radio hobby. And what is more nice for a father which is radio ham...


Attachments:
EF97 12VDC superhet schema.jpg
EF97 12VDC superhet schema.jpg [ 38.18 KiB | Viewed 2453 times ]
EF97 superhet receiver 006.jpg
EF97 superhet receiver 006.jpg [ 79.99 KiB | Viewed 2453 times ]
EF97 superhet receiver 001.jpg
EF97 superhet receiver 001.jpg [ 39.62 KiB | Viewed 2453 times ]
EF97 superhet receiver 004.jpg
EF97 superhet receiver 004.jpg [ 84.89 KiB | Viewed 2453 times ]
EF97 superhet receiver 005.jpg
EF97 superhet receiver 005.jpg [ 63.52 KiB | Viewed 2453 times ]
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 Post subject: Re: 12 Volt EF97 shortwacve superhet
PostPosted: Oct Thu 13, 2016 11:34 pm 
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Joined: Aug Tue 16, 2016 10:01 am
Posts: 68
Well done Nico, clever design, I particularly like the copper/laminate chassis and the homebrew IF transformers.

Jon.


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 Post subject: Re: 12 Volt EF97 shortwacve superhet
PostPosted: Oct Fri 14, 2016 3:56 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1489
Location: Jackson, TN
Nico,

Very creative and clever stuff, please keep your ideas coming.

W need something more than bashing eBay sellers!

Tim


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 Post subject: Re: 12 Volt EF97 shortwacve superhet
PostPosted: Oct Fri 14, 2016 8:12 am 
Member

Joined: Sep Tue 27, 2016 5:53 pm
Posts: 90
Location: Alhaurin de la Torre, Spain
Brother Jonathan wrote:
Well done Nico, clever design, I particularly like the copper/laminate chassis and the homebrew IF transformers.

Jon.


Hi Jon
I use a lot of PCB laminate to make chassis and cases. Easy to work with, strong, ground plane is ideal and you can solder a ground connection anywhere. When the outside is sandpapered and painted, it looks like a factory made box.
Attached som pics of a 80 meter AM transmitter and a tube tester for antique tubes (when I go "tube hunting") with a battery inside. Both build from laminate.


Attachments:
EL84 AM-CW zender 002.jpg
EL84 AM-CW zender 002.jpg [ 138.11 KiB | Viewed 2405 times ]
tubetester 001.jpg
tubetester 001.jpg [ 167.87 KiB | Viewed 2405 times ]
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 Post subject: Re: 12 Volt EF97 shortwave superhet
PostPosted: Oct Mon 28, 2019 7:29 pm 
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Joined: Oct Mon 28, 2019 7:11 pm
Posts: 1
Dear Mr. Nico: I'm experimenting with tube based radios, more specific: shortwave, superheterodyne, 12V, and seems your project it's the more possible to put in practice. I tried to identify the component values but they are blurred. Do you have another image with more resolution that can help identify the values? If you need I supply you an email, I can give you one. Thanks! :D


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