I’ve seen a lot of circuits with tubes at low voltage ( max 20V ), so I wanted to experiment with one too,… and what could be more fun to play with than a radio.

I started from a short-wave radio schematic with an ECL82, in which I replaced the ECL82 with a ECC81, which I knew could work at low voltage, and did a little tweaking with the resistors, and coils.

L1 = 4 turns, L2 = ~40 turns, L3 = 4turns, in this order on a ferrite stick. ( the fun part is to play with these coils and the variable capacitor, until you hear something ).

Note, that all the values are not strict, tweak them as you see fit.

I’ve also had great results leaving aside L1, and connect the antenna by a 100pF capacitor directly in L2.

Have fun.

1. written by: Erwin van Doorn on November 25, 2013 at 12:11 am

Great experiment! However, I don’t understand the removal of the 220k resistor. Maybe it is replaced by a wire, but that way the signal is short-circuited by the 5uF capacitor.

• written by: mihai on November 26, 2013 at 6:38 pm

Thank you, and sorry about that, I cross marked it like it was removed, when instead I’ve replaced it by a lower 100KOhms resistor, as I recall.

• written by: Erwin van Doorn on November 30, 2013 at 4:05 pm

Thanks for the update. I think I’m going to try this schematic.

2. written by: prayudi on March 17, 2014 at 5:10 am

Interesting! Just wondering how did you get calculation for MW band? What if for SW band i.e 40m…?

• written by: Mihai on March 24, 2014 at 2:17 pm

I didn’t calculate. I’ve used the coils and the ferrite bar from an old radio.
The number of turns I’ve given in the article are the ones given in the original description of the schematic. For SW, I think it should be approximately the same number of turns, but instead of a ferrite bar, a small (8…10mm) coil form with an adjustable ferrite core should be used.

3. written by: Sam on August 12, 2021 at 3:30 pm

Output transformer type?

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