Using a pentode as converter (the first stage of a superheterodyne receiver = mixer / oscillator) was quite an old idea and since ~ 1933 used in thousands of radio models.

For commercial and military receivers the pencil tubes development was forced and I would like to introduce the 5840 (European type: EF732) with a circuit of my own design as a one-tube-radio for local and regional AM BC stations. The goal was to listen via loudspeaker (if other noises around are low, because it’s not very loud…), not with headphones. Some years later this was possible with only one transistor, as I had shown in another topic some weeks before.

We have about 100V DC for the anode/plate, so it’s a must to use only one Lectron block for safety. Inside this block are a couple of parts from an old transistor radio. The 100 kΩ trim-pot (not 10 kΩ!) is used to adjust for best amplification, oscillation and a minimum of distortion. IF amplification is missing (no problem, several radio models worldwide didn’t have this stage) und the detector (germanium diode) leads the AF back to the penthode grid-1 for additional amplification. The output transformer has to be of higher impedance (12…20 kΩ), so we use the loudspeaker block of the 4-tubes-superhet again.

A power supply has to deliver 6.3V AC (175 mA, filament) and 100V DC. Many years ago I built this power transformer (size M48B) and wrapped all the isolated copper wires by hand – an exhausting job…

Because no AM radio stations are active on day I used the home-brew AM modulator as a transmitter with very low signal to adjust and test the unit. It’s a good example for minimalism usage of radio parts… One line of the filament voltage and the (-) of the anode voltage is the common “ground”.

The antenna wire is usually 2…4 ft – that was enough when a local station was active in earlier years.

Best,

Günther

• This topic was modified 5 years, 2 months ago by Guenther.
• This topic was modified 5 years, 2 months ago by Michael.