This receiver was found in a 1924 book by Gernsback. The circuit uses three tubes and is powered by 3 batteries of different voltages, common at the time.
Here is a simple source without a transformer for currents up to 50 mA. The voltage depends on the...
We found this circuit in an old QST magazine. The circuit is powered according to the thermostatic relay...
This simple field strength meter is from a transistor book from the 60's. The transistor is made...