Explorer for all ranges (T1098E)

This circuit was found in a Radio Craft from October 1946. Power supply not shown, and tubes are 1.5V filament types.

Variable Filter 70 to 600 Hz (CB1458E)
We found this filter in a documentation for radio amateurs from 1974. The transistors could be the...
OR Gate Using NAND (CB197E)
An OR gate can be improvised using NAND gates of a 4001 as shown here. The diodes are...
Tuned Output 100 kHz Crystal Oscillator (CB12254E)
This circuit was obtained in a 1980 publication. The transistors can be BC548 or 2N2222. The...