This amplified metronome was found in a 90s Popular Electronics. The circuit is powered by a 9 V battery but will also work with 6 V of 4 standard batteries.
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
This old oscillator using germanium transistors has a high impedance headphone as inductor to...
This preamp with several inputs using germanium transistors from the time was found in Japanese...