This two-transistor circuit is from a December 1964 Radio Electronics. Modern transistors can be tested. The headphone must be high impedance or crystal with a 2k2 resistor in parallel.
This protection circuit for standing waves was found in a manual of high frequency circuits and...
This high input impedance amplifier stage was found in a 1963 documentation. The transistor...
This setting appears multiple times on the site from different sources. This is from a 2000 manual...