These transistors are prone to failure. One particular failure mechanism is the growth of tin whiskers from the tin-coated inside walls of the transistor case.
In most cases, germanium transistors can be replaced with modern general-purpose silicon bipolar transistors with no modification to the circuit.
Reasonable silicon replacements for small-signal NPN and PNP germanium transistors are 2N3904 and 2N3906, respectively.
When transistors appear in pairs in the original circuit (differential amplifier, Schmitt trigger) change the pair, not just one transistor.
When a germanium transistor is used with its base-emitter junction DC coupled in parallel with a germanium tunnel diode, you cannot simply replace the germanium transistor with silicon. It will not work because the "high state" voltage across the tunnel diode will not be enough to turn on the transistor. If a germanium transistor in this type of circuit fails, you either have to find a working replacement germanium transistor or modify the circuit to work with a silicon transistor.
- Tin whiskers inside AF114 transistors / Detailed analysis presentation − NASA Electronic Parts and Packaging (NEPP)]
- 321A 2N2207 transistor replacement
- Thread on tin whiskers in OC170/OC171 @ vintage-radio.net / Detail pictures