[Argus25]

One thing it pays to remember about germanium transistors, take the AF127 for example, they have somewhat fragile junctions a max base current for example of only 1mA and a max collector current of 10mA. On the other hand for a BC557 and many modern silicon signal types a max base current peak value of 200mA and a peak collector current of 200mA and an average collector current of 100mA, so they are very hardy to any tester protocols and tests with meters. The designers of modern transistor/fet testers often have not laid eyes on a vintage germanium transistor.

So it is worth knowing if your tester will or will not stress a germanium signal transistor.

As noted the hfe is a wildly varying parameter and good circuit design means the circuit will work with a wide range of hfe values. I would avoid though replacing a transistor type with one that has a much higher fT. For example for an AF127, its 75MHz, but for a BC557 it is 200MHz. Sometimes its possible with this for a lower frequency radio stage to go into VHF oscillation, just due to the inductance of wires and tracks if a very lively high fT device is put in there.

In addition, although you will find you can drop a BC557 in, in place of an AF117 or AF127 and it "works" does not mean it is working as well as it could. Due to the higher base-emitter voltage of the silicon type, the base and collector currents for the stage you drop it into will be lower than for the germanium case, and ideally the bias resistor should be lowered for optimum performance of the silicon type in a germanium's physical circuit.