This circuit is from Mullard documentation from 1969. Transistors support current equivalents. Output transistors must be mounted on heat sinks.
Found in old documentation, this simple test checks both the polarity and the existence of voltage...
Found in Radio Electronics from the 90s, this circuit can vary the frequency in small values...
This circuit can be made usng the TIP32 transistor but must use carbon microphone (not very common these...