Transistor pseudo replacement for the PL81 tube
The circuit shown here could be used instead of the PL81 in its standard configuration according to the data sheet as a replacement in old TV receivers or video monitors.
However, this was not a full replacement of the PL81. The circuit worked just in such standard applications and not in all possible with this tube circuits. The 56 ohm resistor useless converted heating power into heat so that the heater line of the TV receiver was not interrupted. Thus, the circuit, connected to a Noval plug, could be plugged into the tube socket for the PL81. Of course, the BF459 needs a heat sink.
It would be interesting to investigate how such a circuit or a similar one behaves as a single-ended Class A amplifier or as a push-pull amplifier for audio applications. Since the vertical power amplifier is strictly speaking nothing else like an audio amplifier, it should actually work.