Before you start generating signals, it is helpful to have a steady starting point. It is handy to have 5 volts DC, or plus/minus 12, available. You can then design simple bias circuits that will allow the transistors and tubes to operate.
Some devices are designed to run on AC. Many power factor correcting power supply now operate off Full-wave rectified AC. Thyristor controlled power supplies also work off AC. A 400MHz AC power bus was used to create a theoretical computer that could use tunnel diodes.