@chubaka What are rails and what to they do?
Good question. In a Solid state amp there are generally equal positive and negative supplies centered around ground. A 100 watt/8 ohm amp will typically have 50 Volts plus and minus supplies. We call those the rails. It is where we draw the current to send, via the output transistors. to the speaker. The rail voltage will determine the power of the amplifier. 75 volt rails will give you 200 watts and 100 volt rails 400 watts.
Rails were not much talked about until the advent of the SS power amps that were direct coupled to the speaker. In action the positive rail pushes the speaker cone out while the negative rail pulls it in. Again the output transistors determine how much which determines what the speaker reproduces.
In a tube amp we have several power supplies of different voltages and currents. We call the main high voltage B+, a term that goes back to early 1900s radio. A tube amp may also have a negative bias supply, filament supply, driver supply, lots of supplies in tube amps.