Dating a fender bassman 100
The blackface head had a diode recifier, one normal channel and a vibrato channel with no reverb.
The silverface Bandmaster amps got a 5U4GB tube rectifier and the vibrato channel got reverb. Due to this the silverface Bandmaster Reverb is less powerful, has less attack and breaks up earlier than the blackface Bandmaster.
Other configurations are also possible including the Pentode, a tube with 5 active elements providing an additional amplification factor.
This cannot work in the reverse direction because the plate is not heated and cannot emit electrons.
The resulting negatively-charged cloud of electrons is called a space charge.
These electrons will be drawn to a metal "plate" inside the envelope if the plate (also called the anode) is positively charged relative to the filament (or cathode).
Transformers are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits.
This is useful in converting the voltage from a wall outlet, typically 120 or 240 volts, into a higher voltage required by tubes in tube amps .