In this video I compare a Tube and a Solid State Rectifier in the same guitar amp. The rectifier is the part of the amp that turns the AC from your mains plug into DC that can be used in the amp circuit. Tube Rectifiers are less efficient (slower) than Solid State rectifiers so they compress and produce 'sag' and a warmer more compressed tone, whereas Solid State rectifiers are faster, punchier but also colder.
2016 Pure Tone Amps Offset with 5U4 or Diode Rectifier
1971 Marshall 1960A
2008 Gibson Les Paul R9