What are rectifier tubes?
Tube amps require DC (direct current) to work. However, as you know, the power we get from the wall is AC (alternating current). So, a device known as a rectifier is utilized within the power supply of an amp and its job is solely to convert AC into DC - hence the name "rectifier." In most modern tube amps a solid-state device known as a diode does this “rectification”. However, a few "older" amps use a tube as their rectifying device. We happen to Vox and Marshall that makes at leat four (4) of them: the Vox AC30 (GZ34 rectifier tube) AC15 ( 5Y3 rectifier tube), the re-issue JTM45 (GZ34) and the 1962 Bluesbreaker combo (also aGZ34).
Now, as the guitar's signal never actually passes through this part of the amp's circuit, one would think that a rectifier tube has no direct effect on the sound of the amplifier… well, that's the theory anyway! The truth however is different. The thing is, the rectifier tube has a direct bearing on the power supply tube for the other tubes and, due to its relative inefficiency (compared to a solid-state rectifier), it can and will “sag” when the demand for power is higher than it can handle. And, as it happens, a lot of players like the sound and feel that results from this "inefficiency." To hear and feel this "sag" in action, you've got to crank it up and start attacking those strings! Once you've experienced this in action, you'll fully understand why it is called "sag."