There are several modern EL84 amplifiers which have wiring violations on pin 1 of one or more EL84 sockets. NOS EL84s have internal connections between pins 1 and 2 where modern EL84s do not. Sometimes designers of EL84 amps assume that pin one is a safe location for a "tie point", a place to connect a grid resistor and the signal wire from the phase inverter. This will result in a NOS tube placed in such a socket furiously "red plating" and the amp humming. If run this way for long enough, damage to the EL84 tube and/or the component connected to this socket will result.
These amps can be modified by disconnecting and moving this junction off of pin 1. Best options and methods will vary from amp to amp with non PC mounted sockets being the easiest to modify. Contact a qualified amp repair technician to do the work.
Known amps with this problem:
Lexicon Signature 284
Laney VC30
Fryette Memphis 30-Only applies to outer 2 tubes on units with serial numbers (last 4 digits) 0141 or lower.
Crate V18
At least one Mesa amp (but definitely not all), which I can't recall,has this situation.
No problems with:
Bad Cat, Matchless, Dr Z, Fender Blues and Pro Jr, Vox, Gibson Goldtone GA15s, Mesa Lonestars and other amps will work fine with NOS EL84s.