Some modern guitar amp manufacturers have used Pin 1 on their EL84 sockets as "tie points" for connections. This scheme will not allow NOS EL84s to work properly. Some of the amps known for this situation are: Hiwatt DC40, Lexicon Signature 284 and Mesa 50 Caliber. The Laney VC30 seems to have been wired incorrectly as well. If you know of others let me know.

In order for NOS EL84s to work in these amps, the connections from pin 1 of the EL84 sockets must be removed.

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 with no modifications to pin1 but should still be checked for idle current/power. The only way to avoid checking this is to order a "low current", AKA, "cool" pair.