JJ Electronics T-EL34-JJ Vacuum Tube EL34/6CA7 Pentode

Why does it say matched? Does this mean if one burns out you have to replace all four?

Would these tubes do well with a marshall dsl 40 cr?

Can you use these jj electronics t-el34-jj-mp vacuum tubes for a bugera g20 infinium?

Turned my Blackstar amp on, and saw one tube glowing red, while the other is completely cold. Does that indicate a bad tube needing replacement?