I have some NOS metal jacket 6L6s. Can these be used in guitar amps at all? I tried a pair in my Fender Blues DeVille (4x10 model) and they got super hot and it smelled like something was burning. Took these tubes out and put the glass tubes back in (Svetlana 6L6s). No super hot tubes or burning smell. Interestingly, the metal tubes made the DeVille sound even louder than usual. Is this par for the course with metal tubes? Should I use them at all or are they for use in audio equipment or military equipment only?