This is a SUPER clean amp. It was recently retubed with NOS SILVERTONE tubes...I didn't realize there were any still available. At first I thought they might be some sort of JJ or Ruby tube made with NOS bases or something silly like that but it looks like they might actually have been made by RCA: http://www.vintageamps.com/plexiboard/viewtopic.php?f=13&t=83905


