I have three 1960's Custom Kraft tube amps. I love all of them but one of my favorites, a Model 400T came with a 6AU6 in the preamp and one in the trem socket. I've been in touch with Terry Dobbs (Mr. Valco) who doesn't recall preamps being 6AU6 from Valco ever but he stated that there were so many jobbers, it would be hard to say 100%. This amp is driven by a single EL84 and has a EZ80/6V4 rectifier. I have the little sister model from the same era based on the grill cloth and vinyl covering and cardboard like cabinet which is a single 50C5 amp (isolation trannie) a 35W4 rectifier and a 12AU6 in the preamp socket. So I'm wondering if this Model 400T is really supposed to be a 12AU6 in the preamp socket too. How can I tell if it supposed to be? What is the fidifference between these two tubes? Heater only? What would happen if I tried a 12AU6 in this socket? Anything bad? Thanks!