You can use anything with the same pin configuration. All types in the EL34 "family" (E34L. 6CA7, 6550, KT77, KT88) will work in any sockets though you must keep the pairs as the same tube type. Also anything in the 6L6 "family" is fine (6L6, 5881, KT66...). You can use 6V6 but they must be JJ brand because the plate voltage is 460 volts. JJ is the only brand I know of that is designed to work at that voltage. The correct bias settings are in the manual for most tubes.