RCA Model SHF-3

I have an RCA New Orthophonic High fidelity console in which I have installed two Model SHF-3 amplifiers. Does any one here have acces to a schematic or in the very least the tube complement? Both amps are working each being dedicated to their perspective Right and Left channel signals however the one I use for the right channel has developed a hum. It is only audible at high volume levels leading me to believe the problem is in the preamp stage. I'm seeking information on which replacement tube I will need. (I can't make out the writing) I have had a magnavox in the past which used 12AX7 for the preamp but I wanna be sure as this has become my absolute favorite listening device and i expect to listen to it til I'm dead. Any and all advice is welcome. i'm a newbie in the realm of tube Hi-Fi so please forgive my ignorance. Sincerely, Ted Nimmo (audiophile559@yahoo.com)