Vintage Amplifier Service Manual - SOUND INC model MB6P3, MB6P30, MB6R4 & MB6P6 Sound Plus 20-tube 4-channel preamp amplifier (2-power stages) with AM radio tuner.  Manufactured by Sound Inc., of Chicago, IL. Dated 1948.  10 pages. Original old Sams Photofact manual includes schematic, layout, alignment instructions, voltage & resistance test chart, repair parts list and description. This manual would make a sensational accompaniment to your retro audio equipment.