The Sams Tube Substitution Handbook No. 21 by Howard W. Sams & Co., Inc. MPN 21 is a vintage manual focusing on electronic tubes and their substitutions. Published by the renowned brand Sams, this handbook provides valuable information for vintage audio and video enthusiasts looking to repair or restore their electronic equipment. With detailed listings and guidance on tube substitutions, this handbook is a must-have for collectors and hobbyists seeking to preserve the legacy of vintage electronics.