IF YOUR SUB MANUAL LISTS A SUB, GO FOR IT. THERE MAY BE SOME CONSIDERATIONS. THESE WILL APPLY MORE TO BLACK AND WHITE THAN COLOR. I WILL NOTE COLOR ISSUES.
1. DOES YOUR REPLACEMENT REQUIRE AN ION TRAP? THEY DID MAKE ALUMINIZED REPLACEMENTS FOR NON-ALUMINIZED TUBES. YOUR SUB MANUAL WILL SPECIFY THIS. SOME SETS HAD DIFFERENT TUBES LISTED FOR USE. YOU NEED TO BE SURE ABOUT WHETHER YOUR TUBE USES AN ION TRAP. YOU CAN RUIN A NON-ALUMINIZED TUBE IF YOU DON'T USE THE TRAP.
2. ELECTROSTATIC FOCUS? IF YOUR SET USES MAGNETS FOR FOCUS, YOU WOULD HAVE TO MODIFY THE SET TO USE AN ELECTROSTATICALLY FOCUSED TUBE.
3. MECHANICAL ISSUES. THIS APPLIES ESPECIALLY IF SUBBING A GLASS TUBE FOR A METAL ONE. THE SCREEN ON A METAL TUBE IS FLATTER THAN ON A GLASS ONE. YOUR GLASS REPLACEMENT WILL WORK, BUT IT MAY NOT LOOK RIGHT IN THE CABINET. YOU WOULD HAVE TO MODIFY THE MOUNTINGS. YOU WILL HAVE TO MODIFY THE HIGH VOLTAGE CONNECTION. YOUR REPLACEMENT MAY BE LONGER OR SHORTER. WILL THE CHASSIS PARTS MOVE ENOUGH TO ACCOMODATE THE REPLACEMENT (CHASSIS MOUNTED TUBES).
4. DEFLECTION ANGLE. THE DEFLECTION ANGLE ON YOUR REPLACEMENT MUST MATCH THE ORIGINAL. IF NOT, YOU WILL NOT HAVE GOOD RESULTS. THE PICTURE WILL NOT BE RIGHT.
5. SAFETY GLASS ISSUES. YOU CANNOT USE A TUBE WITH A NON-BONDED SAFETY GLASS IN A SET THAT HAS A BONDED GLASS.
6. WITH COLOR TUBES, THE SAME BASICS APPLY. THE 23" RECTANGULAR TUBES, FOR THE MOST PART ARE INTERCHANGABLE. THESE TUBES WERE USED WELL INTO THE SOLID STATE ERA, ESPECIALLY BY ZENITH. I WOULD NOT USE A TUBE INTENDED FOR USE IN A SOLID STATE SET IN A TUBE SET. THE SAME APPLIES TO THE SHARP CORNER 25" TUBES. WITH ROUND TUBES, IT'S SIMPLER. A 21FBP22 (NO SAFETY GLASS) WILL SUB FOR A 21AXP22 OR 21CYP22 WITH LITTLE DIFFICULTY. THE 21FJP22 (BONDED GLASS) ELECTRICALLY WILL WORK, BUT WHAT ABOUT MECHANICALLY? IF YOU HAVE A 21FJP22 MADE BY RCA WITH CATARACTS, THE SAFETY GLASS CAN BE REMOVED AND USED IN A SET WITH A SEPERATE SAFETY GLASS. DON'T TRY THIS WITH A ZENITH TUBE.
IF YOU WANT TO GO THROUGH ALL OF THIS, GO FOR IT. I HAVE SUBSTITUTED WITH GOOD RESULTS. THE EASIEST THING YOU CAN DO IS TO TRY AND FIND AN EXACT REPLACEMENT.