Have you tried contacting Brent Jessee or Jim McShane? Those are to NOS tube experts. They should be able to help you.
MFY Tungsol 6550 tube old versions
In my ARC REF 3 I 'm using a Tungsol 6550 tube with the following code written on the metal base 322NU 3 , it is a smooth grey plate , without holes in the plate .I know 322 is the factorycode of Tungsol and normally the 322 is followed by eq 63=year 45=week 3= shift ?.But I have seen also 322HN-3 and RM-3 .Is there any list of these codes ?
Any help would be wonderfull
Any help would be wonderfull