IBM vacuum tubes

On Tue, 2020-06-16 at 11:04 -0400, William Donzelli wrote: Sure! I'd attach a picture, but.... On the top of the tubes are numbers printed in yellow. A couple typical ones are C392 and C152. On the side, the large IBM logo is flanked by groups of numbers, an example: 62-39 64-17 IBM 317261 188-5 MADE IN U.S.A. This is one of the more legible ones. The two pairs of numbers on the first line at the left appear to vary, the 188-5 and 317261 seem to be constant; but it's hard to be certain. The obvious part number 5965 has some other markings that are overprinted by the IBM logo and numbers. It looks like it could be: 5965 A U.S.A .. .. The pattern of dots is hard to make out because of "IBM" on top of it. The structure inside is a "long-plate" type, with three horizontal ribs. I haven't had a chance to compare the structure to any of the many old dual-triode, medium-mu tubes in my assortment. There might be some clues. It wouldn't surprise me if these were a fairly standard design with some changes to the cathode to withstand being held in cutoff for long periods of time.