Somewhat OT, but maybe helpful info? Tarnsistors :plain vs "A" etc.

Jeff Kruth

Hi Steve!

Way back in the early days of semiconductors, repeatability was a crap shoot. Thats why there were SO MANY early part numbers, a run would be made and they would see what they got, if not up to spec for a 2NXXXX it became a 2NXXXY, slightly different specs.So, as they got better making the :cookie dough" for the transistor recipes, the uniformity, and things like operation over extended temperature range, breakdown voltage, leakage current, etc. became better.A JANTXV 2N2221A would be the ne-plus-ultra replacement for the plain jane 2N2221.Mainly because, I assume "H"type biasing in the circuitry, so small variations in Beta, etc. are swamped out. The JANTXV device has a better "bathtub" curve and guaranteed specs.Heck, a wide variety of devices would probably work in the same slot. Early designers had to work within the wide spec variation of the devices and so made their circuits fairly tolerant. Esp. since they did not want to spend excessive time "tweaking" on the production line. Also a lot of companies had in-coming test depts, to screen the critical parts needed out from the wider variance devices. Usually marked in the manual for "selected". SO relax, you are probably in good shape with "A" devices (tighter specs), unless something is weird, which it probably isnt in a Sulzer. Man, I had SO MANY of these I got from NASA in my early surplus days. Passed them all along. Probably owned 20-25 of them.Regards,Jeff Kruth In a message dated 3/6/2021 9:35:08 AM Eastern Standard Time, writes: Though I follow the group postings regularly, I do not do as much “repair on the bare metal” as many of you do. However, I have some vintage equipment (frequency standards) that use discrete transistors. I have repaired these in the past - usually a transistor in a buffer amplifier or divider stage fails. Fortunately, these are mostly readily available: 2N2221. My question has to do with substitution. Not substituting a different transistor with similar characteristics, but “A” versions of them. This is partly due to availability. 2N2221A transistors seem much more common on the eBay market than the 2N2221. I vaguely recall that you can substitute the “A” versions of transistors for the older (?) ones with no problems. However, before I stock up on replacement transistors, I am interested in the opinions of this group’s members on this. I know this is pretty basic, but surprisingly, my various online searches on this topic always result in sites that either tell me the characteristics of the devices or offer to sell me various replacements (but not simply the “A” versions) - sometimes with very large minimum quantity orders. I have not found (yet) anything that just says, “sure - go ahead and use the “A” version of a transistor or the JAN version in place of the original”. I have used JAN transistors as replacements as they generally have similar electrical characteristics but have a much larger operating temperature range spec. Any advice will be appreciated. Steve Horii