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 Tubes are not the Same and a Mystery Solved.

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Join date : 2010-11-22

Tubes are not the Same and a Mystery Solved. Empty
PostSubject: Tubes are not the Same and a Mystery Solved.   Tubes are not the Same and a Mystery Solved. Icon_minitimeMarch 6th 2019, 5:58 pm

A recent comment in a current post stirred up something folks should remember.
 Some tubes vary greatly in bulb style and are not interchangeable, even it the first part of the number is the same.
Example using a 6A8:
 6A8G is a tall glass envelope with shoulders.
6A8GT is a short, straight sided glass envelope
6A8M is a short metal tube (M= Metal).
6A8MG is a bit taller metal tube ( MG? what the heck is that? MG = Metal Glass. Discontinued after a very short period).

 Often grid leads are too short to replace a G with a GT or M/MG style.
 I have made "grid lead extenders" from dead tubes and old grid caps. Often the extra length in the circuit can cause oscillation, or lack of it.
Of further note:
 When replacing a M/MG style tube with a G or GT style, be sure to add a shield, and make sure it's grounded. The metal tube already has the case grounded.
 Now to further confuse the issue: When replacing a G or GT tube with a metal one, many mfg. would use the pin on the socket that is the grounded case on the M/MG style, as a tie point in the radio, as it was not connected to the tube. Plugging in an M/MG style can damage the radio.
 This can really be a nasty surprise for you.  I had been repairing radios for several years when I experienced this, as no one had told me about this "fun" self induced problem. Until then I always had replaced the tube with an exact same type and designation, but an exact one was not readily available.

Also enter the "entertaining" world of WW2 and the Korean War..
 Tubes became in very short supply, (Even though you might have a black market connection), and it was often impossible to find the proper tube. Subs were done with other pin configurations, and a totally different tube used.
 I ran into this about the 3rd year I was working on radios where one tube was not the correct one in the set. installing the correct tube failed to make the radio work.
 If the "repairman" had just bothered to put a note in the cabinet about the sub, all would have been fine. I would have known what to do. They often made circuit changes, some really drastic, to make the set work.

Enter the MR tube. Yup MR= Material Replacement suffix. Ie: 6J5MR
 These were supposedly "out of spec' tubes the military rejected, but were available for civilian use. Often an entire batch "failed" the "Mil Spec" test, and were branded MR on the tube and box. I have quite a number of these in my collection. The story I heard was the gov't would let perfectly good tubes be branded MR and released to "keep up the morale of the civilians".

 I remember the owner of a radio shop telling me about during the war that some kids  brought in a handful of used tubes and sold them to him  They were all bad when he tested them. Later he went out back of his shop and realized they had been pulled from a set he put out in the pile to take to the dump. Mad Evil or Very Mad He got taken pretty bad on that scam.

 Now also in WW2 in the U.S. the radios of "Possible Enemy Aliens" (Japanese in U.S. concentration, AKA "Internment Camps"- Yeah you probably never heard about these camps. Often they are "forgotten" in the new, "revised" P.C. history books for the schools.), had their radios confiscated and a mandatory disabling of the short wave bands.  This was to prevent "Spies from receiving instructions from Japan".
 I have run across 2 of these sets over the years. One had the coil just disconnected from the band switch, and the other had the coil completely removed.
 Something to look for if the SW band on an old set is not working. It drove me crazy trying to figure out what had happened, until I read an article about this in a WW2 era radio servicing magazine. A mystery solved!
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