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Does anyone know how to tell if an ECC83 Mullard made tube, but with different company branded label, having a date code of B7H2 was made in Aug 1967 or 1977?!! Thanks.
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Yeah, "I63" could be either, and neither would have copper grid rods. What's printed on the tube could also be definitive, especially other date codes, but the main constructional difference will be: look at the metal rod that holds up the halo. If it's straight, with the rim of the halo welded perpendicular to it, it's '67. If the rod is bent at a right angle and the halo is welded parallel to that, it's later. The halo is typically positioned closer to the top mica on the '70s too, but that's less specific.
I guess to be most precise I should say, if the getter ring rod is straight, definitely 1967. I've noticed The bent rod seems to start occasionally '68-'69, then consistently by '70s, like other constructional changes in Philips 12AX7s from a given factory, there are extended periods of overlap. So possible you'd get a bent rod in '67, but unlikely. I looked at a couple of '67 Mullards just now and both have the straight rod. Actually, the halos are already a little lower on those compared to earlier I63s.If it happens to be RCA branded ar another with screened date codes, can post that here for confirmation.
My guess would be that you'll find the straight rod, and it's 8/67.
That was a big help. Appreciate the info
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