IIRC from my days in TV repair many years ago the green stripe was on Matsushita/Panasonic tubes. However, that was several hundred bottles of Scotch ago....
I think the Euro-made tubes have the blackish ring that looks like deposited graphite. The green ring may be just a coat of protective paint over a similar stripe of carbon. If I feel like mutilating one of my 18GB5s, I might consider digging into one of the tubes with the green ring to see if something else is hiding underneath.
PL500 is one tube that has such black/silver ring on the outside ... but not all , and doesn't seems to be a problem with the ones that don't have . That ring can flake off in used tubes , and again no obvious problem .
It is something minor ... the chemical decomposition of the glass , if true motive , is not visible on them .
I was always thinking that the ring is like a shorted turn , preventing some possible oscillations .
It is something minor ... the chemical decomposition of the glass , if true motive , is not visible on them .
I was always thinking that the ring is like a shorted turn , preventing some possible oscillations .
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This topic has been discussed before on this forum. In this post it was mentioned that the EL502/PL502 has the ring connected to its pin 9: https://www.diyaudio.com/forums/tubes-valves/201375-graphite-ring-sweep-tubes.html#post2798581
The Mazda Belvu datasheet for the EL502 says about this: "Afin d'eviter des instabilites de balayage dues a des charges statiques sur le verre, brancher la broche no 9 (bague argentee exterieure) a la masse du chassis."
That means something like: "In order to prevent instabilities of lines caused by static build up on the glass, pin 9 (outer silver ring) must be connected to chassis ground."
The Philips datasheet for the EL500 contains details about the use of the ring. If a metal ring or screen is placed around the tube, it should be earthed and should make galvanic contact with the silver ring on the bulb.
The Mazda Belvu datasheet for the EL502 says about this: "Afin d'eviter des instabilites de balayage dues a des charges statiques sur le verre, brancher la broche no 9 (bague argentee exterieure) a la masse du chassis."
That means something like: "In order to prevent instabilities of lines caused by static build up on the glass, pin 9 (outer silver ring) must be connected to chassis ground."
The Philips datasheet for the EL500 contains details about the use of the ring. If a metal ring or screen is placed around the tube, it should be earthed and should make galvanic contact with the silver ring on the bulb.