On 10/15/23 17:13, Tony Miklos via Jukebox-list wrote:
Working on a Seeburg JL with metal cased RCA 6L6's. (don't recall seeing those before) While trying to find an intermittent very loud snap, crackle, and pop, I get a bit of a shock off the metal case of the one 6L6. Turns out the plate is shorted to the metal enclosure! The base must be a little small because the tube clips weren't grounding out the B+, although it may have been intermittently and that may have been the noise it was making. Put my meter on it and it was about 395vdc to ground, .1 ohms from pin 3 to the metal case.
Interesting! On metal 6L6s and most metal tubes the case is tied to pin 1 and this is usually grounded to chassis in the amplifier.
Double-check the base of the tube and make sure that the center locating pin hasn't broken off or been forced. You might be measuring pin 1. The construction of the tubes has an interior glass envelope that insulates the tube elements from the case.
It's also possible that pin 1 was used as a hot tie-point, but in the era of the JL, metal 6L6s were still somewhat common and I would think Seeburg wouldn't have done that.
Another possibility is that the phenolic base insulation broke down and developed a carbon track between pin 3 and the case.