The 6X6 is indeed a rare tube and goes for a pretty penny when available. The pattern is similar to the 6AF6/6AD6 tube with left and right openings and the pin-out/tech info is shown on the attached figures. This info may be useful if making a substitution. A (much more common and cheaper) 1629 will work but has a 12v heater and a voltage doubler circuit is needed – a suitable one is described in the Radio Wrinkles article at https://www.antiqueradio.org/magiceye.htm. See also https://www.nostalgiaair.org/references/articles/TheFlash/Flash04.htm, https://www.vacuumtube.com/eyetubes.htm, https://www.vcomp.co.uk/tech_tips/eye_tube/eye_tubes.htm and https://www.antiqueradios.org/gazette/eyes.pdf. The following page has photos and animations of many eye tubes ‘in action’: https://www.akh.se/tubes/eyes.htm. Several 1629’s are usually for sale on Ebay.
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