NOS 6U5 Tube fail?
 

My Zenith 7S363 has started developing problems since being electrically restored. I noticed the, formerly bright, 6U5 tube start to noticeably dim. I know these tubes are known to deteriorate but this is unusual to me. Anyone have ideas?

Was the resistor replaced in eye tube socket? It generally affects the opening and closing of the eye, but stranger things happen. Also check the B+ voltage. The set might appear to perform properly, but the B+ could low.
I have the very same set. It needs a new grill cloth and the cabinet can use a little attention. The tuning eye is a little dark as well.

Discussion on brightening eye tubes here..
http://www.nostalgiaair.org/Forums/M...0/M0066090.htm

More discussion here:
http://radiomuseum.org/forum/lightin...eye_tubes.html
As the P-1 phosphor used in these early eye tubes is subject to Columb ageing, the trick is to increase the target voltage for increased brightness *while* reducing the target current to reduce the rate of ageing.
jr

I appreciate the replies to my message. While I've purchased an additional tuning tube and also have a questionable spare I will take your advice seriously. New tubes are no longer manufactured and building a solid-state substitute would be difficult. I believe that a solution based on your advice is possible as does my friend/technician.