The tank coil and cap I was talking about is the larger one on the topside next to the tubes. In post 18 the first picture shows the cap meshed as would be expected and the second picture shows it set to minimum. Did it tune that much differently with the new tubes? A tube issue could cause this so unless you're certain I wouldn't go in there tinkering with the tank circuit. If you do be prepared to work your way though it if things go south.
I wasn't sure if that was just getter material. Some tubes had it on the side and some on the top. I've saw the same manufacturer do it differently.
I think the tube in the lower left corner of the photo near the tuning cap is your bad tube.
The dark color is metal being deposited to glass by the arc. JMHO
I wasn't sure if that was just getter material. Some tubes had it on the side and some on the top. I've saw the same manufacturer do it differently.