[Phlogiston]

I am working on a pile of Zenith TOs. The g500 series and newer uses the 1L6 tube while the earlier versions use the 1LA6 tube.

After restoration I noted failures in the later TOs in the upper freq bands. This is a well known problem with failing 1L6s. The interesting thing is that the failure occurs after several minutes of warm-up. At first the tube will work as an osc. in the upper bands.

Testing the tube initially on a Hickok 533 indicated tubes around 300 and 240 (GM) on the osc. section. These are not really hot, but OK. At the time of failure these readings drop to around 150 to 200 (GM). Then after cooling for a couple of minutes the reading returns to 300 and the tube will work again - for a while.

Keep in mind that the 1.5V COMMON CATHODE tube never really gets warm to touch.

Gas would not do this. The tube is loosing it's emissions, probably due to age, but, that does not explain the return to functionality.

All voltages were tested and were right-on. Fil voltage drop across the tube was 1.55V.