[Phlogiston]

Yes all voltages stable and at spec.

I know that a tube can act this way when it is old. The longer it runs the hotter the plate and grids get which can cause unwanted emissions.

The interesting thing about this tube is that it really never gets hot - ever - new in box (unless you put the box in the oven) or used in radio. AND the tube never stops working. The amp/mixer section is fine. The failure is that the osc just wont run above about 11mhz (oscillator) when hot. Performs just fine on broadcast band.

Replacement with a good tube cures the issue - but the old tube also tested good. So much for tube testers - right?

A possible moral to my story is: Be cautious when purchasing a used 1L6, or just buy new ones - until you can't anymore.

Anybody got a new 1L6?

P.S. These battery mini-tubes all can be a little flakey. While working on this I noted that the radio made a few notes immediately on turn-up, kind of like plucking the taunt dial string. I turned the radio over, with it on, to measure voltages and after 30 sec or so, the fil in the 1S5 went open. Replacement worked and no more "ting, ting, ting on start-up.