[R-390] OT: Leakage testing a 1L6

[Scott, Barry (Clyde B)]

Can someone give me some general advice for checking leakage in a
low-voltage tube?  I have a Zenith Transoceanic with 1L6 for the converter
tube.  It isn't working properly and checking the control-grid voltage shows
zero volts between cathode and control-grid.  This should be approximately
-5 volts.

I checked the voltages to the cathode and the control-grid with respect to
B- and they show the same voltage (approximately 2.7 volts).  I began to
suspect something with the tube and put it in my Hickock 752A.  One of the
"Leakage Test" positions showed a definite reading.  This particular test
position checks (among other things) cathode to first grid.  I assume I have
a bad tube.

The thing that puzzles me is I don't get a reading between cathode and
control-grid with my ohm meter.  Is this to be expected?  Can it be the case
where the cathode must be powered up in order to see a short (leak?) here?
I know some heaters are known to sag when hot and touch other electrodes but
I figured in a 1.4 volt tube, this wasn't very likely.  Is the leakage
something other than one electrode touching another?

By the way, one of the symptoms the radio displays is that it works (rather
weakly) for about 30 to 45 seconds and slowly fades into nothingness.  I
sort of assume this is heat-related and the leakage is "building up" after
warmup, but I'm just guessing here.

Thanks for the OT help!

Barry - N4BUQ