[NT]

On Apr 11, 2:33*am, "Phil Allison" wrote:
"Roger Jones"

( snip long story)

** The EL84 lost vacuum.

First, at low air density the tube will conduct DC heavily from plate to
cathode via ionised air *- *causing a loud hum, accompanied by a purple glow
from inside the glass.

The heater current goes up as heat is now being dissipated into the
surrounding air keeping the temp low and hence resistance low.

As the tube fills with air, there will be no more conduction but the glass
will get hot by convection of heat from the cathode.

If you cannot see any crack in the glass, there may be a loose or corroded
pin that let the air in around it.

... *Phil

Heaters glow in vacuum, but not in air, since air conducts heat away.


NT