Thanks, Ian. That's a very clear and concise description of the issue.

This may be overkill, but since it costs nothing I'm thinking of four
six-hour cycles, allowing the tube to cool to room temp or so between
cycles.

Since you're here, Ian, I'll slip in an unrelated question. What do you
think about using regulated DC for the heater on an 8877 ? I ask because
I'm thinking of using a couple of spare controller I/O pins and a
home-made R2R network to ramp up a series-pass regulator as part of the
overall controller startup.

On the other hand, I suppose a relay and appropriate power resistors
would be simpler... ;-)

Thanks,
Jim, N7CXI

Ian White GM3SEK wrote:
ken scharf wrote:

As for the flash over in those ceramic metal tubes, maybe some trace
radioactive elements in the ceramic has something to do with this?

No, it's just the result of very slow outgassing from the metal and
ceramics.

To handle this problem, all vacuum tubes are constructed with some kind
of 'getter', a chemically activated surface that combines with stray gas
molecules and keeps the vacuum clean. In small glass tubes, the getter
is the silvery metal film (usually barium) deposited on the glass, but
such metals are too volatile for use in transmitting tubes.

The getter used in transmitting tubes is a metal such as zirconium or
tantalum, which needs to be hot in order to function. In large glass
tubes the main getter is typically the dull grey coating on the outside
of the anode, which is intended to run very hot. In addition, there are
often secondary getters in cooler locations such as the grid and
heater/filament structures. However, the anode of a metal/ceramic tube
is always quite cool, so the main getter has to be in the hottest place
available, typically on top of the cathode post.

All this means that transmitting rubes need to be *used* in order to
keep the vacuum clean. Tubes that haven't been used for a while may need
to be fired up with heater/filament only, to give the getter a chance to
function before high voltage is applied. Cooling is also required, of
course. How long this takes will depend on the individual tube, but 24
hours is usually enough to reduce the risk of high-voltage flashover.


Just to be clear, a leaky seal is a completely different problem.
Outgassing is typically worst with brand-new tubes, but will eventually
settle down. In contrast, a leaky seal is continuous. Although the
getter may be able to handle a very tiny leak for a while, the leak will
eventually win.