[Milsurplus] "Blue Glow" mystery.

[Richard Brunner]

If the blue glow is within the tube, that's gas.  If the blue glow is on the 
surface inside the glass envelope that's electrons hitting the glass, and 
indicates a good vacuum.  When a tube is "cut-off" that does not mean there 
is absolutely NO electron flow.  As the tube approaches cut-off, it 
approaches it as a limit, and the current is reduced to a very small amount, 
maybe a few mils or microamperes.  Electrons may also find unintended paths 
around tube parts uncontrolled by the grid, etc.

As an extreme example, early radar modulators used big triodes, passing 
amperes for a few microseconds with a tube voltage drop of maybe 1500 volts. 
and a few milliamperes when cut off at 15 kv.  The result was the tubes 
dissipated almost as much power when cut-off as when conducting.

X-Rays need much more voltage drop, 10 kv comes to mind for soft X-Rays. (of 
course I'm an expert.....) You also need the right kind of target/anode.

Richard, AA1P


>>    The dark blue glow that seems to be right at the glass
>> is caused by x-rays from the tube.  Its commonly seen in
>> tubes like audio output tubes with fairly high voltage on
>> the plates. Some older tubes had a black coating on the
>> inside of the glass to stop it.
>
> That's my big question and point.
> You are of course correct about the blue glow,
> but why is this one happening in a cut-off tube
> with no current flow, then disappearing when
> current is flowing?  That's opposite of what
> I've seen before.