R.Measures wrote:
>>2. I let the amp run in stand-by for 8 hours to de-gas the tube.
>
>
> I have never observed that the measured leakage-current through the
> vacuum in a 4-400A, 8163, 3-400Z or 3-500Z decreases by operating the
> filament. Neither Eimac or Amperex state that such a thing occurs.
> Does anyone know where such a story originated?
Misunderstanding about the way that the chemical getter cleans up the
gas atoms that are released from the inside surfaces of a 'vacuum' tube
if it is not used for several years. (NB this is not the same as a leak
from the outside atmosphere - it happens in good tubes that are
perfectly sealed.)
If there is too much gas inside the tube, it may trigger an arc. This
might happen pretty soon after you start to drive the tube hard, or in
bad cases it can happen when you first aply B+. Trouble is, you can't
tell in advance whether you're going to get away with it, so it's wise
to take precautions when bringing an amp back into service after a long
layoff.
Transmitting tubes use a getter material that needs to be heated in
order to mop up the gas atoms effectively. In most indirectly heated
metal/ceramic tubes, the hottest location is on the cathode post, so
that's where they put the getter, and it will start to work if you run
the heater.
But the getter in the big glass tubes is on the anode, so you have to
run some anode current to heat it up. As Rich says, it doesn't seem to
be a major problem with glass tubes (possibly because glass has a much
lower rate of outgassing than most metals or ceramics). The best thing
to do seems to be to run old glass tubes up at reduced anode voltage if
possible, and if you feel there may be a risk, and not push them too
hard too soon.
(Side question: does anyone know where they put the getter in a directly
heated metal/ceramic tube like the 3CX1200?)
73 from
Ian G/GM3SEK
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