Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>
Tesla list wrote:
>Original poster: FutureT@xxxxxxx
>In a message dated 2/10/07 8:45:03 PM Eastern Standard Time,
>tesla@xxxxxxxxxx writes:
>
>>Hi Cameron,
>>
>>John's site (futuret) has pictures of a (3) 833a VTTC sparking wildly
>>powered by a 7500volt Pole pig. Tip: Bert Hickman told me there is a way to
>>"ping" the tubes with a 10KV adj HV supply to increase their voltage
>>tolerance.
>>
>>Jim Mora
>
>Jim,
>I tried that "spark knocking" or "debarnacling" method on some
>833A's to try to increase their voltage handling capability. It turns
>out that the ones I tried had an internal spark gap section which
>tended to arc at a relatively low voltage. I think I tried some other tubes
>also. I used an HV supply that went up to 60kV or so. In any
>case I was not successful in raising the voltage standoff capabilities
>of the tubes. The HV supply I used was 60Hz AC. I don't think
>the desired arcing (slight "ticking") occurred while debarnacling
>until I reached 35kV or more.
>The 833A's can withstand somewhere from 5400VAC to
>7200VAC depending on loading, plate redness, grid drive, etc.
>I never run them over 5400VAC these days. Some 833A's
>can withstand more voltage than others. Grid to filament
>arcs are an ever-present danger too. Debarnacling should
>be able to help to prevent that also, I suppose.
>John
John and all,
"Conditioning" is a technique used with high voltage/high power
vacuum tubes and particle accelerators to carefully eliminate high
E-field points that would otherwise initiate vacuum breakdown and
(potentially damaging) arc-overs. By using an adjustable HV supply
and a limited amount of energy (typically via a small HV capacitor
charged through a high impedance source), microscopic protrusions
("barnacles") can be safely removed from the tube's various internal
elements without otherwise damaging the tube. For vacuum tubes, this
is done with no filament power applied, with the stressing voltage
applied between various tube elements. Stressing can be performed
using either an AC or DC voltage source. Since field emission occurs
from the negative-most electrodes, using AC insures that all stressed
electrodes will be conditioned.
By slowly increasing the voltage during the conditioning process, the
device can be brought to the point where it can safely operate
significantly higher than the rated voltage (15% above, and sometimes
higher) to insure no spark overs at the rated voltage. This is
standard operating practice for high power ceramic transmitting
tubes. A conceptually similar conditioning process is used for HV
ignitrons and thyratrons. A good paper on the subject ("Conditioning
of Large Radio-Frequency Power Tubes", can be found on Eimac's site
as Application Note #21:
http://www.cpii.com/eimac/PDF/AB21.pdf
BTW, about 2 weeks before our December Lichtenberg production run,
the 150 kW electron beam accelerator at the facility we use suffered
a catastrophic vacuum loss when the hot titanium exit window got
sprayed by from a water leak. After extensive repairs, the
accelerator tube required extensive conditioning. This involved
considerable controlled sparking (at lower beam currents) so that it
could eventually operate properly at the desired operating voltage
(~5 Million volts) at full power. The conditioning process took well
over a week to complete...
Bert
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