Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx> Tesla list wrote:
Original poster: FutureT@xxxxxxxIn 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 MoraJim, 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.pdfBTW, 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 -- *************************************************** We specialize in UNIQUE items! Coins shrunk by huge magnetic fields, Lichtenberg Figures (our "Captured Lightning") and out of print technical Books. Visit Stoneridge Engineering at http://www.teslamania.com ***************************************************