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Re: Max anode voltage?

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>
Hi John,

 > Original poster: FutureT@xxxxxxx
 > I don't know exactly what voltage that particular tube can handle
 > (ignoring the specs I mean), but I can
 > give my results with an 833A which is rated for 4kV max plate
 > voltage.  I've used a raw AC power supply of 5kVAC on the plate,
 > and things were OK but on the edge.  Arcs began to occur in
 > the tube when I tried higher voltages.  To some degree the tank
 > load affects how much voltage the tube can withstand.
 > Individual variations between tubes can affect their voltage
 > standoff capabilities.  In some cases I may have had too much
 > voltage on the grid also which may have contributed to problems.
 > I generally run the 833A's on 5kVAC in any case.
 > If a plate lead choke/resistor is used to feed the plate, this
 > will limit the current in the event of an arc-over and protect
 > the tube usually.  Still, internal arcs are not good for the tubes.
 > For more powerful systems, a crowbar circuit can be used to
 > shut down the power quickly.
 > I've applied about 4.5kV to an 845 tube which is rated at
 > 1200 volts.  That too was on the edge.
Wow almost four times its rated voltage
With 10% more filament voltage you can probably double the saturation
So you may have been able to get 8x the power for intermittent use.
You confirm that internal arcing is one of the limits to the anode voltage.
I will just have to crank up the voltage while I check for arcs then back
off a bit.
Perhaps run the tube in for a while at it max voltage.
Robert (R. A.) Jones
A1 Accounting, Inc., Fl
407 649 6400

Hi Bob,

If you're really going to push these tubes, you can significantly increase their operating voltage beforehand by conditioning them before use. Conditioning (or "spot knocking") removes high points, whiskers, etc. that can become initiating points for arcing through field emission. The technique is sometimes referred to as "debarnacling"... :^)

Debarnacling is done with no filament power applied. HV AC (or sometimes DC) is applied through a 100k - 400k power resistor between pairs of elements. Sometimes a small (1000 pF - 3000 pF) capacitor is placed across the elements being conditioned in order to provide a controlled amount of "blasting energy" without damaging the tube. The HV supply current is carefully monitored during the process, and the high voltage supply voltage is slowly increased until sparking first occurs. The applied voltage is then slowly increased to allow sparking to condition the tube until the desired voltage is reached. Although debarnacling is most often used for 10 kV+ power tubes, it also works for smaller ones as well.

You can get all of the gory details (and lots of other great tube information) from Eimac's site:

The specifics of debarnacling are covered in Eimac Applications Bulletin #21:

Good luck,

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