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Re: 813 VTTC Problems



Original poster: Steve Conner <steve@xxxxxxxxxxxx>


How exactly should the G2 and G3 grids of these tubes be connected to the circuit? And are they actually supposed to draw current?

G3 connects to the cathode. G2 (aka screen grid) connects to a positive bias supply and can draw significant current. 4kV is far more than should ever be applied to the screen- it's usually nearer 200-500V. Check the tube datasheet for suggested values of screen supply voltage. dropper resistor, and to get an idea of how much current the screen will draw in use.


Some tubes work OK as triodes with G1 and G2 connected together and driven as a control grid. But G1 can draw a lot more current than G2 and overheat sometimes.

If the plate supply to a tube like this is halfwave rectified pulsating DC (as some VTTC builders use) then the screen supply should be halfwave rectified too. Otherwise the screen can overheat since it draws excessive current when there is no plate voltage.

Steve Conner
http://www.scopeboy.com/tesla/