Screen grid voltage.

[C.K. B]

Hello Forum. 

When I trace a beam power pentode (tetrode) and set the screen at constant 170V and my plate (and grid) voltage start at 0V, does the screen voltage start at 0V or at 170V? If the screen voltage starts with 170V, that would be bad for the tube. Can someone clarify this?

Thanks.

[Martin Manning]

I believe the sequence is that the g1 voltage is set, and then the g2 and anode voltages are pulsed simultaneously. In the case you describe, the g2 voltage pulse is constant at 170V while the anode voltage pulse sweeps from a couple of volts up to the maximum specified, and that sequence repeats for each specified g1 voltage. In the process of making a complete trace of the anode characteristic, the case where the anode and g1 voltages are near zero,  and the g2 voltage is high will occur. This condition only persists for about 1 ms, so there is no risk. 

[C.K. B]

Thank you Martin.

I'll tick the S2 current display, right Y vertical, and see what is going on.

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[C.K. B]

I did some tracing with screen current measurement. The screen current is as high as plate current while plate voltage is low. Tested is here a Russian SV83 (6P15P) pentode with screen voltage set @ 170V. Under static condition, the tube ( screen grid) would be destroyed shortly. I wonder how they did it before pulsed testers? Destroy a tube for each curve?

[Martin Manning]

Early curve tracers produced oscillograms by stepping and sweeping voltages  You can read the history of that at Phillips here: https://www.dos4ever.com/uTracerlog/tubetester.html#intermezzo