Curve Tracer Screen Current

[BRSHiFi]

How much current should I make a HV DC supply able to supply for powering screen grids for curve tracing?

For a time period, the B+ will be below the screen. What happens here?

[HollowState]

Under normal operating conditions the screen of a pentode will draw very low current. If the plate B+ falls below the screen voltage, the screen current can increase to much higher levels. In this case the screen can become damaged by overheating from exceeding it's dissipation rating. You should not let this happen for more then a couple of seconds. Typical operating screen current is around a couple of milliampers or less. Beam power tubes can be a little higher. If you're going to use a separate screen supply, 25mA should be plenty.

Transmitting tetrodes that operate with a much wider voltage differential between plate and screen will exhibit negative screen current in many cases. (4CX250B, 4-400A & etc.) I think this is because the space charge between the plate and screen is more negative at the screen end with respect to the source. At least this is something I have witnessed many times while testing such tubes on my industrial tester. As a tube ages the screen current will tend to go positive. This may act as an indication of hard use in some circumstances.

Also when testing these high voltage tubes the screen requires a low source impedance supply. Without it the screen will want to "pull up" to the plate level. This will immediately drive the tube over the hill by drawing tremendous current and quickly damaging it if left that way. A 10K ohm power resistor across the supply (screen to ground) cures this problem. I have never seen this happen to any receiving grade power tube.

[Merlinb]

Screen current for a pentode is usually about one fifth the anode current. For a beam tetrode, about one tenth. Except of course, when the anode voltage is very low and the screen current shoots up in either case.

I'm working on a curve tracer design at the moment, and I am just going to use an LR8 for the screen supply (10mA limited), with a big 47u decoupling cap to soak up the higher pulses. But I only need it to test preamp valves and very small (e.g. table top radio) power valves. When it's finished I will try powering some bigger bottles though, and let you know when the 10mA screen supply isn't enough.

[Sorento]

Originally Posted by kingneb

For a time period, the B+ will be below the screen. What happens here?

many data sheets actually show what happens, its the dashed lines in the plate curves like in this example of a sweep tube:
http://frank.pocnet.net/sheets/093/6/6BQ6GA.pdf

[kevinkr]

The uTracer screen and plate supplies as an example can both deliver peak currents of up to 250mA. (Note this is not the continuous rating as the tracer pulses these supplies while measuring.)

Perhaps a screen supply with about 1/3rd the current rating of the plate supply might be sufficient to prevent the supply from limiting screen current inadvertently?