Science/Engineering/Technical forums

I have an x-ray cathode heater supply that is just a variac powering an isolated 60hz 2.5VAC transformer, but rather than grounding one side of the filament, it goes through a 1M variable resistor for variable anode current adjustment.

So what I'm wondering, is how does this work. Let's say I have 6kV on the anode supply, with the current limit at max, and a current flow establishes at 100uA (a realistic scenario), this is only 100V drop across the resistor. Now, if I were to cut the limit knob to zero and set the supply to 5.9kV (which seems like it should be the equivalent), it would be drawing about 3mA.

So, quite simply, how does this work? How does biasing the heater a couple dozens of volts above ground manage to cut the current way down?

Tyler
This is used to bias the Wheinhelt electrode (Iknow that isn't the spelliing)
this electrode acts as a grid but also does useful focus work if applied right
Depending on the design, voltages from a very few tens of volts to a few hundred is all this electrode needs. No current is drawn from this electrode so the resistor drop sets the applied bias
very common in electron microscopes to stabilise the emission current from the electron gun

In the case John is correct (almost certainly and not surprising either ), looking into some old books on vacuum tube or CRT design would answer all your questions, Tyler. We have those here in our online library if you can hack the downloads -- the best one is large indeed, but there are a few with useful pictures of what's going on with a control or focus grid which can control beam currents quite nicely while draing little or no current by itself. It's a common vacuum tube design to put some resistance in the cathode, where current IS drawn to allow the control grid to be easily made negative vs the cathode. Done correctly, it's self stabilizing, as increasing the cathode current makes the voltage drop across its resistor go up, biasing the grid more negative, which cuts down the electron flow and restores the original current -- nice setup for a lot of things.

The English call vacuum tubes "valves" for a real good reason, they beat us on that terminology -- because that's what they are, valves for electron flow. And your X ray tube is most probably a triode, as John says, so pretty much all the inner workings are the same, though the end purpose is different.

This one is probably the most complete (ever) out of what we have here, but it's large (148 mb). This is where you can go look for more. Shrades beam power tubes has some nice illustrations that kind of get it across. Termans Electrical engineer's handbook is the best I've seen but we don't have it online. You can still find them used and cheap though, and they are a very good electronics education on the fundamentals that haven't changed, really. He has one of the best electrostatic electron lens sections in print. Older (pre 1970) ARRL handbooks have a lot of good stuff on tubes as well as basic electronics stuff. The newer ones are from an era where sadly, most hams don't build their own gear anymore.