If you have an old TV set etc., you can take its CRT (and several other things as well, see "Where to get...") and test your own high voltage on it. The CRT has some contacts on its "neck" and one on the side for the HV input. First, you have to try out which two of the "neck" contacts are for the cathode heating (use about 6V for this and try until the cathode glows dimly red - this takes a while). Then connect your positive HV output to the separate CRT input and the other one to one of the heating contacts. A more or less bright spot should appear on the screen, depending on your voltage and output current.
NOTE: If you have a positive hv output and the negative side earthed, you can use any kind of source for the heating voltage. To make sure, you should connect the earth output to an actually earthed water tap etc. If you have a negative hv output and the positive side earthed, still connect the negative one to the heating, but you must use a battery for the heating voltage and isolate it against earth, as it's on full hv potential!
If you have a curcuit diagram of your TV set, you will (hopefully) have no difficulty using the other electrodes (such as focus). If you saved the deflection coils (on a plastic carrier on the "neck"), you can also try these to move your spot by feeding them with suitable ac currents.
Meanwhile, I DID succeed in producing X-rays with a rectifier valve. See X-ray stuff!
Every CRT emits a certain amount of xrays. These have to be shielded to protect people in front of the TV :-). To achieve this, the screen of a CRT is very thick (1cm) and made of lead glass. Therefore you won't be able to measure any xrays (with simple geiger counters) with voltages below 25kV and low currents. But you might try to get a higher current (and higher xray intensity) through the CRT; this depends mainly on what current your hv source can deliver without voltage drop. With a TV cascade, this won't probably much more than is used anyway in a TV set and therefore not enough to overcome the shielding. Also, with a too high electron beam current, the screen might become too hot locally and the glass be damaged. The other possibility is to use a higher voltage to give the xrays more energy, but with this you'll get isolation problems in the CRT.
Due to the difficulties mentioned, I haven't succeded yet, but if you do, I'm very interested!
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