Sounds like it's gone to air to me - it might be the PSU, but your description tells me it's more likely to be dying. It will get dimmer and dimmer and stop working altogether.

David.

There are two things that can fail in a tube - the power supply and the vacuum tube itself. If the power supply fails, it's usually fatal. It's not unheard of for it to fail partially - such as losing screen voltage - but it can happen. You can also lose PC voltage. You can also lose MCP voltage.  But these things are very rare - Usually PSU failures are binary - it either works or it doesn't.

However, when the tube fails, it will can do several things... It can burn up (emission point travel ) or get shading or it can shade over the entire tube at once ( often accompanied by severe shading to one side ) - under these circumstances, the tube simply doesn't maintain it's sensitivity anymore. You can take it out in the morning and get a great, albeit faint, image. However under darkness, it exhibits almost no gain. Also, the tube just keeps getting darker over a period of months, until no image is present.

It is this kind of fault I suspect from the OP's description.

More than likely PC poisoning is occuring also, and the tube is effectively beyond repair. If this is the case, then the power supply is salvageable, but that is all.

( though I do mention the PSU is salvageable because even dead tubes have some value on Ebay )

You'd need to take it to an expert to find out for certain, and I'm pretty sure the costs of doing that make the attempt of questionable value...

Regards

David