Seyl
If you have an old tube radio you can disassemble it and use some of it's parts The problem with tube radios (aside from them being more rare than tube organs)is that with radios you have to be able to go through and isolate the power amp from the rest of the parts. Radios have a ton of extra parts that completely ******** it up for use for anything but radio use. With the tube amp from an organ, it's already set up for instrumental input, all you have to do is pull it out and add a jack to the input.
Keep in mind though this is all theory right now. I've heard of this being done many times but I've never read a description on the process involved, but I believe it IS in fact that simple.