I recently got hold of a motorized roller inductor that is made PROPERLY. This is a MIL-SPEC unit that has been on stock for many years. It features a silver foil that is rolled on to a ceramic former, pretension, a shortcut cylinder that prevents eddy currents and arcing on the unused coil section under QRO operation. Due to that the unused coil is shorted and that the shorting is not a single turn, that the foil is wide, this coil has a very high Q.
I just got myself a new roller inductor. This one is designed on the principle of a silver foil that is rolled away from and onto a ceramic form with guides for the silver form. The ceramic coil forms the coil. There is a large shorting cylinder that the unused silver foil is rolled onto. The effect of this is to significantly increase the Q of the inductor. For high power QRO applications there may either be arcing from the end of the unused part of the coil or heat loss in this part of the coil. How the unused coil is completely shorted with an inner conducting cylinder and the unused part of the coil has no flux thru it.
PA0FRI and EA6AFJ has made some cool low power HF tube amps with the old and thrusty PL519. Here is a picture taken by EA6AFJ.
This is actually a nice amplifier that should be quite easy to build for the experienced engineer and experimenter. The benefit of using lower anode voltages is that the tank circuit capacitors can be of a low cost variable type. The PSU can also be integrated more easily in the same cabinet as the amplifier circuit. You can check out more information over at PA0FRI’s webpages: http://pa0fri.home.xs4all.nl/Lineairs/Frinear150/fri150eng.htm