Care and Feeding of Vacuum Tubes (Valves)

On 03/13/2013 04:52 PM, R SMALLWOOD wrote: They can be a lot of fun. Lately, I've seen circuits using JFETs to provide a constant current load, resulting in some fairly spectacular gain factors. You can abuse the hell of of most tubes electrically--just watch the heater voltage. It was very common for amateurs to run power tubes (particularly horizontal output types because of their low cost) with the plates glowing cherry red. It shortened their life, but such stuff used to be cheap. I liked 6BG6s for that kind of thing. 6L6Gs got similarly abused. When 1625s (WWII surplus version of the venerable 807 with a 12V heater) were cheap, all sorts of things were attempted. There was even a grounded-grid modification where the beam-forming electrodes were fished out of the base (a hacksaw was handy) from their connection with the cathode and brought out separately to an unused pin. Fooling with some of the oddball circuits is also worth investigating. For example, try building an electron-orbit (Gillmorell/Barkhausen) oscillator, where the control grid is very positive with respect to the cathode and the plate is negative with respect to it. Before fancy microwave tubes, it was one way to get high-frequency oscillation. Unless you're into high-power stuff, you can get away with 250VDC or less as the DC supply. High-power transmitters get into the kilovolt+ range and can be quite deadly. --Chuck