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12-24-2021, 12:51 PM
Hi Steve
First I will re-iterate my usual recommendation that tubes under foot are not a good idea. That does not stop people from doing it, however
Most of the very tiny tubes were meant for use in harsh environments, so you have that working in your favour. Your tube pedal will likely have to rely on mains power rather than batteries, so that relieves you of some constraints, as well. The biggest electrical concern with tubes is the heater power required. since it literally is just "wasted" as heat. Of course, without that heat the cathode would not get to a temperature that allows liberation of electrons and for the tube to function, so the heat and power for the heater is a necessity.
The plate voltage often does not have to be too high. If you want "tubey" voltages, use an AC wall-wart for power and then internally use a reverse-wired small PT. You can get up to 200V using the smallest devices. The amount of plate power required for one or two gain stages is low, as you likely won't even be at 1mA per plate. The small PT will restrict the voltage conversion, which you will see is not the usual factor of root-2. This has to do with the basic design of tiny PTs compared to larger ones. If you use something greater than 10VA, then root-2 magically works in this application and the raw Va can be about 3200Vdc.
As far as selecting circuit values, you will see examples using quite low resistances and extremely high resistances. Since the tubes are optimised for lower plate voltages, and often for high-frequency use, lower values are what the tube expects and this is reflected in the values that arise using the plate curves to test or find load lines. Fortunately with tubes there are generally no "wrong" circuit values as they are very forgiving and robust. This leaves it open for you to begin with intuitive values and go from their, taking an empirical design route, measuring results, or listening, and making value changes then re-evaluating the new circuit.
Have fun
First I will re-iterate my usual recommendation that tubes under foot are not a good idea. That does not stop people from doing it, however
Most of the very tiny tubes were meant for use in harsh environments, so you have that working in your favour. Your tube pedal will likely have to rely on mains power rather than batteries, so that relieves you of some constraints, as well. The biggest electrical concern with tubes is the heater power required. since it literally is just "wasted" as heat. Of course, without that heat the cathode would not get to a temperature that allows liberation of electrons and for the tube to function, so the heat and power for the heater is a necessity.
The plate voltage often does not have to be too high. If you want "tubey" voltages, use an AC wall-wart for power and then internally use a reverse-wired small PT. You can get up to 200V using the smallest devices. The amount of plate power required for one or two gain stages is low, as you likely won't even be at 1mA per plate. The small PT will restrict the voltage conversion, which you will see is not the usual factor of root-2. This has to do with the basic design of tiny PTs compared to larger ones. If you use something greater than 10VA, then root-2 magically works in this application and the raw Va can be about 3200Vdc.
As far as selecting circuit values, you will see examples using quite low resistances and extremely high resistances. Since the tubes are optimised for lower plate voltages, and often for high-frequency use, lower values are what the tube expects and this is reflected in the values that arise using the plate curves to test or find load lines. Fortunately with tubes there are generally no "wrong" circuit values as they are very forgiving and robust. This leaves it open for you to begin with intuitive values and go from their, taking an empirical design route, measuring results, or listening, and making value changes then re-evaluating the new circuit.
Have fun
