Important Notice I just updated the BOM for the passive 2649 psu to include a revised 250 ohm Heater trimmer that will work out adjusting to 4V 100ma consumption, so if you stick a 39 ohm between pin 4-7 and you adjust the voltage to 4.0V, the initial 100 ohm value got at 4.4V at 100ma at minimum , so this pot will work for those lucky that will try an AC701 someday, keep in mind that it has been mostly calculated for higher current higher voltage 5840W tube , but this pot will cover the range better for both type tubes, Just a precaution for slowly ramping heaters on gentle tubes,

Best,
Dan,