Original poster: "Jim Mitchell" <electrontube-at-sbcglobal-dot-net> Hi, I was testing the voltage to the filament on my 813 tube in my VTTC, and I noticed that it was like 4v, when the 813 has a 10v filament! I am wondering, is this a bad thing? The filament does get hot, and the VTTC works, but it doesn't work optimum, could low heater voltage cause this? Regards - Jim Mitchell