I've been trying to experiment with vacuum tubes and am having issues with the heater. I'm just using a simple triode gain stage using a 12AX7, with a 6.3V DC heater. According to the datasheet, the heater should draw about 300mA at that voltage. However, using a multimeter, I found that there was little to no current (less than 1mA) flowing. Also, I measured the resistance of the heater and found that it was infinite (Ohm's law led me to expect about 20 ohms). At first, I though the tube might have been burned out, so I tried several other tubes but got the same result. Any ideas what's up? Thanks in advance.