Just like old times...Very nice project.
I remember Radio Shack had lifetime guaranteed tubes, and they wouldn't replace them if they were marginal. So, I'd pop the tube into a different socket for a much higher filament voltage, let it glow like a light bulb till it died (about 1-2 minutes), and get my new tube. My father and I had a code-phrase "socket 58", which I think was for a 35w4 rectifier. If any of you folks remember Cash Electronics in Torrance, that's where I'd fry my tubes, then take them around the corner to Rat Shack.