[Adam]

So what's important is the difference in voltage from a given point in the heater string in reference to the B- (not so much the value of the actual voltage drop across the tube). And at the voltage-midpoint of the heater string, this will at most reach 1/2 the value it reaches on the ends. So 6V tube will have a 6V drop across it no matter where in the string it is, but if you measure AC voltage from one end of the heater of the tube to the B-, it will be greater if the tube is towards one edge of the string than towards the middle, and the increase in that voltage difference would be the cause of greater hum.

It sounds great with just the one 1N4007 I have in there, but I'm tempted to try this and see what happens. Maybe over winter break, as I'm running out of free time as the summer is coming to an end. I want to take it apart again anyway to touch up the alignment and see if I can improve the sensitivity on AM.