This should be doubled and closer to 12.6VDC right?
Does this mean one of the two diodes in the sel rectifier is shot?
Yep, that would be the case on the bad diode, but not on the output voltage (under load) - that is, per the manual, 11 VDC. Replace both the caps and the rectifiers with silicon. I use 1N4007 as my general-purpose replacement in these circumstance.
There is a bit more to it, however. There is much less forward-drop with a silicon diode than with a selenium diode. Per the schematic, the OEM design had the filament voltage at 11 VDC under full load. If you are getting 6.7 V from one (1) diode, that suggests you would be getting something on the order of 13.4 V if both were working - not good.
a) Install the two new diodes - mind polarity.
b) Measure the actual voltage-to-ground with the tubes in place - for the few minutes necessary, no harm will come to them.
c) Using the calculator (linked), determine the ohms and wattage required to make the correct output voltage. Go UP
to the nearest next watt. Don't go extra-heavy as that resistor will be a last-resort fuse if needed.
d) Note that you are drawing 150 MA per filament in series (per tube). This will help you calculate the resistor wattage.
I just went through this on a PAS 3x that recently crossed the bench, and has, now about 500+ hours on it since, playing in my office. http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html