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Given the explanation of the B+ source that bob posted, if B++ is right then PS diodes should not affect the B+ .
It looks like B+ is obtained by a balance of net currents and voltage drops through V-2,4,6, 10a 11A, and 12 (if I'm reading correctly). Basically B+ is created by a voltage divider created by the impedance of each tube connected to it....It is like the audio output B+ divider/dropper concept I thought it was, on crack (ie they took it too far).
To figure out the low B+ issue you are pretty much going to want to examine the biases of all the tube sections it feeds look for bad tubes, and bad passive components. You will want to look for bias voltage changes in tubes that are not immediately explained by changes to the B+ rail.
Tubes shown in Bobs diagram between B++ and B+ will be conducting too little and or tubes shown between B+ and the Common B return/b- will be conducting too much.
SS sets that use this type of multi-stacked parallel B+ (I hear 2 way radios do this a lot) can be a b!tch to troubleshoot.
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