low voltage on transformer secondary (schematic attached)

Hi guys, I have a question.... I am working on a Ampeg SVT bass head  and it has been worked on a lot in the past.  I noticed that it has an (extra) transformer and after scratching my head I see that it is hooked up in parallel with the secondary of the power tranny that provides voltage to the plates of 2 12bh7s and grid 4 of the 6550 power tubes. With the extra tranny (which is rectified and filtered) disconnected the voltage is about 100 volts to low, but connected it is pretty close to right on spec. Also disconnected amp barely works, but connected it sounds great. The output of the extra tranny is connected to diodes 11 and 14, the lead connecting them has been cut.  So the amp works, but I would appreciate any comments, especially concerning what could make the output of the tranny low.

Thanks,
David

My guess would be one of the windings on the original transformer is open circuit, and rather than swap the whole transformer some one came up with this mod. Without the extra transformer you are getting half wave rectified ht off one side of the tranny. Does it hum more with the extra tranny removed?

Thanks Radardoug I will test that and see.