That bias supply is unconventional (polite way of calling it weird).
They are trying to make 1 circuit do 2 jobs ... in a way that having trouble on one causes *big* problems in the other..
Bad engineering.
Main question is: will you use this in the original organ, as intended ?
In that case, connect the missing part and everything should go back to normal.
Yet, as I said, having a design that when you disconnect the preamp it kills the power tubes isn't my idea of reliability.
The 25+25VAC winding + diodes SD701 make a DC supply with unloaded voltage of 35V DC.
It's a *floating* voltage, not directly referenced to ground.
Now, if you ground it through R701 and connect the other end to some "mystery" point which sits at +15V , which "eats" 107mA, and voltage drops to 30V DC, then one end of C702 will also sit at +15V and necessarily the lower end will be at -15V.
A very "iffy" design, depending on a delicate balancing act to work at all.
Personally I would disconnect C107 "+" end from connector S701 pin 2, would ground it through another 140 ohm 5W resistor, so as to have a balanced and symmetrical load (basically I'm replacing the mystery elements at the other end of the connector) , now I will have the proper -15V bias and call it a day.
Good luck and post results.