WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
It's there to HF balance the two sides of the push-pull to make up for imbalances in though output transformer. The DCR, leakage inductance, and capacitance are different from one half to the other because of the winding geometry.