[Mondial]

The bottom 6F6 gets its grid drive signal from the screen grid of the top 6F6. If you look carefully at the schematic, the screen of the top 6F6 is not bypassed and is fed B+ through a 3500 ohm resistor. With this connection, an audio signal is created across this resistor, which is inverted in phase with respect to the top grid signal. In effect the top 6F6 screen acts as the plate of a triode phase inverter stage and the .01 uf cap couples this inverted signal to the lower 6F6 grid.

As far as the popping sound, it seems that you may have a low frequency oscillation or "motorboating". Check to see that all the electrolytic filter caps are connected correctly with respect to ground and polarity. Note the 10 uf bias bypass cap has its Positive terminal connected to the chassis.