Problem with tremolo on a twin reverb

[John Loque]

Hello,
i put a new optocoupler in a twin reverb recently and the tremolo was working back. But today, i try it and it does not work anymore. It does not oscillate.
So i had to re-think the oscillator principle, and actually i don't anderstand what makes it start oscillating. Why this -50 VDC wire near the grid ? Why this 2M2 resistor value?
Thanks...

 

[Rikaro]

The negative bias voltage is applied to both grids and puts both triodes in cutoff, thus stopping the oscillation.
The pedal switch kills the bias voltage by shorting it to ground. This starts the LFO.