Blackface Tremolo Circuit

[Fletcher Munson]

I'm wondering why the Fender blackface tremolo circuit has negative voltage applied to the oscillator. Can anyone help me understand what purpose this serves?

To be clear, I'm talking about the connection that runs from the junction of the bias diode/cap/pot to the 2.2M resistor. I don't have an amp in front of me to measure, but there has to be about 40-50 negative volts there. What does that do for the cathode-biased oscillator?

[Helmholtz]

THe negative voltage biases the oscillator tube into cutoff and thus stops the oscillation.
The footswitch grounds the 1M grid leak resistor and kills the negative bias, so oscillation starts.

[Fletcher Munson]

Awesome explanation. Many thanks!