AB763 Tremolo question

[G._Hoffman]

Could someone explain to me why the tremolo oscillator in the AB763 is hooked up to the negative bias power supply?  The tremolo is coupled through the opto, right?  So, we're not looking at a trem that plays with the bias.  And the trem oscillator tubes are both cathode biased.  So, what is it for?


Gabriel

[sluckey]

Some LFO phase shift oscillators are slow to start up. Using the negative bias voltage like this will quick start the oscillator when the neg. bias is removed by the footswitch.
 
The negative bias insures the oscillator will be completely turned off when the footswitch is open (or not plugged in). Closing the footswitch kills the bias voltage to the oscillator tube, allowing it to oscillate. The abrupt change from hard cutoff snaps the oscillator into operation very quickly.

[G._Hoffman]

Great, thanks.  That means I can ignore it for my new amp, which has a digital LFO.


Gabriel