fender ?

[shooter]

I'm looking over a dual showman schematic, wondering what the logic - reasoning is to reference the trem osc to the bias voltage is, vs just ground reference

[eleventeen]

The bias volts feed is not the "reference" for the trem circuit, it serves the function of hard-shutting off the oscillator upon closing the footswitch. There are still your normal R+C bypass deals to ground under the cathodes; those form the circuit reference.

[PRR]

It is, as said, a hard shut-off; it also kick-starts the slow oscillator when you ground the -37V feed and throw a big bump into the R-C network.

[sluckey]

vs just ground reference

To add a bit more... The footswitch must be closed for the oscillator to work in that AB763 circuit. IOW, those two 1M resistors will be connected to ground when the oscillator is working.

[shooter]

thanks gentleman! 
I'm in talks to do a restoration on one, n that circuit wouldn't let me sleep well!  an MI, um, green beer and it's bedtime!

thanks again
dave