Tube Driver Schem from AMZ

[swinginguitar]

What are the circles with "VR" in them? It says to connect them together?

[Ice-9]

The points marked VR are 1/2 supply voltage and are used to bias the opamps to half supply voltage. They should all be connected together. The two 22k resistos form a voltage divider and the mid point of it will give you the correct half supply voltage.

[swinginguitar]

got it - as in Vr (big v little r)?

to make this a clean preamp, would one simply drop out the 2nd op amp (looks like the first opamp is the buffer, and the grit gain comes from the 2nd opamp)?

[panterafanatic]

The grids on the tubes are biased at zero volts, the 2nd triode looks like it's biased positively to me. If you stick a resistor from the cathode to ground this should set the cathode more positive than grid, which should result in less distortion.

As for VR, think of it like this, it's a reference voltage. It allows the signal to "float" on top of half of the power supply voltage giving you the most headroom while working with opamps. It's needed for the non-inverting to set it centered while the inverting input goes to ground. Whereas the 2nd opamp gain stage doesn't need it because the signal is inverted (flipped upside down), and you just set the non-inverting input to the reference voltage and the signal will naturally rest in between the two supply rails (voltages).