Electronic – Need help reading a schematic, is this a 220k log b pot

Given that you've said R7 is set to 0 ohms, the two fixed terminals of R6 are shorted together. When the wiper is at either extreme, its resistance to either fixed terminal will also be 0 ohms. When the resistance is 50%, both halves will be in parallel. Given that this appears to be a 50 k pot, we'd expect

$$R_{eq}=\frac{R_{max}}{2}||\frac{R_{max}}{2}=\frac{R_{max}}{4}=\frac{50000}{4}=12500$$

just as you measured.

The first tube is an anode gain stage bootstrapped with the signal taken on the cathode of the next stage. This is a positive feedback that tries to keep the current flowing trough R5 constant. This turns R5 to a virtually huge impedance load as seen by U1A. This is good because the ECC83 has a high internal impedance (rounded to 60k) so it needs a high load not to loose gain.

Since the current flowing through the tube becomes nearly constant, it compensates the fact R3 is not decoupled which results in a dramatic raise of the stage amplification. Using this bootstrap method you can bet it will give a gain between 80 and 100.

The direct coupling between the anode gain stage and the cathode follower is also a good thing in a guitar amp since, depending on the bias of the cathode follower, its grid may steal current from its neighbour causing nice H2 distortion.

Maybe a solution for being able to switch between high gain and low gain, would be a switch in the bootstrap loop of the second pair of ECC83. The filters behind the first couple of triodes will decide the output level of the amplifier.