Need help adding gain overdrive/control to my tube guitar amp design
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.