Once you internalize the approach used, you'll be able to solve similar problems in the future (or avoid them entirely).
We need to re-cover the important part of the lesson: Disrupting Direct Current makes a Big POP; try to switch only AC Signal Volts to avoid popping noise.
If you want the option to parallel V1, then we want that section to be passing plate/cathode current all the time. That avoids Disrupting Direct Current, which is the idle current flowing cathode-to-plate for that section.
Instead, we want to switch the signal at the grid of the section that we will activate/de-activate.
- The section being switched needs a resistance from grid-to-ground that is always connected. 1MΩ is fine, or you could use larger if you're concerned about the input impedance when it is paralleled with the other section.
- You will already have a wire from the Input Jack to the V1 section that is always-on. Leave it in place.
- Add a switch from the always-on grid to the other V1 grid. Closing the switch adds this 2nd section in parallel to the first one.
What to do with the Plate Load Resistor? Or the coupling cap?
- I would probably use whatever plate load you typically use (maybe 100kΩ, maybe 220kΩ).
- Add a coupling cap from the switched-tube-section's plate to wherever the other-section's coupling cap goes. That way, you don't need to worry about strange interactions.
- Tweak load resistor & cap-value to taste.
You can get as-much (maybe more) gain-change by switching the cathode bypass cap on V1.
- Leave the cathode resistor always-connected.
- Connect the cathode bass cap to the cathode as-normal.
- Add a resistor (10-22kΩ, or more if you like) between the bypass cap's Negative Terminal and Ground.
- Add a Single-Pole Single-Throw switch across that 10-22kΩ resistor. When the switch is Open, gain is lower. When the switch is closed, gain is higher.
The resistor makes sure there is always a DC Volts charge on the bypass cap, so shorting the resistor to bring the cap in-circuit makes for quieter switching.