You can just start with the 50 watt board.
Most of these ideas should be easy to implement. Take the .68uF gain boost cap on the second 12AX7. Instead of simply soldering it across the 820 ohm resistor on that stage's cathode, you'd run a wire from the ground side of the resistor to the push-pull switch, then the capacitor goes from the other lug of the switch to the other end of the resistor. It's out of the circuit until you pull the switch.
Same with changing feedback resistors. Use a 100K on the board for mucho power amp gain. Run a wire from one end of that resistor to the switch. Run a second 100K resistor from the other side of the switch back to the other side of the board-mounted resistor. When you pull the switch out, the second 100K resistor is added in parallel to the board-mounted 100K, resulting in only 50K of resistance, more feedback into the amp, and a tighter cleaner sound.
The bright cap works the same way across the volume pot. CHeck out some blackface Fender amp schematics (which usualy also show a wiring layout) for examples of how to wire the bright cap and switch.
Lots of possibilities, LOTS. If you have enough push-pull switches you could use them to change the tone stack for different response (I'd have to wake up more to figure the wiring on that exactly.) Could be a bunch of fun!!
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Power transformer
I was thinking about the amp and a question came up - since the Laney presently runs two 6L6 tubes (and four 12AX7 tubes), will I need a different power transformer to convert to EL34 tubes? My understanding is that EL34 tubes require more (filament?) current than 6L6 tubes.
Or should I keep the 6L6 tubes, rather than convert to EL34s?
Or should I keep the 6L6 tubes, rather than convert to EL34s?