If you don't need as much gain as the 5687 delivers( u=~16 ), the 12B4 is my next option. It takes a second 9-pin socket, and ought to have 6 or 7, instead of 2 LED's to bias. That's about *IT*.The heaters will take ~25% more curent, at 0.6 amp instead of 0.45 amp of the 5687. Both will run at 12.6 V.
I offer this as an alternative to the gymnastics a lot of ForePlay owners go through getting a reasonable amount of volume knob sensitivity. We're not stuck with 12AU7's so there is no need to act like it at the design table.
Putting in a second socket later oughtn't to be too difficult. Take out the valves before drilling more holes. Reserve a bit of real estate with a Sharpie....
regards,
Douglas
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