We’ve captured the unique qualities of these amps in the Formula № 5. For this circuit we felt cascaded JFET gainstages yielded the best result. They have a softer sound than MOSFETs. JFETs also have much more natural attack/decay than diode clipped rigs which are pingy on the attack and fizzy on the decay. We chose capacitor types that enhanced the midrange and burnished the treble.
This versions has been modified slightly to make one of the stock resistors switchable. With the 2M2 in circuit you have "Formula No 5 mode", with it removed you have higher gain "More mode".