The Formula No. 5 was inspired by old tweed amps. The unique qualities of these amps were captured in the Formula No. 5. For this, 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. The chosen capicitor types enhance the midrange and burnish the treble.
Tweed amp sound
Tone circuit functions as a bright cap or treble rolloff depending on Gain.