Why Different Bassman Circuits?

[TheSpinDoctor]

Fender has produced a variety of Bassman amp circuits over the years. This question is specific to 50-watt amps. The Normal channel seems to have less noise than the Bass channel. The AB165 and the AA 371 circuits have the same number (2) of gain stages for both the Bass and Normal channels. The AA864 and the 6G6-A circuits have a third (and fourth for the 6G6-A circuit) 7025 gain stage only for the Bass channel. The power amp section used 6L6GCs for the AB165, AA371, and AA864 amps. The 6G6-A used 5881 tubes. What benefit was achieved by the additional preamp gain stage?

 

[LetItGrowTone]

A bass doesn't need more voltage gain than a guitar (and they weren't into high gain), but maybe they felt it needed more compression, also incidentally provided by more triode stages.

(That fourth one in the G6G-A, V1b, isn't for voltage gain but for current gain to drive the tone stack, as you probably already know.)