The baffles in Leslie cabinets were constructed from hardwood prior to 1970.After that year, baffles were constructed from particle board.The silverface Bandmaster amps got a 5U4GB tube rectifier and the vibrato channel got reverb. Due to this the silverface Bandmaster Reverb is less powerful, has less attack and breaks up earlier than the blackface Bandmaster.As usual the later silverface models were modified to become cleaner and more powerful.Leslie introduced two speeds, fast and slow, in 1963 with the Model 125 Leslie. After going through a variety of names, Don Leslie officially named his speakers Leslie speakers in 1946.Beginning in 1946, all Leslie speakers were stamped with a production code.Yes of course some have the mid knob and bright switch while others don’t.All these ab763 amps with reverb has the gain stage in the reverb recovery circuit which the non-reverb amps don’t have.
The Leslie sound was prominent on many rhythm-and-blues and rock 'n' roll records.
The reverb amps will start to break up around 4-5 on the volume knob while the non-reverb amps stay clean to approx 6-7.
This is caused by the lack of gain stage in the reverb recovery circuit which contributes to tube distortion and compression in the preamp section when the amp is pushed.
Guitarists such as Eric Clapton and Stevie Ray Vaughn plugged their guitars into Leslie speakers to achieve the Leslie effects. The speakers have production codes stamped on them, as do the speaker cabinets.
In addition, a number of other clues provide helpful information. Foil CBS tags indicate that the Leslie cabinet was built after 1966. A typewritten metal tag was used from the mid to late 1970s. Another visual clue is the material used to construct the baffles.