The Tremolux was the first Fender amp with a built in effect, tremolo.
[1] In 1957, the rated power output of the Tremolux was increased to 18 watts when it changed to a fixed bias design and a 5U4 rectifier.
Its rated power output was increased to 20 watts, its output tubes were a pair of 6BQ5/EL84 (the only Fender amp of that time to feature them) and they were operated at voltages in excess of their rated maximums and under fixed bias, but was soon changed to 35 watts and a pair of 6L6GC power tubes with a GZ34 rectifier (circuits 6G9-A and 6G9-B.
)[2] There were 10", 12" and 2×10" speaker cabinet configurations (the 2×10" being the majority of amps produced).
In 1964, the cream tolex was replaced with black tolex with silver grillecloth, and the circuit was changed to AB763, which used an optoisolator to couple the tremolo effect, replacing the earlier direct bias-modulation circuits.