sentences:  Amps produced after the end of 1969 saw a change in circuitry, the removal of the drip edge and the rectifier was changed from a 5AR4 to a 5U4GB along with a change in bias resistor value, a "boost" pull switch to the volume control pot was added in 1977.In 1980 and 1981 the Silverface version was cosmetically changed back to the Blackface.It was discontinued in 1981.