Cathode biased amps do not have an adjustable bias supply. Screen resistors connect the screen grids of the output tubes to the B+ power supply. The cathodes of the output tubes of a cathode bias circuit connect to ground through a cathode resistor and most often also through a bypass capacitor. The amount of current flowing through the output tubes is often adjusted manually by a potentiometer that fine tunes the output voltage of the bias supply. In simplest terms, this starts the flow of electrons from the cathode to the plate of the output tube, providing amplification. The bias supply provides a small negative voltage through the bias resistor to the the control grid of the output tube. In a fixed bias circuit, the cathodes of the output tubes are connected directly to ground. The output stage of an audio amplifier may either have a fixed bias circuit or a cathode bias circuit. If too little current flows through the tube, the tone from the amp will be bland and lifeless. If too much current flows through a tube, the tube may run red hot and fail prematurely.
In addition to the connection with the Beatles, the AC-100 MkI is loved for its touch sensitive response and uncompressed tonality.ĪC-100 "80-100 Watt" Amplifier - Serial 215 - At a Glanceīias is the amount of current that flows through a tube. When the Beatles returned to tour America later in 1964, John Lennon and George Harrison were also playing their guitars through cathode biased, thin edge AC-100 heads. The cathode biased, thin edge AC-100 has special historical significance as it was the amp model used by Paul McCartney when the Beatles arrived in America in February 1964.
Serial #215 had a thin edge cabinet, red control panel, a one piece logo and brown grill. The aluminum ID plate on the back panel of the AC100MkI head shown on this page displayed serial #215 (serials started at #101), making it a earlier example of the model.
The balance were equipped with a "thick edge" head cabinet made of 3/4" plywood.
Rated at ~80 watts RMS, about 125 of these AC-100 heads were equipped with a "thin edge" head cabinet made of 3/8" plywood. While JMI produced nearly 2200 AC-100 heads, only about 350 featured the original cathode biased, NFB "80-100 Watt" circuit. Given the identification number OS/036 and named the "80-100 Watt Amplifier Circuit," it detailed the cathode biased, no negative feedback (NFB) power amp circuit used in the earliest AC-100 "MkI" heads. JMI issued the schematic for the circuitry of the first generation AC-100 head on September 9, 1963.