Vox AC-100 Mk I "Cathode Biased" Head - 1963-65
Serial #215 - A Look Under the Hood

AC100 MkI Chassis - Amplifier Serial #215
Part numbers shown above reference to the JMI "80-100 Watt Amplifier Circuit" schematic OS/036

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The 80-100 Watt Amplifier Circuit
JMI issued the schematic for the circuitry of the first generation AC-100 head on September 9, 1963. 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.

While JMI produced nearly 2200 AC-100 heads, only about 350 featured the original cathode biased, NFB "80-100 Watt" circuit. 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. The balance were equipped with a "thick edge" head cabinet made of 3/4" plywood.

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. Serial #215 had a thin edge cabinet, red control panel, a one piece logo and brown grill.

The Beatle Connection
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. 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. In addition to the connection with the Beatles, the AC-100 MkI is loved for its touch sensitive response and uncompressed tonality.

AC-100 "80-100 Watt" Amplifier - Serial 215 - At a Glance
  • First generation schematic: JMI Drawing No. OS/036 "80-100 Watt Amplifier" dated 9/9/1963
  • Red control panel
  • Cathode biased output stage, no negative feedback
  • ~80 Watts RMS Output Power
  • Diode rectified power supply
  • Plug style mains voltage selector
  • "Top Boost" tone control circuit
  • 12AU7 phase inverter circuit
  • Thin edged head cabinet with brown diamond grill, no corners, Vox logo handle and small one piece Vox logo

  • Cathode vs Fixed Biased Output Stage
    Bias is the amount of current that flows through a tube. If too much current flows through a tube, the tube may run red hot and fail prematurely. If too little current flows through the tube, the tone from the amp will be bland and lifeless.

    The output stage of an audio amplifier may either have a fixed bias circuit or a cathode bias circuit. In a fixed bias circuit, the cathodes of the output tubes are connected directly to ground. The bias supply provides a small negative voltage through the bias resistor to the the control grid of the output tube. In simplest terms, this starts the flow of electrons from the cathode to the plate of the output tube, providing amplification. 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.

    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. Screen resistors connect the screen grids of the output tubes to the B+ power supply. Cathode biased amps do not have an adjustable bias supply. Bias is self-regulated by the relationship of the input signal with the cathode and screen resistors.

    JMI dropped the cathode biased AC-100 head in June 1965 in favor of the fixed bias OS/036 AC100/2 Amplifier circuit.

    The photos near the top of the page reveal that four 270Ω cathode resistors (R26, R27, R33 and R34) and four cathode bypass capacitors (C13, C14, C15 and C16) are located on the lower tag strip of the AC-100 MkI circuit. Four 1kΩ screen resistors (R28a, R28b, R32a and R32b) were mounted between the lower tag strip and pin five of the output tube sockets.

    As output tubes have to draw quite a bit of current to bias themselves, cathode biased amps such as the AC-100 tend to be a little less powerful and generate more heat than fixed bias amps. For this reason, the AC-100 Mk1 produces about 80, not 100 watts.

    It is easy to spot if an AC-100 head has the cathode bias circuit by looking through the ventilation screen on the bottom of the amplifier (see image at left). If the lower tag strip has four matching resistors and capacitors, it is a cathode biased amp.

    The AC-100 Mk1 Chassis
    The chassis design of the JMI Vox AC-100 combined a pressed steel horizontal base with a vertical assembly made of aluminum. The design of the chassis was adapted from the AC-15 and the AC-30.

    The steel base of the chassis supported the power supply and output amp sections of the AC-100. The use of steel in the chassis base not only provided strength but also electronically isolated the high voltage (and hum producing) section of the power supply and output amp from rest of the amplifier. The power transformer and output transformers were located at opposite ends of the chassis base to provide proper balance. "L" brackets mounted to the top of the power and output transformers strengthened and supported the vertical aluminum portion of the chassis.

    The sockets for the EL34 power tubes were suspended in a tray below the top of the steel base. This improved air circulation inside the cabinet and provided additional clearance between the cabinet and the tops of the power tubes.

    The vertical aluminum section of the chassis enclosed the preamp circuitry and secured the control panel. Aluminum is even less likely than steel to pick up hum and oscillations from the power supply and output amp section of the amp. This served to further electronically shield the preamp from the power supply.

    Diode Rectified Power Supply
    The AC-100 "80-100 Watt" power supply circuit included a power transformer, a plug style mains voltage selector, two fuses, four BY100 silicon diodes, two large filter or "smoothing" capacitors and a 10H 250 mA choke.

    The AC-100 "80-100 Watt" chassis was equipped exclusively with a Woden 72191 power transformer. The primary, or input side of the transformer had five taps. When combined with the control panel mounted plug style voltage selector, these taps allowed the AC-100 to accommodate the various mains voltages throughout the world. A 3A control panel mounted fuse (FS1) protected the primary side of the power transformer from current surges.

    The secondary side of the power transformer had three windings. A 350 VAC winding powered the B+ circuit. One center-tapped 6.3 VAC 6.5A winding powered the tube filaments for the EL34 output tubes while a second 6.3 VAC 2A center-tapped winding powered the preamp tube filaments. This separation of the preamp and output tube heater windings is quite unusual. It virtually elminated any chance of interaction between the preamp and output amp stages. I cannot suggest another guitar amplifier that used this dual heater design.

    The AC-100 head was the first tube amp design from Vox that did not include a valve rectifier. The AC-100 utilized a bridge of four BY100 diodes (MR1 - MR4) for DC rectification. AC-100 amps with the "80-100 Watt" circuit were normally equipped with a unique horizontally oriented 10H 250 mA Woden choke (p/n #79213). This choke is easily spotted through the upper ventilation grill and is a certain indicator that the head is a first generation "80-100 Watt" AC-100 Mk I amplifier (see photo at right).

    The choke was straddled by two twin 50uf filter capacitors (C18 and C19) wired in parallel, yielding 100 uf on each to smooth the AC ripple in the 430 VDC B+ power supply. An internally mounted 1A fuse protected the B+ power supply from damage caused by internal short circuits.

    Preamp Circuit
    The preamp section of the AC-100 "80-100 Watt Amplifier Circuit" included two tubes, V1 and V2.

    The input jacks connect to V1, a 12AU7 (ECC82) tube. The 12AU7 is a dual triode tube with a gain factor of 17. This means that either of the ECC82 triodes has the potential to amplify the signal input seventeen times. Vox used only one of the two triodes for the first gain stage of the AC-100 preamp, the second triode was not utilized.

    V2 was a 12AX7 (ECC83), another dual triode tube. The triodes of the 12AX7 have a gain factor of 100, meaning each will amplify the signal input one hundred times. In the AC-100 circuit, the first triode was used as an additional preamp gain stage. The second triode powered the "Top Boost" tone controls (shown at left), identical to the circuit in top boosted AC-30 amps.

    Phase Inverter
    All amplifiers using a "push-pull" circuit design, such as the AC-100, need to have a phase inverter circuit. The phase inverter converts the audio signal from the preamp into two equal but opposite waveforms. Each waveform from the phase inverter feeds one side of the push-pull output amplifier. The output tubes then connect to opposite sides of the primary of the output transformer. The original and inverted signals emerge from the output transformer as a single amplified signal. Tube V3, a 12AU7, powers the AC-100 phase inverter circuit.

    Four-Pin XLR Power Cable
    The AC-100 utilzed a detachable power cord that featured a standard regional AC plug on one end and a four-pin female XLR connector on the other. While this may not be the case on your amplifier, Vox normally connected the "Neutral" or white wire from the power cable to pin one of the four-pin XLR plug. The "Hot" or black wire was connected to pin four of the four-pin XLR plug. The ground (green) wire was connected to pins 2 and 3 of the four-pin XLR jack.

    WARNING - There is no way to know if your Vox amplifier will match this wiring scheme. Please consult a trained professional service technician for assistance with the power cable. The Vox Showroom accepts no responsibility for personal injury or damage to your amplifier from this information.



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