Vox AC-100 MkII "Fixed Bias" Amp Head - 1969
Cabinet Serial #2098 - Chassis Serial # 2290 - A Look Under the Hood

AC100 MkII Chassis - Amplifier Serial #02098, Chassis Serial #C2290
Part numbers shown above reference to AC100 MkII Schematic - OS/167

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The AC-100 MkII Amplifier Circuit
JMI produced about twenty two hundred AC-100 heads from 1963 through 1969. About four hundred of the earliest AC-100 amps were constructed with the cathode biased "80-100 Watt" output circuit dated September 1963. These were replaced by the fixed bias AC-100 "100 Watt Amplifier" circuit in May 1965. The AC-100 fixed bias circuit was again revised in June 1965, renamed the "AC100/2." The final printed revision of the fixed bias AC-100 circuit was penned in July 1967 and covered primarily post JMI amps produced through late 1969. The amp was now known as the "AC100 MkII." At chassis serial #2290, the amp shown on this page may be one of the last AC-100 amps produced.

AC-100MkII Serial 2098 - At a Glance
  • Chassis Serial #C2290, produced near the end of production
  • Fourth generation schematic: VSEL Drawing No. OS/167
  • Chassis mounted heat shield behind EL34 output tubes
  • Ground shield beneath input jacks
  • Gray control panel
  • Transformer driven fixed bias supply
  • No negative feedback
  • 100 Watts RMS Output Power
  • Diode rectified power supply
  • Four individual main filter capacitors with bleeder resistors
  • "VSP" rotary style mains voltage selector
  • "Top Boost" tone control circuit
  • High gain 12AX7 phase inverter circuit
  • Thick edged head cabinet with black diamond grill
  • One pin corners, Vox handle, small one piece Vox logo

  • The AC-100 MkII Chassis
    The chassis design of the JMI Vox AC-100 Mk II combined a pressed steel horizontal base with a vertical assembly made of aluminum.

    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. Later AC-100 chassis, such as the one shown on this page, had a three sided metal shield to isolate the output transformer from the heat generated by the output tubes.

    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 (see photo above).

    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 (eddy currents) from the power supply and output amp section of the amp. This served to further electronically shield the preamp from the power supply.

    The chassis serial number was stamped on the aluminum section of the chassis above the speaker terminal strip (see photo at left).

    Fixed Bias NFB 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. While the earliest AC-100 amps had a cathode or "self biasing" output circuit, the AC-100 MkII had a fixed bias output circuit.

    The cathodes of the output tubes of a cathode bias output 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 gain and frequency of the input signal with the cathode and screen resistors.

    In a fixed bias output circuit, the cathodes of the output tubes are connected directly to ground. A bias supply provides a small negative voltage to the control grid of the output tube. In simplest terms, this bias voltage starts the flow of electrons from the cathode to the plate, providing amplification.

    The fixed bias supply in serial AC-100 #C2290 was unlike that installed in prior AC-100 amps. Earlier fixed bias AC-100 amps tapped the one half of the B+ power supply as the source for the bias circuit (re: schematics OS/036 and OS/167). Zener diodes were then used to regulate the -210VDC B+ bias voltage down to -35 VDC. In contrast, AC-100 serial #C2290 was built near the end of production. It used one leg of the center tapped 6.3 VAC filament heater winding as the source for the bias supply. A small step-up transformer mounted to the tube heat shield (see photo at right) increased the 3.15 VAC supply from the filament tap to a voltage appropriate for the fixed bias circuit.

    To date, I have not found any factory documentation for this unusual fixed bias circuit. The final documentation for the AC-100 circuit appeared on Vox Sound Equipment Ltd. schematic #OS/167. This schematic included revisions to the AC-100 schematic through December 1969 but did not include the fixed bias circuit shown on this page. This opened the possibility that AC-100 #C2290 was producd in 1970.

    The change in 1965 from a cathode biased to a fixed biased output stage at last allowed the AC-100 to live up to a 100 watt RMS rating. While the cathode biased AC-100 actually produced just 80 watts, the fixed bias AC-100 amplifier chassis featured on this page tested at nearly 125.

    No Negative Feedback
    Like the AC-30, the output stage for the AC-100 MkII did not incorporate negative feedback (NFB). Amps with NFB take a small amount of the output from the output transformer and direct it back into the preamp to improve fidelity and reduce distortion. Amps without NFB, such as the Vox AC-30 and AC-100, offered higher gain and a smoother audio transition from clean to overdrive.

    Diode Rectified Power Supply
    The AC-100 MkII power supply circuit included a power transformer, a rotary voltage selector, two fuses, a bridge of four BY100 silicon diodes, a CZ4 Brimistor, four filter or "smoothing" capacitors and a 19H 100 mA choke.

    The primary, or input side of the power transformer had five taps. When combined with the control panel mounted rotary 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 360 VAC 595 mA winding powered the B+ circuit. One center-tapped 6.3 VAC, 6.5A winding powered the tube filaments for the EL-34 output tubes while a second 6.3 VAC, 2A center-tapped winding powered the preamp tube filaments and indicator lamp. 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 without a tube rectifier. The AC-100 utilized a bridge of four BY100 diodes for B+ rectification. The remaining AC ripple in the B+ power supply was smoothed by a 19H 100 mA choke that was straddled by two pairs of 200uf filter capacitors (C15 through C18). Each pair of filter capacitors was wired in series to yield a 100uf capacitance. An internally mounted 1A fuse (FS2) protected the B+ power supply from damage caused by internal short circuits.

    The schematic for AC-100 MkII called for a Brimar CZ4 "Brimistor" in the B+ power supply. A Brimistor was a thermally sensitive resistor encased in a heating element. It was designed to guard against a potentially harmful "switch-on" current surge in the amp. The Brimistor was mounted to the top of the chassis base.

    The electrical resistance of the Brimistor was highest when the amp was cold. This allowed the Brimistor to have the greatest effect of limiting "in-rush" current as the amp was first turned on. As the amp warmed, the heating element inside the Brimistor caused the resistance of the thermally sensitive resistor to drop. After 30 seconds, this drop in resistance allowed the Brimistor to pass the full operating B+ voltage to the circuit.

    As was so often the case, the Brimistor originally installed in AC-100 serial #2290 failed and was removed from the B+ power supply. It was replaced by a wire that bridged the terminals that once held the Brimistor (see photo at left).

    Preamp Circuit
    The preamp section of the AC-100MkII included two tubes, V1 and V2.

    The input jacks connect to V1, a 12AU7 (ECC82) tube. The 12AU7 was a dual triode tube with a gain factor of 17. This means that either of the ECC82 triodes had 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 had a gain factor of 100, meaning they 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 feeds one side of the push-pull output amplifier. The output tubes then connect to the primary side of the output transformer where the original and inverted signals emerge as a single amplified signal. Tube V3, a 12AX7, powered the AC-100 phase inverter circuit.

    Earlier AC-100 heads utilzed a 12AU7 tube in the phase inverter circuit. As explained above, the 12AU7 would amplify the signal seventeen times but the 12AX7 would increase the signal one hundred fold. The change from a 12AU7 to a 12AX7 in the phase inverter circuit contributed to an increase in output power.

    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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