Vox AC-100 MkII "Fixed Bias" Amp Head, Black Control Panel - 1968
Cabinet Serial #1948 - Chassis Serial # 1858 - A Look Under the Hood

AC100 MkII Chassis - Amplifier Serial #1948, Chassis Serial #1858
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 September 1965, then named the AC100/2. The final printed revision of the fixed bias AC-100 circuit was made in July 1967, shortly before the demise of JMI. The amp was now known as the AC100 MkII. This is the version of the AC-100 amplifier featured on this page.

The aluminum ID plate on the back panel of the AC100 MkII head shown on this page displayed serial #1948, and was probably built in 1968. In addition to the serial found on the the back of the cabinet, JMI also stamped a separate and unique serial number, #1858, on the chassis (see photo at left).

AC-100MkII Serial 1858 - At a Glance
  • Fourth generation schematic: VSEL Drawing No. OS/167, dated 7/7/1967 through 12/4/1969
  • Chassis mounted heat shield between EL34 output tubes and output transformer
  • Ground shield beneath input jacks
  • Black control panel
  • Zener regulated fixed bias output stage, no negative feedback
  • 100 Watts RMS Output Power
  • Diode rectified power supply
  • Two dual element main filter capacitors
  • "VSP" rotary style mains voltage selector
  • "Top Boost" tone control circuit
  • 12AX7 phase inverter circuit
  • Thick edged head cabinet with black diamond grill, one pin corners, Vox logo handle and small one piece Vox logo

  • Black Control Panel
    It was reported in "Vox Amps - The JMI Years" by Jim Elyea that AC-100 amps with black control panels appeared between serial numbers #227 and #336. However, black control panels also surfaced occasionally on later AC-100 amps, such as serial #1948 shown on this page.

    North Coast Music acted as the US distributor of Colorsound pedals in the late 1990s. Former JMI lead engineer Dick Denney built these Colorsound pedals at his home in Kent. One day I had a long phone conversation with Dick Denney about the JMI days. I asked Denney about the occasional use of black control panels on AC-100 amps. He told me that JMI would temporarily switch to black control panels if they ran out of stock of the more commonly used red or gray control panels. He indicated that JMI retained a stock of black panels for short-term use to keep the flow of merchandise heading to the dealers. He also recalled that some artists preferred the black control panels so some were kept in stock for use on special order amps. Contrary to views expressed by others, there is no reason to suspect or evidence to suggest that the black control panel on #1858 is not the original installed by JMI.

    Unlike red and gray AC-100 control panels, the nomenclature on black AC-100 control panels included lines pointing to the input jacks and the pilot lamp (see picture below).

    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.

    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.

    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. An adjustable 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 of the output tube, providing amplification. While most fixed bias amplifiers include a trim pot to manually adjust the bias voltage of the output tubes, the AC-100 MkII included a unique circuit that eliminated the need for this manual adjustment.

    This AC100 MkII tapped one half of the B+ power supply to provide a 210VDC source for the bias supply. The 210 volt bias supply voltage was further conditioned by passing it through a pair of BY100 diodes, an 8uf filter capacitor (C14), two 10k resistors (R32 and R33) and a pair of zener diodes. Zener diodes have the unique property of regulating the amount of voltage that they pass. The zener diodes in the AC-100/2 were calibrated to limit the negative bias voltage to -35 VDC. This biased the EL34 output section without the need for manual adjustment.

    Like the AC-30, the output stage for JMI Vox AC-100 MkII does not use 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, offer higher gain and a smoother transition from clean to overdrive.

    Diode Rectified Power Supply
    The AC-100 MkII  power supply circuit included a Parmeko power transformer, a rotary voltage selector, two fuses, four BY100 silicon diodes, a CZ4 Brimistor, two large 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 dual element 200uf filter capacitors (C16 through C19). Each dual element filter capacitor 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.

    Despite the fact that it was included in the AC100MkII schematic, it appears that the CZ4 Brimistor was never installed in AC-100MkII serial #1948. The chassis stand-off mounting points for the Brimistor show no evidence that anything had ever been soldered to them (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 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 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, powers 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 utilized 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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