Vox AC-100 Mk I "Cathode Biased" Head - 1963-65
Serial #430 - A Look Under the Hood

AC100 MkI Chassis - Amplifier Serial #0430, Chassis Serial Altered by JMI from #1084 to #1135
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," this schematic detailed the cathode biased power amp circuit used in the earliest AC-100 heads produced by JMI.

JMI produced about 350 cathode biased "80-100 Watt" AC-100 MkI heads from late 1963 through early 1965 before a redesigned, fixed bias version of the amp was introduced in mid 1965. The aluminum ID plate on the back panel of the AC100MkI head shown on this page displayed serial #430 (serials started at #101), making it a later example of the model. Serial #430 had a thick edge cabinet, gray control panel, one pin corners, a one piece logo and black grill.

In addition to the serial found on the aluminum model ID plate on the back of the cabinet, JMI also stamped a separate and unique serial number on the chassis. The chassis serial number for AC100MkI #430 was originally stamped #1084, but JMI obliterated this number, changing it to #1135 (see photo above).

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.

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 "80-100 Watt" AC-100 MkI circuit. Two 330kΩ bias resistors (R20 and R21) are located on the upper tag strip as part of the phase inverter circuit.

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. If the lower tag strip has four matching resistors and capacitors, it is a cathode biased amp.

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

Diode Rectified Power Supply
The AC-100 "80-100 Watt" power supply circuit included a power transformer, a rotary 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 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 350 VAC winding powered the B+ circuit. One center-tapped 6.3 VAC winding powered the tube filaments for the EL-34 output tubes while a second center-tapped winding powered the preamp tube filaments.

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 (M1 - MR4) for DC rectification. AC-100 amps with the "80-100 Watt" circuit were normally equipped with a horizontally oriented 10H 250 mA Woden choke (p/n #79213). JMI changed to a vertically oriented Woden 19H 100mA choke (p/n #76854) in 1965, as noted on the later schematic for the AC-100/2 amplifier. The "80-100 Watt" AC-100 chassis shown above was built in 1964 and had the early Woden 72191 power transformer but the later 76854 19H Woden choke.

The choke was straddled by two 100uf filter capacitors (C18 and C19) 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 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 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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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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