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).
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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.
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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 FeedbackLike 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.
