JMI Vox AC-50 Mk II Dual Channel, Tube Rectified Amplifier Chassis
"A Look Under the Hood"

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The metal portion of the AC-50 Mk II chassis was adapted from the AC-50 Mk I. The base of the chassis was made of formed steel, supporting the transformers and power tubes. A vertical piece of aluminum, bent into the shape of an inverted "L," was bolted to the steel base and to the tops of the transformers to support the preamp and control panel sections of the amplifier.

This unique Vox chassis design yielded many benefits. The steel base section of the chassis provided the strength and rigidity to support the heavy power and output transformers. The output tubes were mounted vertically to this steel chassis base, facilitating proper ventilation and providing an excellent heat sink for the tube sockets. Locating the hand wired preamp circuitry on the inside of the grounded vertical aluminum structure electronically isolated the sensitive, high gain preamp circuitry from stray electronic fields eminating from the transformers and power amplifier circuitry. The chassis was structurally rigid and not easily susceptible to damage.

The chassis was bolted to a 3/8" thick seven ply baltic birch slider board, allowing the chassis to slide in and out of the cabinet on rails like a drawer.

The AC-50 Mk II featured two channels, "Normal" and "Brilliant." Both channels featured "Top Boost" circuitry, Vox parlance for amps with individual treble and bass controls. The circuit diagram can be found on the JMI OS/053 AC-50 Mk II schematic, available from North Coast Music.

The AC-50 Mk II was the third generation of the AC-50 circuit design. JMI developed a series of five AC-50 circuit revisions (dual input single channel, quad input single channel, dual channel tube rectified, dual channel diode rectified and high gain dual channel - diode rectified) and three styles of head cabinets (thin edge small box, thick edge small box and thick edge big box) in the period between 1964 and 1967.

Summary of the Preamp Circuit - Each channel used one half of the same 12AU7 (V1) as the first gain stage. The 12AU7 is a low gain tube, having an amplification factor (mu) of only 20. It appears that Vox chose this tube for the first position in the AC-50 preamp to keep the preamp gain at low to moderate levels. This suggests that Vox was trying to design the AC-50 Mk II to be a "clean" amp as this low gain circuitry limited the preamp from overdriving into distortion.

The "Normal" channel had slightly less gain and a bit more headroom than the "Brilliant" channel. This was due to different bias points chosen by Vox for each half of the 12AU7.

Even though the first gain stage of the "Normal" and "Brilliant" channels share the same 12AU7 tube, the channels are voiced differently. The coupling capacitor used between the plate of the 12AU7 and the 470k "Normal" channel volume control had a value of .022 uf. The coupling capacitor used between the plate of the 12AU7 and the 470k volume control of the "Brilliant" channel was 500 pf.

The .022 uf capacitor used in the "Normal" channel was large enough to allow the entire audio spectrum to pass. However, the 500 pf silver mica coupling capacitor in the "Brilliant" channel acted as a high pass filter, gradually rolling off frequencies below a 680 hz. cut off point. A 100 pf treble bypass capacitor wired to the load and wiper terminals of the "Brilliant" channel volume control further accentuated high frequency response, especially at lower volume levels.

The output from the initial "Normal" channel 12AU7 gain stage was then fed serially through both stages of a 12AX7 tube labeled as V2. The output from the initial "Brilliant" channel 12AU7 gain stage was fed serially through both stages of another 12AX7 tube, labeled on the schematic as "V3." The first side of either 12AX7 was used for gain, the second side powered the "Top Boost" tone control circuits.

A third 12AX7 tube (V5) served as a phase splitter, dividing the preamp output signal into positive and negative components to supply the push/pull EL-34 output amp stage.

Power Amp
Two EL-34 fixed bias power tubes provided about 45 watts RMS total output power.

To extract maximum audio power for the AC-50 Mk II, Vox pushed the "B+" or plate voltage on the EL-34 output tubes to 480 volts. While high by today's standards, this plate voltage was still within the "safe operating range" for the Mullard and Brimar tubes Vox originally installed at the factory in these amplifiers. However, many modern EL-34 tubes will not tolerate plate voltages that exceed 400 to 425 volts. Installing EL-34 tubes that are not rated to tolerate the 480+  plate voltage of an AC-50 Mk II can create premature failures.

A single 250k pot, located underneath the chassis, adjusts the bias for the EL-34 output tubes. The schematic for the amp indicates that the amp is properly biased when the pot is ajusted to provide -32 to -40 volts.

Power Supply
Tube Rectification
- The AC-50 Mk II retained the power supply designed for the AC-50 Mk I model. It included a power transformer with input taps for five mains voltages (115, 160, 205, 220 and 240 VAC), a series of 32 and 16 uf 500 volt filter capacitors and a 8H 100 ma choke. A unique feature of this power supply was a small neon lamp that helped to regulate and stabilize voltage and current in the bias circuit for the output tubes.

The 115 volt tap on the AC-50 transformer was intended for the US market. Unfortunately, JMI didn't understand that the US mains voltage wasn't 115 volts, it was 120 volts. While a five volt difference from 115 to 120 VAC on the primary side of the transformer might not seem a big deal, it will result in a nearly 5% voltage increase in the secondary. This pushes the B+ plate voltage to over 500 VDC. Some in the US (including myself) have overcome this issue by setting the control panel voltage selector to 240 volts and connecting the AC-50 Mk II amp to an external 120 to 240 volt step-up transformer.

This tube rectified power supply exhibited a sag in voltage when driven hard. The audio compression that is a natural byproduct of this momentary voltage sag in the power supply is preferred by many guitarists.

The AC-50 Mk II was one of the last amp models introduced by Jennings Musical Industries with a tube rectified power supply. Most of the 1966 Vox UL series and all of the 1967 "Solid State" series (Conqueror, Defiant and Supreme) featured diode rectified power supplies. After 1966, Vox did not introduce another amp with a tube rectified power supply until the Korg/Vox era, when the AC30TB/TBX arrived in 1994.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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