The Vox AC-50/4 Mk VI "High Gain" Diode Rectified Head

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The clean and brilliant guitar tone produced by Vox
amplifiers played an important role in many of the hit records produced during 1964 and 1965, the early years of the "British Invasion." During this period, the Beatles, the Rolling Stones, the Dave Clark 5, the Hollies, the Animals and U.S. group Paul Revere & the Raiders, among others, took the music world by storm. Their success helped to make Vox the dominant brand of guitar amplifiers in the world at the time.
Vox was so proud of the brilliant frequency response produced by their signature "Top Boost" tone circuitry that they gleefully proclaimed their amplifiers could produce "glass shattering treble" in a 1964 product catalog.

By 1966, however, musical tastes were evolving. The bright, poppish guitar sounds from the early "British Invasion" were being replaced by a darker, overdriven tone. This new "bluesy" tone was developed by guitar amps that incorporated a high gain preamp design. Such preamp circuitry would intentionally overdrive the amp into distortion, a concept previously avoided by Vox.

While Vox did not seem at first to respond to these changing tastes in guitar tone, upstart amp manufacturer Jim Marshall did. Marshall owned a music store in London that was producing high gain 50 and 100 watt amplifiers in the basement of his shop.

One of Jim Marshall's earliest amp customers was guitar legend Eric Clapton. Clapton and his band "Cream" used Marshall amps to redefine guitar tone with the release of the "Fresh Cream" LP in 1966. Others would soon follow suit and use high gain amps, including Jeff Beck, Pete Townshend and Jimi Handrix.

During this same period, Vox introduced their hybrid UL series in 1966 and an all solid state line in 1967. Still following the "bright and clean" design philosophy that had worked so well for JMI a few years earlier, neither of these new Vox amp series garnered much enthusiasm in the market place. To make things worse. the Royston Group, the UK conglomerate that purchased JMI Vox several years earlier was financially spinning out of control and on the road to bankruptcy. It would soon take Vox down with them.

Vox hadn't realized it yet, but Marshall would soon replace them as the largest UK manufacturer of guitar amps, a title Marshall would retain for many years to come.

It is in this context that we look at the AC-50 Mk IV, the fifth and final AC-50 design from Jennings.  

The Design of the "High Gain" AC-50 Mk IV Preamp
Aside from a higher gain preamp, the AC-50 Mk IV was nearly identical to the amp it succeeded, the AC-50 Mk III. The Mk IV circuit was found on Vox schematic OS/163, dated April, 1967.
The AC-50 Mk I, Mk II and Mk III preamp circuits all incorporated a 12AU7 tube in their preamp section. The 12AU7 is a low gain tube with an amplification factor of just 20. The low gain design of the 12AU7 helped to form the basis for a very "clean" sounding AC-50 amplifier.

These earlier two channel AC-50 amps used one half of a 12AU7 in the first preamp position of the "Normal" channel, the other half was used in the first preamp position of the "Brillliant" channel. The bias points selected for the 12AU7 in the "Normal" channel provided slighly less gain than those selected by Vox for the "Brilliant" channel. Both halves of the 12AU7 utilized a 25 uf cathode bypass capacitor.

The "high gain" AC-50 Mk IV replaced the 12AU7 tube that had been used in the first position of the preamp of all prior AC-50 models with a 12AX7. The 12AX7 has an amplification factor (mu) of 100, five times greater than the 12AU7. The use of a 12AX7 tube in the first position of the Normal and Brilliant preamp circuits increased the gain and harmonics over prior AC-50 models. However, due to the bias design of the Mk IV preamp circuit, not all of that gain potential was realized.

Both halves of this 12AX7 used identical plate load and cathode resistors, making gain equal in both channels. Unlike all prior AC-50 models, cathode bypass capacitors were not employed in either channel. By removing the cathode bypass capacitors in this first stage of the Mk IV preamp, negative feedback was increased, somewhat suppressing gain. Removing the cathode bypass capacitors probably also made the circuit a bit more linear.

From this detailed description of the circuits, I hope to make it clear that one cannot simply replace the 12AU7 in a Mk I, Mk II, or Mk III with a 12AX7 to make the equivalent of a AC-50 Mk IV.

Even though the first gain stage of the "Normal" and "Brilliant" channels share the same 12AX7 tube, the channels are voiced differently. The coupling capacitor (C1) used between the 12AX7 plate and 500k volume control of the "Normal" channel (VR1) had a value of .022 uf. The coupling capacitor (C12) used between the 12AX7 plate and 500k volume control of the "Brilliant" channel (VR6) 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 used in the "Brilliant" channel acted as a high pass filter, gradually rolling off frequencies below 700 hz.

The output from the initial "Normal" channel 12AX7 gain stage was then fed serially through both stages of another 12AX7 tube, labeled on the schematic as "V2a" and "V2b." The output from the initial "Brilliant" channel 12AX7 gain stage was fed serially through both stages of another 12AX7 tube, labeled on the schematic as "V6a" and "V6b." The first side of either 12AX7 (V2a and V6a) was used for gain, the second side (V2b and V6b) powered the "Top Boost" tone control circuits.

A fourth 12AX7 tube (V3 on the OS/163 schematic) 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. No cathode bypass capacitor was used on V3, another revision from the AC-50 Mk III circuit.

Power Amplifier
The power amp featured two EL-34 output tubes running in fixed bias mode, producing about 45 watts RMS output power.

It can be a challenge to install modern EL-34 output tubes in the AC-50. Many modern EL-34 tubes are too tall to fit in the clearance between the chassis and top of the cabinet.

To extract the maximum audio power from a pair of EL-34s, Vox pushed the plate voltage to 460 volts. This 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 safely tolerate plate voltages that exceed 400 to 425 volts. Installing EL-34 tubes that are not rated to operate at the 460+  plate voltage of an AC-50 can create premature failures.

Dual Amphenol three-pin XLR "round top" jacks were provided in a recessed rear panel for speaker connection. A speaker impedance selector allowed the choice between 8 or 16 ohm total speaker loads.

To connect to a single 16 ohm cabinet, the selector plug should be in the 16 ohm position and the speaker system could be plugged into either jack. If either one 8 ohm or two 16 ohm cabinets were
connected, the selector plug should be moved to the 8 ohm position. The AC-50 does not accept speaker loads less than 8 ohms, so pairs of 8 ohm speakers should never be connected to the amp.

A red "Warning" placard located above the speaker jacks advises that the amplifier should never be operated without a loud speaker connected.

Power Supply
The AC-50 Mk IV utililzed a slightly modified version of the solid state, diode rectified power supply from the AC-50 Mk III. While some may prefer the audio compression that is a natural byproduct of an amp with a tube rectified power supply, the diode rectified power supply in the Mk IV also offered some advantages. Diode bridges are highly dependable, reducing the potential for power supply failures and eliminating tube rectifier replacements. Removing the rectifier tube from the power supply also eliminated the heat it would have generated. Additionally, the added efficiency of a diode rectified power supply offered additional audio headroom, improving the performance of the amp for bass guitar.

The Mk IV power supply also incorporated a special time delay switch called a surgistor. This device acted as an automatic "standby" switch, allowing the tube filament heaters to warm the tubes for about twenty seconds prior to applying the 460 volt B+ supply to their plates, extending tube life. Read more about the surgistor in the AC-50 Mk III "under the hood web page.

A rotary voltage selector switch allowed the amp to accept local mains voltages of 115, 160, 205, 220, or 245 volts. This feature made it possible to accomodate the AC power standard in most countries.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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