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The AC-50 Mk III utilized the classic Vox amp chassis design. 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. The AC-50 Mk III serial number imprinted on the inside of the chassis pictured above was 0788. The serial number stamped on the serial plate of the AC-50 Mk III amplifier with this chassis was 4218.
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 III 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/072 AC-50 Mk III schematic, available from North Coast Music. This article will make numerous references to the part numbers from this schematic.
The JMI OS/072 AC-50 Mk III circuit was drawn on January 7, 1965, as noted on the schematic. This date establishes that JMI Vox developed a progression of four major circuit revisions (dual input single channel, quad input single channel, dual channel tube rectified and dual input diode rectified) and three styles of head cabinets (thin edge small box, thick edge small box and thick edge tall box) for the AC-50 in just over a year.
Summary of the Preamp Circuit - The AC-50 Mk III preamp is nearly identical to the preamp used in the amp it replaced, the two channel AC-50 Mk II "tube rectified" model.
Each channel used one half of the same 12AU7 (V1 in the OS/072 schematic) as the first gain stage. The 12AU7 is a low gain tube, having an amplification factor 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 III to be a "clean" amp as this low gain circuitry limited the preamp from overdriving into distortion.
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 (C1) used between the 12AU7 plate and 470k volume control of the "Normal" channel (RV1) had a value of .022 uf. The coupling capacitor (C15) used between the 12AU7 plate and 470k volume control of the "Brilliant" channel (RV6) 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, essentially rolling off frequencies below the 680 hz. cut off point.
The JMI OS/072 schematic indicated that Vox once planned to include a "Brilliant" control panel switch to the "Brilliant" channel of the AC-50 Mk III. JMI posted this circuit modification to the OS/072 schematic on December 1, 1965. This feature was apparently never implemented into production.
The output from the initial "Normal" channel 12AU7 gain stage was then fed serially through both stages of a 12AX7 tube (amplification factor of 100), labeled on the schematic as "V2a" and "V2b." 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 "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 third 12AX7 tube (V3 on the OS/072 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.
Two EL-34 fixed bias power tubes provided about 45 watts RMS total output power. A feature unique to the AC-50 Mk III were the externally mounted bias adjustment trim pots located next to each output tube socket. The output tubes are properly biased when these bias trim pots are adjusted to provide 2.2 VDC across each cathode resistor (R27 and R28).
Diode Rectification - The AC-50 Mk III abandoned the GZ-34 tube rectifier used in the three previous versions of the model. The tube was replaced with a bridge of four BY100 solid state diodes. The power supply also included a 3h 100 ma choke, mounted inside the chassis beneath the rotary voltage selector switch.
The change from tube to solid state rectification on the AC-50 Mk III offered some advantages. Replacing a GZ-34 rectifier with four relatively inexpensive silicon diodes offered a significant cost savings to Vox. 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.
Due to their highly efficient nature, amps with silicon diode rectifiers tend to exhibit less power supply "sag" when driven hard. This tends to give diode rectified amps more clean audio head room, making the AC-50 MK III a better choice for use with bass guitar. However, it is this loss of power supply "sag" that causes the AC-50 Mk III to suffer as a guitar amplifier. The natural compression that occurs in tube rectified guitar amps is preferred by many guitarists.
Brimistor - Diodes are "instant-on" devices, they do not need a warm up period as a GZ-34 rectifier tube would. This presented Vox with a new design issue that would need to be addressed.
Tubes in a guitar amplifier prefer to be gently warmed up for a number of seconds prior to having the B+ (plate) voltage applied to their anodes. Applying the full B+ voltage may cause damage to a cold tube upon startup. The GZ-34 rectifier tube used in prior versions of the AC-50 required about twenty seconds to warm up and produce B+ voltage. This delay afforded the other tubes an opportunity to warm up prior to receiving B+ voltage. However, the diode bridge in the AC-50 Mk III needed no warm up time. The B+ voltage would be instantly applied to cold tubes.
Fender addressed this issue by adding a "Standby" switch to their amplifiers. A "Standby" switch interrupts the B+ voltage. Fender suggested the amp be turned on for twenty seconds with the "Standby" switch off to allow the filament heaters to warm up the tubes. After twenty seconds it was safe to flip the "Standby" switch to the operational position.
Vox solved the problem by installing a surgistor in line with the B+ power supply. A surgistor was a time delay switch comprised of a heating element and a set of electrical contacts mounted to a thermally sensitive bi-metallic strip. The surgistor used by Vox in the AC-50 Mk III was manufactured by the UK electronics manufacturer Brimor under the trade name of "Brimistor."
When the AC50 Mk III amp was cold, the surgistor switch contacts were "open." When the amp was turned on, the filament heaters started to warm the tubes. At the same time, the heating elements in the surgistor started to actuate the heat sensitive bi-metallic strip. It would take about twenty seconds for the surgistor heater coils to completely deflect the bi-metallic strip and close the contacts on the switch. This enabled the B+ voltage to flow to the plates of the now fully warmed tubes.