The Vox T.60 Head - A Look "Under the Hood"



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JMI produced three iterations of the Vox T.60 head. The earliest version was detailed on a schematic entitled "A62/030 - Circuit Diagram for Amplifier Model T60," introduced in October 1962. It went through several modifications that resulted in a revised circuit design, the "A/027 T60 Amplifier
Circuit," released in January 1963. A final version of the T60 schematic, "OS/062 T60 Amplifier Circuit" included several more circuit refinements and covered production from July 1963 through June 1966. The T.60 bass amp was replaced in 1966 by the Vox UL430 and UL460 amplifiers.

A62/030 Circuit Diagram for Amplifier Model T.60 - October 1962 - January 1963
Preamp Stage
- The initial preamp gain stage was powered by two OC44 germanium transistors noted as TR1 and TR2 on JMI schematic A62/030. A third OC44 germanium transistor labeled TR3 provided gain recovery after the tone controls. The preamp section also included resistors R1 through R17 and capacitors C1 through C8. This all solid-state preamp section was hand-wired onto a portion of the tag strip mounted to the inside bottom of the chassis.

Driver Stage - The driver stage included a fourth OC44 germanium transistor (TR4) which acted as a "predriver." TR4 was mounted to a portion of the hand wired tag strip not occupied by the preamp circuit. An OC35 germanium transistor (TR5), mounted to the outside of the chassis pan, powered the power amp driver stage. The output from the collector of the driver transistor was then sent to an interstage transformer (T2) mounted to the inside of the chassis pan. The interstage transformer acted as a "phase splitter," dividing the audio waveform into positive and negative components to drive the "push-pull" amplifier output stage. The driver stage also included resistors R18 through R21 and capacitor C9.

Power Amp Stage - The power amp stage utilized an OTL (output transformerless) push-pull solid-state power amp circuit powered by a pair of germanium OC35 transistors (TR6 and TR7). The power rating of the T.60 power amplifier was about 30 watts RMS, 60 watts peak. The power amp circuit also included resistors R23 through R28.

Power Supply - The primary winding of the mains transformer (T1) for all versions of the T.60 power supply had five taps (115VAC, 160 VAC, 20 VAC, 225VAC and 240VAC) to allow the amplifier to accept local voltage of most countries. These taps attached to a manually adjustable five-way voltage selector. The mains transformer also incorporated one center tapped secondary winding to power the internal circuitry of the T.60 amplifier.

The A62/030 power supply utilized a two diode version of a full-wave rectifier design commonly used in the early days of solid-state circuits. This configuration required that the secondary winding of the mains transformer be center tapped, essentially splitting the secondary into two smaller windings. While simple and inexpensive to implement, this twin-diode rectifier circuit was inefficient as it only allowed only half of the full voltage available across both halves of the winding to be utilized at any given moment. Full-wave rectification using two diodes and a center-tapped transformer went out of favor with the advent of diode bridges. Diode bridges did not require a center-tapped secondary winding and used the full electrical potential of the secondary winding.

Two 0.1 uf 600 volt diode bypass capacitors (C11 and C13) minimized noise and protected the amp against voltage transients. A pair of large 2500uf electrolytic capacitors (C10 and C12) filtered and smoothed the remaining AC ripple from the power supply.


A/027 Circuit Diagram for Amplifier Model T.60 - October 1962 - January 1963
Preamp Stage
- The design of the T.60 preamp depicted on JMI schematic A/027 was identical to the design of the preamp offered on JMI schematic A62/030.

Driver Stage - The design of the T.60 driver stage depicted on JMI schematic A/027 was identical to the design of the driver stage offered on JMI schematic A62/030.

Power Amp Stage - The power amp stage depicted on JMI schematic A/027 was identical to the design of the power stage offered on JMI schematic A62/030.

Power Supply - The power supply of the A/027 T.60 circuit advanced from a two diode center tapped full-wave rectifier circuit to a four diode center-tapped full-wave rectifier circuit. The additional pair of diodes allowed the amplifier to take full advantage of the electrical potential of the secondary winding. This essentially doubled the available voltage in the power supply and boosted the RMS power output of the amp.

The 0.1 uf diode bypass capacitors (C11 and C13) from the A62/030 circuit were eliminated from the A/027 design.

OS/062 T60 Amplifier Circuit - July 1963 - June 1966
Preamp Stage
- The preamp circuit depicted on JMI schematic OS/062 was identical to both prior T60 schematics. The O62/030 schematic added connection points for an external vibrato unit included when this circuit was incorporated into the JMI Vox Lightweight 30 amp.

Driver Stage - Predriver transistor TR4 changed from the OC44 used in the two prior T60 circuit designs to an OC72. As the hfe, or amplification factor, of the OC72 transistor was 70 while the hfe of the OC44 transistor was 100, one could expect a small loss in gain from T.60 amps using an OC72 as a predriver.

Power Amp Stage - The power amp stage depicted on JMI schematic OS/062 was identical to the design of the power stage offered on JMI schematic A62/030 and A/027.

Power Supply - There were several small changes to the power supply circuit. A 2.2uf to 4uf shunt capacitor was added in parallel to each of the main 2500uf filter caps (C10 and C12). A zener diode (Z1) regulated the supply voltage for the driver and preamp stages.








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