The Vox 760 Head - An "Under the Hood" Look at the Upper Chassis
Power Supply and Power Amp Circuits (1966)



Vox 760 Serial #1001

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While exact production numbers remain unknown, the 760 may be the rarest of Vox amps.

The schematic for the 760 power supply and amp section (OS/115) was released on March 3, 1966. The 760 preamp schematic (OS/117) was dated April 25, 1966 with a revision on November 15, 1966. Most if not all 760 amps were made in 1966.

While investigating the serial numbers posted in various web sites and speaking to various 760 owners, I have found that the existing serials seem to be concentrated between #1001 and #1100. While as many as one hundred fifty to two hundred 760 amps may have been produced in 1966, the number of surviving 760 amps has diminished over the years. Some may have been abandoned over service issues such as transformer failures. Additionally, Vox scrapped a number of 760 amps that were returned to the factory by dissatisfied customers. These amps were often exchanged for the all solid state amps introduced by Vox in 1967.

If you own a 760, please email your serial number to the Vox Showroom at voxshowroom@aol.com so that I may continue to refine the production count.

Chassis
The 760 chassis was comprised of three parts. The chassis base was a rectangular steel box with an open bottom. It enclosed the preamp circuitry. The steel upper chassis was riveted to the base. It was formed to create an electronically shielded "valley" to mount the transformers and other power supply components. The upper chassis then extended over the top of the KT88 output tubes, serving as a heat shield for the cabinet. An anodized aluminum front panel completed the chassis. It included the control panel and front escutcheon and was fastened to the base and upper chassis.

Transformers
The power and output transformers for the 760 were manufactured for Vox by Drake. The output transformer was rotated 90° to the power transformer to minimize 60 hz hum.



Figure 1 - CZ4 Brimistor

 760 Power Supply Components
The 760 power supply included the power transformer, the power supply tag strip and a number of filter capacitors. These components were all located in the "valley" of the upper chassis.

The "primary" side of the power transformer connected to the wall current. The primary had five input "windings" that allowed the 7120 to operate on 115VAC, 165VAC, 205VAC, 225 VAC or 245VAC mains voltages. The correct mains voltage was manually selected from a five position rear panel rotary switch.

Four "secondary" power transformer windings provided the operating voltages to the 760. The first winding provided the B+ supply to power the tubes. The second winding supplied 6.3 VAC for the tube heaters and lamps. The third winding powered the solid state preamp. The fourth winding provided the screen bias voltage for the output tubes. Let's take a look at the power supply circuitry in detail.

B+ Power Supply
The 760 B+ power supply started at the HT winding on the power transformer (see Figure 2). The HT winding stepped up the local mains voltage to 450 VAC. A bridge of four BY100 diodes mounted to the power supply tag strip converted the output from the HT winding to pulsed DC (direct current).

Following the diode bridge was a device called a "Brimistor" (see Figure 1)." A Brimistor was a thermally sensitive resistor encased in a heating element. It was designed to guard against a potentially harmful "switch-on" current surge in the amp.

The electrical resistance of the Brimistor was highest when the amp was cold. This allowed the Brimistor to have the greatest effect of limiting "in-rush" current as the amp was first turned on. As the amp warmed, the heating element inside the Brimistor caused the resistance of the thermally sensitive resistor to drop. After 30 seconds, this drop in resistance allowed the Brimistor to pass the full operating B+ voltage to the circuit.

The 760 power supply also had two 200 uf 350 volt capacitors (C56 and C57). These were wired to each other in series, allowing them to act as a single 100uf primary filter capacitor rated at 700 volts. This combined 700 volt rating was more than adequate to accommodate the ~550VDC from the rectifier bridge.

Each of the 200 uf filter capacitors had a 220k one watt resistor (R100 - R101) straddling their positive and negative terminals. These resistors ensured that the voltage from the 760 power transformer was shared by both filter caps. They also served as "bleeder" resistors, discharging potentially lethal stored voltage when the amp was off.

AC Filament Heater Supply
A 6.3 VAC winding powered the tube heaters supplied voltage to the power, standby and front panel illumination lamps.

DC Supply for the Solid State Preamp
A 20VAC winding in the secondary of the 760 power transformer, four OA200 diodes and a 500 uf capacitor created the 25VDC power supply for the solid state preamp circuit.

KT88 Screen Bias Supply
A 107VAC winding in the secondary of the 760 power supply, a BY100 diode, capacitors C54 and C55 and a 3.3k ohm resistor created the -143VDC screen bias supply. Bias was adjusted by VR13 and VR14, located just behind the tubes.


Figure 2 - Condensed Version of the 760 Power Supply Schematic


KT88 Tube
760 Power Amp Stage
The major components of the 760 power amplifier stage were V1, a 12AX7 tube, V2-V3, a pair of KT88 power tubes, and the output transformer, labeled "7312."

Introduced in 1956 by the General Electric Company plc (GEC) and noted for its high power, low distortion and wide frequency response, the KT88 tube was favored by many top end manufacturers of hi-fi equipment. McIntosh introduced their legendary MC275 audiophile power amplifier in 1961 with four KT88 tubes in the output section.

The KT88 was initially produced by Marconi Osram Valve, a subsidiary of GEC. The KT88 was similar to the American designed 6550 tube but the KT88 offered lower total harmonic distortion.

While the KT88 could accept plate (anode) voltages up to 800 volts, Vox took a more conservative route for the 760. The 760 power supply delivered ~540 VDC at idle to the plates of the KT88 tubes, yielding a clean and distortion free 60 watt RMS output. While sounding great for bass, many felt that the KT88 based power amp in the 460 and 760 was just too clean for guitar.

The bias current for the KT88 tubes was adjusted by two variable resistors (VR13 - VR14).

The 760 and 460 utilized an 12AX7 (V1) in a paraphrase phase inverter circuit.

The 760 and 460 employed negative feedback to reduce distortion in the power amp stage. The signal for the negative feedback circuit was provided by the 16 ohm tap of the output transformer. The cathode of the phase inverter tube (V1) reintroduced the negative feedback back into the audio signal.

Comparison to the AC-30
It is interesting to compare the power amp stage of the amp that defined Vox tone, the AC-30, with the 760. The AC-30 used four EL84 small bottle, low power output tubes in a cathode biased circuit with no negative feedback, yielding a compressed tone rich with harmonics. The 760 used two KT88 big bottle, high power output tubes in a fixed biased circuit with an abundance of negative feedback, producing a very clean and uncompressed tone.




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