The Vox 715 and 710T Head - An "Under the Hood" Look at the Upper Chassis
Power Supply and Power Amp Circuits (1966)


Vox 710T Serial #2048

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While it might seem strange that Vox might offer the same amp with two model numbers, this was the case with the UL Series 710T and the 715.

The schematic for the 710T and 715 power amp sections (OS/113) was released on February 28, 1966. The 715 preamp schematic (OS/117) was dated April 25, 1966 with a revision on November 15, 1966. Most if not all 710T and 715 amps were produced in 1966.

While investigating the serial numbers posted in various web sites and speaking to various 710T and 715 owners, I have found that the existing serials seem to be concentrated between #2001 and #2100. While many more 710T and 715 amps may have been produced in 1966, the number of surviving amps has diminished over the years. Some may have been abandoned over service issues such as transformer failures. Additionally, Vox scrapped a number of 715 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 710T or a 715, 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 715 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 EL84 and ECC83 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 UL710T and UL715 were manufactured for Vox by Drake.


Figure 1 - Unused Location for EZ81 Tube Rectifier

 715 Power Supply Components
The 715 power supply included the power transformer (labeled "7239"), the components on the power supply tag strip and a number of filter capacitors.

The "primary" side of the power transformer connected to the wall current. The primary had five input "windings" that allowed the 715 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.

Three "secondary" power transformer windings connected to the internal circuitry of the 715. One winding provided ~25 volts to power the solid state preamp. A second winding supplied 6.7 VAC for the tube heaters and lamps. The third, or "high tension" (HT) winding, powered the B+ supply.

B+ Power Supply
Metered end to end, the HT winding of the 710T and 715 power transformer produced ~580VAC. However, the HT winding had a center tap (C.T.) that split the voltage into two opposing or "out of phase" ~290VAC circuits. The HT windings from the power transformer were fed into a full wave rectifier, a series of four smoothing capacitors and two step down resistors. The B+ power supply provided ~370VDC and ~340VDC respectively to the plates and 100 ohm screen resistors of the EL84 output tubes. It also supplied ~290 VDC to the 100k plate resistors of the ECC83 phase inverter.

The original design for the 715 B+ power supply included an EZ81 rectifier tube, as depicted on JMI schematic OS/113 (also see Figure 2 at right). As JMI ramped up for production of the 715, at least one hundred metal chassis were ordered with a hole for the EZ81 tube socket. However, a last minute, cost saving redesign of the 715 B+ power supply eliminated the EZ81. It was replaced by a pair of BY100 silicon diodes (see Figure 3 at right). To my knowledge, the redesigned diode rectified B+ power supply remains undocumented in JMI service manuals.

As the first run of chassis had already been punched for the rectifier tube, a round cover plate was fabricated to cover the now unused tube socket hole (see Figure 1 above). It appears that less than twenty amps were produced with an EZ81 tube rectifier.

Aside from a change from a tube rectifier to two diodes, the tube and diode rectfied power supplies are identical. It is also worth noting that all of the other hybrid UL models also had diode rectified B+ power supplies.


Figure 2 - Tube Rectified B+ Power Supply - UL715 and UL710T


Figure 3 - Diode Rectified B+ Power Supply - UL715 and UL710T


EL84
715 Power Amp
Like the fifteen watt Vox AC-15 they intended to replace, the Vox 710T and 715 both utilized two EL84 output tubes (V2 - V3). The output stage in all three amps was configured in a Class A "self biased" circuit with no negative feedback (NFB).

1966 was a heady time for Vox. Most of the best bands were playing Vox gear and JMI felt compelled to produce the best sounding and loudest amplifiers in the world. This desire caused JMI to push many of the electronic components in their amplifiers to the limit and in some cases, even beyond.

In a data sheet from UK tube manufacturer Mullard, the suggested maximum plate voltage for pairs of EL84 tubes was 300VDC. The AC-15 produced a 310VDC plate voltage, just slightly greater than suggested by Mullard. The 710T and 715 produced a 370VDC plate voltage, almost 25% in excess of the recommended maximum. This higher voltage increased the audio headroom of the 710T and 715 but shortened the life of the output tubes.

Additionally, the filament or "heater" winding of the 710T and 715 was 6.7VAC, about 7% greater than the recommended 6.3VAC.

Phase Inverter Circuit
The 710T and 715 utilize an ECC83 (V1) in a paraphrase phase inverter circuit.





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