Vox Defiant and Foundation Power Supply and
50 Watt Power Amp Module - A Look "Under the Hood"

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The Early Perils of Solid State Amps at JMI
Jennings Musical Industries (JMI), the UK manufacturer of Vox, introduced their first solid state amplifier, the T.60, in 1962. Lightweight, compact and powerful, the T.60 bass head (seen directly below) was even briefly adopted by Paul McCartney of the Beatles.

It didn't take long for JMI to notice that far too many germanium output transistors were failing in the power amp stage of the T.60. JMI discovered that the T.60 output transistors were prone to go into "high frequency oscillation." Simply stated, the output stage of the amp developed a very high pitched squeal beyond the range of human hearing. This oscillation drove the transistors past their safe operating range, causing them to overheat and fail. The T.60 went through several design revisions attempting to address this problem to no avail. Due to this unresolved issue, the T.60 head was discontinued in 1965. JMI would be reluctant to produce another solid state amplifier.

Meanwhile, Thomas Organ in America had started to design and build their own Vox amp designs. Thomas Organ was a proponent of solid state modular amp construction using printed circuit boards. Using this philosophy, Thomas designed a three channel transistorized modular preamp chassis for guitar and a second two channel transistorized modular preamp chassis for bass.

Thomas Organ then developed 30, 60 and 120 watt modular solid state power amps that would connect interchangeably with either the guitar or bass preamp via a nine pin connector. The Super Beatle, Royal Guardsman, Buckingham, Viscount, Sovereign, Westminster and Scorpion amplifiers were all constructed using combinations of these preamp and power amp modules. The 60 watt power amp module from the Royal Guardsman, V1181 Westminster and Sovereign is shown at left.

In October 1965, JMI lead engineer Dick Denney visited the Thomas Organ Labs in California to witness first hand the new Thomas Vox solid state amp designs. As a result of his observations, Denney was sold on the benefits of a modular solid state preamp/control section. Due to transistorization, Thomas Organ was able to include an incredible number of effects in their solid state preamp/control section. The circuitry generated little to no heat and required very little current to operate.

However, the engineers at JMI remained skeptical about the dependability of solid state power amplifiers. The memories of the T.60 were just too fresh in their minds. JMI delayed the introduction of another solid state power amp section for a year by first introducing the "UL Series" hybrid amps in 1966. The UL models for guitar had a modular dual channel, solid state preamp that featured reverb, tremolo and distortion. EL84 or KT88 tubes powered the output stages. A Vox UL730 guitar head is pictured at right.

The UL amps became the stepping stones between the tube models Jennings produced through 1965 and the modular, all solid state amplifiers introduced in the April 1967 JMI price list.

JMI Vox Goes Solid State

Following the lead of Thomas Organ in America, JMI included new and totally solid state Vox amps in their 1967 product catalog. Additionally, JMI also produced an eight page pamphlet entitled "Why We Use New Silicon Transistors in Vox Solid State Amplifiers" in support of this new product line.

JMI had great reservations about the dependability of a high powered solid state output amplifier stage. Thomas Organ helped to overcome their concerns by selling thousands of US made Vox amps with solid state power amp modules during 1966. The US designed Vox power amp modules did not suffer the same failures JMI experienced with the T.60 amplifier several years earlier.

When Thomas Organ engineer Sava Jacobsen designed the solid state modular output amplifier circuits for the new US Vox amp models, high powered solid state audio amp design was in its infancy. Jacobsen's cautious power amplifier designs included an extra margin for safety.

In a 1994 letter written to North Coast Music, Jacobsen discussed some of the design considerations for the Thomas amp solid state amp circuits. He explained that the silicon output transistors used in the higher powered Thomas Vox amps were less prone to overheating, unlike the germanium transistors previously used by JMI. Jacobsen designed large three sided steel heat sinks for the output transistors to further guard against thermal overrun and failure to the output stage.

Building on the experience gained by Thomas Organ, JMI developed four solid state power amp modules with silicon transistors that would be incorporated into seven new amplifier models. JMI included even larger extruded aluminum heat sinks than those used by Thomas Organ.

To further protect their new 50 and 100 watt RMS amplifier modules from thermal runaway, JMI intalled a thermostat (Otter G Manual Safetystat, shown at left) on one of the heat sinks. The thermostat would shut down the power amp should it overheat.

50 Watt Power Amp Circuit
The audio signal from the preamp section was cabled to pin 3 of the octal socket mounted on the power amplifier module. From the octal socket the preamp signal entered the driver stage of the power amp. The power amp driver stage included a 2N3054 NPN transistor (TR15) and an interstage transformer (T2).

The interstage transformer served two purposes. It served as a "phase splitter," dividing the audio waveform coming from driver transistor (TR15) into positive and negative components to drive a pair of 2N3442 power amp output transistors (TR16 and TR17). The isolation transformer also guarded against output transistor failures caused by voltage spikes coming from the preamp.

The output transistors were direct coupled to the speakers, eliminating the need for an output transformer.

While the 50 watt Defiant and Foundation amplifier module used 2N3442 output transistors, the power amp modules for the Virtuoso, Conqueror, Dynamic Bass, Supreme and Super Foundation amplifiers incorporated 2N3055 output transistors.

Power Supply Circuit
The power supply circuitry was also located in the power amplifier chassis. The power supply converted the local mains AC voltage from the wall socket to the internal DC voltages needed to operate the amp.

The primary side of the power transformer (T1) connected to the local AC wall current. The primary winding had 245, 225, 205, 160 and 120 VAC taps. These taps connected to the rotary voltage selector to allow for proper selection of local mains voltage.

The secondary side of the power transformer had three windings. The first winding was rated at 26 VAC and 0.5 amps. Four DD003 diodes (see photo at right) and a 2500 uf capacitor (C65) converted the 26 VAC output from the transformer to 35.25 VDC to power the preamp and power amp driver circuits.

The second winding was rated at +36 VAC and -36 VAC at 1.7 amps. A 5B20T rectifier bridge (see photo at right) and two 2500 uf capacitors (C63 and C64) provided +47.5 VDC and -47.5 VDC taps, or a total of 95 VDC, to supply the output amplifier circuit.

The third winding, rated at 6 VAC and 0.3 amps, supplied the indicator lamps.

An octal socket, mounted on the power amplifier chassis, received a hard wired cable with an eight pin plug from the preamp module. This socket supplied the 35VDC operating voltage for the preamp on pin 1, chassis ground on pin 8, audio signals on pin 3 and connections for the indicator lamps on pins 5, 6 and 7.

The input receptical for AC mains power cord was located on the back panel of the power amp/power supply module (see photo at left). The AC socket shown in the photo at the top of this page is not original. Replacements for the original mains plug are still available from Farnell in the UK. Click here to go to the Farnell web site page that offers this unique plug.

A rear panel mounted rotary mains voltage selector allowed the Vox Defiant or Foundation Bass to be adjusted to the local AC mains voltage.

The 50 watt modular power amplifier/power supply module was constructed on a two piece chassis.

The chassis pan (see photo above) was a rectangular five sided stamped steel structure measuring ~21w" x ~4.5d" x ~2h." Finished in either a passivated (yellow) finish or in zinc plating, the chassis pan supported the transformers, power supply and power amp driver circuits. Four threaded inserts stamped onto the bottom of the chassis pan provided attachment points to secure it to the head cabinet.

The second member of the chassis was the vertical heat sink support (see photo above). Finished in zinc plating, the steel heat sink support was a "sub modular" asembly, mounting either two or four finned power amp heat sinks. The 50 watt Defiant and Foundation Bass, the 35 watt Conqueror and Dynamic Bass and the 20 watt Virtuoso required two power amp heat sinks, but the 100 watt Supreme and Super Foundation Bass amps required four.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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