Vox Supreme and Super Foundation
Power Supply and Power Amp Module
"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 120 watt power amp module from the Super Beatle, V1182 Westminster and Churchill PA 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 introduced new 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.


100 Watt Power Amp Circuit
A 2N3054 NPN transistor (TR15), mounted to the chassis pan, powered the power amp driver stage to increase the level of the audio signal coming from the preamp module. The output from the power amp driver stage was then sent to an interstage transformer.

The interstage transformer served two purposes. It served primarily as a "phase splitter," dividing the audio waveform into positive and negative conponents to drive the "push-pull" amplifier output stage. Secondarily, driving the output transistors with a transformer made them virtually immune to failure casued by voltage spikes coming from the preamp.

The output from the interstage transformer was then directed to the output amplifier stage. A pair of 2N3054 driver transistors (TR16 and TR17), mounted to the rear facing aluminum heat sinks, increased the gain going to the output transistors. The 100 watt "push-pull" output stage used two pairs of 2N3055 output transistors (TR18, TR19, TR20 and TR21, see diagram at top of page).

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 mains voltage selector to choose the correct local AC 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 (C73, see diagram above) 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 +53 VAC and -53 VAC at 3 amps. A 5B20T rectifier bridge (see photo at right) and two 2500 uf capacitors (C70 and C71, see diagram above) provided +77 VDC and -77 VDC taps, or a total of a whopping 154 VDC to supply the output amplifier circuit.

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


The preamp 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 33VDC 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 rear control panel of the power amp/power supply module. Replacements for the cable end plug are still available from Farnell in the UK. Click here to go directly to the web page in the Farnell web site that offers this unique cable end.

A rotary mains voltage selector, also mounted on the rear control panel of the power amp/power supply module, allowed the Vox Supreme or Super Foundation Bass to be adjusted to the proper AC voltage for most countries.


Chassis
The 100 watt modular power amplifier/power supply module was constructed on a two piece chassis.

The chassis pan (see photo at top of page) was a rectangular five sided stamped steel structure measuring ~21w" x ~5¾" x ~2t." 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 on the bottom of the chassis pan provided attachment points to secure it to the head cabinet.

To accommodate the added size of the power transformer, the chassis pan for the 100 watt module was 1¼" deeper than the 4½" deep chassis pan found on the 50, 35 and 20 watt solid state modules (see photo at right).

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

The steel heat sink support bolted to the top of the chassis pan and was further supported by a pair of diagonal steel braces.




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