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 Vox
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Jennings Musical Instruments, the UK manufacturer of Vox, introduced their first solid state amplifier, the T.60, in 1962. Lightweight, compact, and powerful, the T.60 bass amp was even briefly adopted by Paul McCartney of the Beatles.
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It didn't take long for JMI to notice that far too many T.60 amp heads were being returned for warranty repair. The germanium output transistors in the T.60 were failing at an alarming rate. Even worse, JMI was initially unsure why these failures were happening. After some research, it was discovered that the output transistors in the T.60 were prone to go into "high frequency oscillation." Stated simply, the output power stage of the amp would develop a very high pitched squeal, beyond the range of human hearing, at maximum output. This would drive the transistors past their safe operating range until they overheated and melted down. The T.60 went through several design modifications to address this problem, but it was never totally resolved. The T.60 quietly left the Vox catalog in 1965.
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 philosphy, Thomas designed a three channel transistorized modular preamp chassis for guitar and a second two channel transistorized modular preamp chassis for bass. Thomas Organ engineer Sava Jacobsen 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.
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 Jennings 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 amp section by introducing the "UL Series" hybrid amps in 1966. These amps had a solid state preamp and a tube power amp.
The UL amps became the stepping stones between the tube amplifiers JMI produced through 1965 and the modular, all solid state designs Vox introduced in 1967.
JMI Vox Goes Solid State....and Broke
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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. Jennings 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 Vox amps with solid state power amp modules during 1966. The US designed Vox power amp modules did not suffer from the same problems that JMI experienced with the T.60 amplifier several years earlier.
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When Thomas Organ engineer Sava Jacobsen designed the solid state modular output amplifier circuits and chassis layout for the new US Vox amp models, high powered solid state audio amp design was in its infancy. Jacobsen was very concerned about dependability and durability.
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 silicon transistors were less prone to be affected by overheating, a problem that proved to be destructive to germanium solid state devices. Regardless, Jacobsen designed massive heat sinks for the silicon output transistors to ensure that thermal overrun would not cause a failure of the output stage.
JMI decided to use even larger and more efficient heat sinks than those used by Thomas Organ. JMI hoped that these larger heat sinks would make their output amps even more dependable and stable.
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Jacobsen designed the 120 watt Thomas Organ Vox Super Beatle audio output amp with four matched 2N3055 output transistors in a "push-pull" OTL (output transformerless) circuit. A 2N3054 and a 2N2924 transistor served as a predriver for the output stage. An interstage transformer isolated the power amp from the preamp to protect the power amp from electronic surges or faults in the preamp.
The power amp designed by JMI for the Supreme and Super Foundation was similar to that in the Super Beatle. Like the Super Beatle, the 100 watt JMI amp module also used four matched 2N3055 output transistors in a "push-pull" OTL circuit arrangement. Like the Thomas design, the JMI amp module also included an interstage isolation transformer. Unlike the Thomas design, the JMI circuit had three 2N3054 transistors rather than just one. Two were mounted to the heat sinks and used as predrivers for each half of the power amp. The third was used to increase the gain coming from the preamp prior to entering the interstage transformer. JMI also mounted a thermal sensor to one of the heat sinks that would shut the amp down should it overheat.
The power supply was also located in the power amplifier chassis. The power supply converted the local mains AC voltage from the wall socket to the DC voltages needed internally to operate the amp. An octal socket, mounted on the power amplifier chassis, received a hard wired cable with an eight pin plug from the preamp module. This connection supplied operating voltages to the preamp and sent audio signals from the preamp module to the power amp.
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The input receptical for AC mains power cord was located on the back panel of the power amp/power supply module. Unlike the Thomas Organ design, the three pin AC cable connector was both removable and grounded. 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 voltage selector, also mounted on the back panel of the power amp module, allowed the Supreme or Super Foundation amps to be adjusted for various mains voltages throughout the world.
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Vox introduced their new solid state amp series in August 1967 at the British Music Trade Fair held at the Russell Hotel in London. Shortly after the trade fair was completed events would transpire that would put the entire future of Vox in question.
The Royston Group operated a conglomerate of companies in the UK, including Jennings Musical Instruments. JMI founder Tom Jennings sold a controlling share of his company to Royston in 1964. In September 1967, Royston unexpectantly fired Tom Jennings and most of his original JMI staff. This occured less than thirty days after Vox introduced their new solid state amplifiers at the Russell Hotel. Tom Jennings, Derek Underdown and even Dick Denney (who had left somewhat earlier of his own volition) were no longer associated with Vox. This was not the end of the bad news.
Another of the companies in the Royston Group was competing for a contract to develop and manufacture an aeronautical flight recorder. The development effort proved to be very costly. Royston was taking all the profits from the other businesses under their umbrella, including JMI, to fund the work on the data recorder. Royston "bet the entire farm" on winning the contract for the flight recorder.
Unfortunately, they did not win the contract and the all of the companies in the Royston Group fell into bank receivership. Even though Vox had always been one of the profitable subsidiaries for Royston, they found themselves in bankruptcy in early 1968. By mid year, Vox once again was back in business as an independent firm under the name "Vox Sound Equipment Limited."
Read more about this story in a 1968 article from British Beat Instumental magazine entitled "Vox aims to sell you Solid State."
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URL: http://www.voxshowroom.com/uk/amp/SSpower_amp_hood.html
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