JMI Vox Echo-Reverberation Unit -"Cliff Richard Reverb" (1962 - 1968)
A Look "Under the Hood"


The "under the hood" story about the Vox Echo Reverberation unit cannnot be told without closely examining the efforts of JMI to violate the patents of the Hammond Organ Company
for their Type 4 reverb system. For this reason, this account starts with a detailed look at the superbly engineered Hammond Type 4 reverb tank. It continues with an explanation of how JMI modified the Type 4 design to sidestep the patent and escape paying a license fee to Hammond.


Hammond Type 4 Reverb Pan, circa 1967

The Hammond "Type 4" Reverb Tank
The Type 4 reverb system was designed in 1960 by Alan Young, an engineer at Hammond Organ. After design patents were secured, Hammond included their new reverb system in many of their home organs. Hammond also licensed the Type 4 reverb system to other manufacturers of audio equipment. In 1961, Fender Musical Instruments licensed the Type 4 design for use in their amplifier line, starting with the 6G15 Fender Reverb Unit. It didn't take long before the Hammond Type 4 reverb system was adopted throughout the guitar amplifier industry. Hammond licensed the Type 4 reverb system to many manufacturers, including Gibson, Thomas Organ (manufacturer of Vox in America), Ampeg, Kustom, Acoustic, Marshall, Shure, Peavey, and others.

The Hammond Type 4 reverb tank is still in production, now made by Belton in Korea. It continues to be included in the designs of top guitar amplifier manufacturers.

Components of the Type 4 Reverb Tank
The outer chassis of the original Type 4 reverb tank was made of steel and was ~16 3/4" long, ~4 1/4" wide and ~1 1/4" tall. The outer chassis should be grounded to the amplifier to minimize electronic interference.

The inner channel was suspended in the inside of the outer chassis on four support springs. The delay line components (drive transducer, receive transducer and transmission spring) were mounted to the inner channel.

The drive transducer was comprised of a small coil with a laminated steel yoke. Two small permanent magnets were suspended in the air gap of the laminated steel yoke.

The transmission springs attached to these magnets. Both of the 14" transmission springs were made from two conjoined 7" segments. Each of the four 7" spring segments were wound with a unique wire diameter and pitch angle. This mix of dissimilar transmission springs created a more natural sounding reverb.

A portion of the audio signal from the amplifier was introduced to the coil of the drive transducer. This created an alternating magnetic field in the coil that was transferred to the air gap of the yoke. The fluctuations in the magnetic field caused the magnets of the transmission springs to vibrate, converting an electronic audio signal to movement in the transmission spring.

These vibrations were delayed and reflected within the transmission springs as they traveled from the drive transducer to the receive transducer.

Like the drive transducer, the receive transducer also included a coil with a laminated steel yoke and a pair of suspended permanent magnets. The receive transducer converted the delayed vibrations from the springs back to an audio signal by electromagnetic induction. The delayed or "wet" signal returning to the amplifier from the receive transducer was then blended with the original or "dry" signal, creating the auditory illusion of reverb.

A label inside the Type 4 reverb pan stated: "Notice - Purchase of this device does not confer a right to make, use or sell the system covered by U.S. patent no. 1,967,447 and Canadian patent 655,242."

It was clear that Hammond was not only ready to defend the patents for the Type 4 reverb system, but also would require a license fee from a manufacturer building a product that included their invention. While Leo Fender was willing to license the Hammond Type 4 reverb system, Tom Jennings, president of Jennings Musical Industries was not. Jennings sought to develop a spring reverb system for Vox amps that would alter the Type 4 design sufficiently to avoid prosecution from Hammond.




Vox Two Spring Reverb Pan From a 1962 Vox Echo Reverberation Unit

Dual and Single Spring Vox Reverb Delay Lines
Like the Hammond Type 4 tank, the delay line Vox designed for the stand alone Echo-Reverb unit and some AC-30 SRT heads used a pair of transmission springs to create the reverb effect. The transmission springs in the Vox delay line were made from two conjoined segments, again like the Hammond Type 4. Each of the four transmission spring segments were wound with a different pitch angle and thickness of wire. This mix of dissimilar springs caused the reverb to have a more natural ambiance.

A simpler, single spring delay line (shown at right) was included in the some Vox amps, including the UL700 series.

The drive transducer, receive transducer and transmission springs were mounted to a long steel channel, similar to the inside channel of the Hammond Type 4 reverb. This steel channel was suspended inside the chassis of the Vox Echo Reverb on two rubber bands.

Vox Single Spring Reverb Pan with an ACOS 071 Cartridge

The Piezoelectric Effect
While the transducers in the Hammond Type 4 were electromagnetic, the Vox reverb transducers utilized the piezoelectric effect. The piezoelectric effect is divided into two types, direct and inverse. The direct piezoelectric effect states that an electrical charge will be created in certain crystals as the result of mechanical stress. The inverse piezoelectric effect states that certain crystals will increase or decrease in length in response to the application of an electrical charge.

Vox developed their reverb pan to include standard crystal phonograph cartridges as drive and receive transducers. The transmission spring replaced the stylus or "needle" normally found in the phono cartridge. The "Drive" phono cartridge would use the inverse piezoelectric effect to convert the electrical audio signal from the amp into mechanical vibrations in the spring. These vibrations would be converted back to an electrical signal by the "Receive" cartridge using the direct piezoelectric effect. The signal returning to the amplifier from the receive transducer was then blended with the original signal to create reverb.

It was this application of the piezoelectric effect that allowed Vox to circumvent the Hammond Type 4 patents.

The ACOS 071 crystal phono cartridges Vox included in their early delay lines produced a maximum output of about one volt. In 1964, Vox switched to the Sonotone 2T (shown at right), a similar crystal phono cartridge.

As time and experience would show, crystal phono cartridges were too fragile for use in reverb pans. The stresses of driving down a bumpy road were often enough to cause cartridge failures. Farfisa faced similar problems with their sixties era portable organs. They as well used crystal phono cartridges in their delay lines.

It is now nearly impossible to locate crystal phono cartidges to repair a defective Vox Echo-Reverb. The production of replacement crystal phono cartridges ended in the early seventies with the development of superior sounding magnetic phono cartridges. One cannot substitute a magnetic cartridge for a crystal cartridge in a Vox reverb pan due to voltage incompatibilities.

Vox Single Spring Reverb Pan with a Sonotone 2T Cartridge

Circuit
The Vox Echo-Reverberation circuit featured four tubes, dual channels and point-to-point hand wired construction. The delay line, including transducers and transmission springs, was suspended inside the chassis on two rubber bands.

Tube and Control Functions
Please refer to the photo near the top of this page for tube locations.

One half of V1, a 12AX7, was the gain stage for the first channel. The other half of V1 was the gain stage for the second channel. V1 worked in conjunction with the 1/4" inputs and the channel 1 and 2 level controls.

Both triodes of V3, a 12AU7, powered the signal to the drive transducer. V3 worked in conjunction with the Reverberation Length control. Advancing the Reverberaton Length control increased the amount of signal delivered to the drive transducer.

One half of V4, another 12AX7, powered the return signal from the receive transducer. V4 worked in conjunction with the Reverberation Level control. The Reverberation Level control balanced the output of the "wet," or delayed signal with the "dry," or original signal.

V2, an EZ80, was the rectifier tube for the DC power supply.

Photo Credits
My thanks to Mike Handley for the chassis photos of his Vox Echo Reverberation.





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