Jennings Amplifier Products and the Wurlizter Phase Shifting Vibrato 1957-1960

Glen Lambert

You may already have read elsewhere that Dick Denney came into the Jennings fold in 1957, having previously been known to Tom Jennings in the war years, bearing a prototype guitar amplifier that Jennings was keen to bring into his new product line. What we don't often hear is that Denney also came with a keen interest in a phase shifting Vibrato effect, likely having become familiar with it either from an examination of a 1950s Wurlitzer console Organ, or from an article such as the one featured in 'Radio & Television News' of April 1954. Even though the Vibrato circuit was originally designed for organ and accordion, Denney had no doubt noted the outstanding effect when an electric guitar was run through it, and had become keen to implement it in some form in his designs. With his background in sales of Organs and so on, Tom Jennings was presumably most pleased to run with this concept.

The phase shifting vibrato was in the picture from Denney's earliest work with Jennings. His guitar amplifer prototype was to become "Amplifier Model AC.1" (schematic drawn out in March 1958) and would include the effect integrated into the chassis. However, while the AC.1 amplifier was still in development Jennings was apparently keen to rush out a product to get the ball rolling. So Denney's first design release was not an amplifier, but came in the form of discrete effects box specifically aimed at the accordion market. It was called the "Vibravox unit", and appeared as early as 1957. There is no known surviving unit to examine, but from study of schematic OS/001 it is almost certain that it is the phase shifting vibrato circuit originating from the 1953 Wurlitzer model 44 Organ. It had a virtually identical schematic down to the 6 settings which provided for three variations in speed and three in depth, exactly per the Wurlitzer circuit. In all the amplifier iterations designed by Denny in the following years, an effect channel of this type was either a standard or add-on feature. Prior to this Derek Underdown's "G Series" amplifiers were the work-horse manufactured by JMI, but they had no built-in effects.

JMI schematic, Vibravox, OS/001, detail

The first iteration of the "Vibravox" unit, pictured in a Jennings advert in "Accordion Times", January, 1958. Images of two later iterations can be found . For further comments, see the end of the section on OS/001, below.

Wurlitzer schematic, detail

A detail of the vibrato circuit in a Wurlitzer organ. The circuit derives ultimately from one published in "Radio and Television News", April 1954, pages 52-53 and 90, by Richard H. Dorf: "The Universal Vibrato". A PDF of the issue can be .

Schematic OS/001 - the Vibravox

The main information panel of schematic OS/001 - "VIBRAVOX. For Unit 2 AC1-2 Amps. OS/001" - suggests it was not only the schematic for the Vibravox Unit marketed by JMI to accordionists (various adverts in issues of "Accordion Times"), but the circuit incorporated in the AC.1/15 amplifier. As already noted, it was a phase shifting vibrato based on a Wurlitzer Organ. As the design had been patented by the Wurlitzer engineers, three key recognisable sections of the Wurlitzer circuit were secreted into epoxy sealed capacitor cans and the contents never divulged outside JMI. The contents were omitted from the Schematic and only referred to in a note on the upper left corner as "Hermatically Sealed Cans No. 1, 2 & 3".

JMI schematic, Vibravox, OS/001, detail

Upper left information panel, JMI OS/001.

For a description of the internal arrangement of the standalone unit, we have an eye-witness account from around 1960 - see towards the end of this section.

The Vibravox Unit 2 also included a "Tremulant" switch, which basically shorted one side of the phase shifter push/pull modulator to ground, transforming it into a very nice sounding Tremolo effect. This idea was not taken from the Wurlitzer circuit, legend being that Denney accidentally discovered it when a screwdriver dropped into a Vibravox unit that was running open on his workbench, shorting part of the circuit. This must have happened early in development as even the first Vibravox version from late 1957 had the Tremulant switch.

The Tremulant switch is the one major detail that distinguishes the Vibravox from the original Wurlitzer circuit and is one of the true innovations in the Jennings early product.

JMI schematic, Vibravox, OS/001, detail

Detail of JMI OS/001. "Tremulant" switch = Vib/Trem. selector.

Like most amp designers Denney was using tried and true circuits from datasheets, tweaking values to suit his requirement. However, the idea of shorting away one side of the Vibravox modulator stage in the Vibravox unit to create a standard Tremolo effect is something that could well have happened exactly as the account says, with a screwdriver falling in and shorting out part of the modulator tube base. A side effect of the component values used to achieve the phase shift is two opposing networks with one side tending to empasis low frequencies, and the other, higher frequencies. The shorting switch (with a 0.1uf capacitor added to remove the transient click or thump) was initially connected to short away the lower (fuller) frequency side of the modulator, meaning when it was activated only the higher frequency (thinner) side remained for the tremolo signal path.

With the loss of the phase shifting topology the audio signal came through now as a simple Tremolo effect. The fact that the tonally fuller side was the one chosen to be shorted away suggests the accidental nature of the design. This was probably the side that the screwdriver landed on and Denney was careful to replicate it. The switching configuration was carried all the way through to and including the 1960 designs of the AC30/4, AC30/6 and AC15. It wasn't until some years later that it was realised the shorting capacitor could be moved to the tonally thinner side of the modulator to produce the very same Tremolo effect, but now with the fuller signal allowed to remain. This modification doesn't appear to have been documented in the List of Changes but was quietly introduced in all JMI product with a Vib/Trem channel by 1964.

One can demonstrate the two different 'tones' of the Vib/Trem channel. With the footswitch in the off position, use the Vib/Trem switch with a guitar or other audio running. You will hear a change in tone as you switch from Vibrato to Tremolo setting. If you have an early amp the audio will become noticably thinner, if you have a later amp the audio you will hear a slight reduction in high frequency.

Vibravox control panel

The controls on the panel consisted of a 3-way speed selector and a simple on/off toggle for selecting Vibrato or Tremulant (Tremolo) effect. The first Vibravox unit released appears to have included a 3-way depth selector per the original Wurlitzer circuit. Unit 2 simplified this to a preset resistor which could range from 470k to 1M, selected for optimal setting at the time of assembly. A 1/4" jack was included for a footswitch to enable or disable the effect, but was not a true bypass. Schematic OS/001 is undated but assumed (as it is number 1) to have been drawn prior to AC.1/15 from March 1958.

The standalone Vibravox unit

JMI schematic, Vibravox, OS/001, detail

Detail from the Jennings advert in "Accordion Times", January 1958.

Note on the front of the unit the white push button "bedside lamp style" Tremulant selector switch, and the differently coloured power supply chassis at rear, which was supplied if you ordered your unit in complete standalone mode, i.e. if power were not supplied by the octal plug built into the Jennings amplifiers.

JMI schematic, Vibravox, OS/001, detail

Detail from OS/003, the schematic for the AC2/30 amplifier, showing the octal socket and legend.

Although no Vibravox units have come to light far, we do have an eye-witness account of one from c. 1960:

"I was working as an apprentice in a large factory at the time and did repairs "under the bench" to radios, record players and TVs at home. I was about 20 at the time and had been building record players, starting during my time at school, so had a good knowledge of electronics, mostly self taught and with help from a couple of TV engineers who lived locally. I think the unit came from one of the guys at work. We had many budding musicians working there at the time."

"Vague recollection that it [the Vibravox unit] was about 6"x10"x3", with a sloping front panel across a shorter end. Mains powered with the valves near the back end away from the controls."

"The valves were mounted on a panel across the chassis, glass on one side and parts on the other. All the parts had been painted with white gloss to prevent easy copies being made. I suspect it was made in the garage at the back of the original shop on Dartford Road. Often used to chat to the guys working there when I went passed and the doors were open to get some fresh air."

David noted elsewhere that:

"I remember getting a Vox standalone tremolo unit to service about 1960 and although I lived about 300 yards away from their works, and Tom Jennings brother lived in my road, they would not help with a circuit diagram. All the parts had been covered in paint making it impossible to diagnose the fault. I recall that there were about 5 valves and mostly ECCxx types. The fault was cured with new valves but I never did work out any sort of circuit."

Amplifier Model AC.1 - the AC.1/15

"Amplifier Model A.C.1", marketed as the AC.1/15, was the amplifier that Dick Denney was brought in by Jennings Musical Industries to produce. It was a deluxe two channel amplifier and designed to push the EL84 outputs to the limit. The schematic, which has no number, is dated 4th March 1958. The HT is given as 340v, the cathode biasing resistor 130ohm. This would likely have been driving the output power to levels approaching 18W, well in excess of EL84 ratings. The "Vibravox" was a standard part of this amplifier, and implemented in the main chassis. On the AC.1/15 schematic the Vibravox circuit is noted as being on a separate diagram (OS/001) with labels for each interconnection. It featured 8 and 15ohm speaker outputs. This box type chassis was housed in a square TV Front Cabinet with cream diamond embossed covering, the speaker a single 12" Goodmans Audiom 60. The OS label for the schematic has not survived but we could imagine it was OS/002.

An early AC1/15 chassis.

AC1/15 serial number 3708.

Schematic OS/003 - the AC.2/30

Undated; entitled "A.C.2. 30 Watt Amplifer": a two channel amplifier with increased output power. It had an HT of 400-430v, two EL34 valves in its output stage, with conventional adjustable bias from a -30v rail. Output was a nominal 30 watts into a single 16ohm speaker. The preamp had two 12AU7 pre-amp tubes and an octal socket for powering a radio tuner or Vibravox. Cabinets were relatively narrow and tall with a split front: a grille in the lower half to protect the speaker; and above, a margined area with large VOX lettering arranged in variable formats. There was a single solid handle on the side. Production numbers were relatively low. The AC.2/30 appeared as early as November 1957 in an advert in "Melody Maker" magazine, followed in December 1957 by one in "Accordion Times". An AC.2/30 is seen in a photograph of a live performance at the Embassy Ballroom in Kent, 1958, Dick Denney present in the band.

The AC2/30 in a Jennings advert in "Accordion Times", December 1957. Note that the AC1/15 is briefly described: price 42 guineas. For more on the AC2/30, .

Schematic OS/004

OS/004 is a revised version of Derek Underdown's G1/10 circuit: two EL84s in cathode bias (130R instead of 125R); a cathode bypass capacitor at V1 (stil a 6BRT). There are other slight changes elsewhere. No date is given, but probably early 1958.

Schematic OS/005 - "AC/15 Amplifier No.2" - the AC15 (2nd circuit)

Drawn on 4th December 1959, this represents a simplification of the AC.1/15 design. For around nine or so months (from late 1959 to September 1960), two types of amplifier were built according to this schematic: a 15 watt model with AC15 on the serial number plate; and a 10 watt model with AC10 on its plate.

Detail of OS/005. Click for a larger image as ever.

The AC15 build in 1960

Pictures for general reference of a two-tone AC15 in red and blue/grey. Although the pictures are a little small (click for slightly larger ones) one can see the layout of the control panel - 2 channels, a volume and tone control for each - and the "Vibravox" controls inset in the back panel (in the third picture immediately above the bell of the Goodmans Audiom 60 speaker.

The EL84 based output was tamed down to a nominal 15W by the inclusion of 100ohm resistors in series with the connections from the OT anodes to the EL84 plates. The two channels each had their own tone control. The main Channel was now pre-amplified by the hifi spec. EF86 pentode, which was capable of supplying a greater drive to the power section, although this was attenuated somewhat by the 3-pole high pass filter required to remove thumping from the Tremolo oscillator.

Upper chassis and control panel of an AC15 built according to schematic OS/005, mid 1960. The ECF82 valve can be seen in its can more or less centre. A vibration-dampening socket was provided with a view to stopping the valve picking up extraneous noise. In company with EF86s, ECF82s are considerably more sensitive than ECC82s and ECC83s.

The Vibravox circuit (as laid out in OS/001) was extensively trimmed down by the removal of the phase-shifting section, leaving only the parts needed for Tremolo. This came in the form of an optional add-on unit - somewhat confusingly termed "Vibravox" (see the pictures below) - supplying a plain Tremolo effect to the EF86 driven main (white input) Channel.

Preamp of the AC15. The tremolo unit on this amp plugs into the B9A socket on the preamp tagstrip. In some amps the connection was hard-wired.

Above, a tremolo unit with its original B9A connector; and an instance of an AC15 with a hard-wired tremolo unit.

The Tremolo circuit made clever use of the unusual ECF82 tube, which was a "dual" tube: on one side, a pentode with enough drive to modulate the tremolo frequency directly onto the anode of the EF86; on the other, a medium power Triode, which was used as the pre-amp of the normal Channel.

Detail from OS/005 showing the elements reserved for the add-on rear panel Tremolo unit.

Note in the detail above that the contents of the optional rear add-on "Vibravox" are enclosed by a dotted line. It is interesting to see that JMI retained the over-arching name "Vibravox", even though the circuit was greatly reduced. The controls on the back panel were a 2Meg Tremolo Speed pot, Tremolo Deep/Soft push switch (commonly seen on bedside lamps of the era), and a 4-way paxolin wafer socket for connection of a footswitch.

The "Vibravox" tremolo unit in situ in an AC15 from 1960. Note that the contacts of the footswitch socket have been jumpered. See below, for pictures of a footswitch that survives intact, together with a detail of the pin arrangement of the plug.

The AC10 build in 1960

In relation to the AC15, key elements are generally smaller all round, chassis included. A smaller power transformer provides a lower HT voltage, and therefore a lower power output = 10 watts. Correspondingly, the output transformer is smaller too - frame-mounted, along the lines of a choke - but its impedance is the same as that of the transformers in the AC15s that were built to this circuit.

AC10 serial number 3236, detail of the chassis.

In the preamp: a single tagboard with a B9A socket for plugging in the rear-panel tremolo unit (should one have been ordered), and to its right, relatively low down on the chassis, the third preamp valve (the 12AX7). In the amp below, one can see that the socket has been blanked off, as no tremolo unit is present.

Preamp of another AC10. Note in this case the B9A socket is blanked off. One can see the 12AX7/ECC83 valve socket to the right of the board.

The "Vibravox" tremolo back-panel unit on an AC10 - exactly the same format as the unit in the AC15 pictured above.

Above, the pin arrangement of the footswitch plug (with paxolin retainer); and an example of a footswitch that survives with an AC10 from 1960.

Had Jennings become nervous about using the full Wurlitzer phase shifting effect, or was it simply a cost-cutting measure after noting that more guitarists than accordion players were using their amplifiers, guitarists favouring the simple tremolo effect?

OS/006 - the AC6

OS/006, the AC/6 Amplifier schematic, drawn on the 8th December 1959. This was Jennings' first practice amplifier. It had a single 12AX7 pre-amp and a single EL84 output. It had a lower powered EZ80 rectifier and likely delivered less than 5W into its single 3ohm speaker.

A JMI pricelist of mid 1960 listing the AC6 at 25 guineas.

In 1961 the AC6 was redesignated the AC2 (OS/009), and illustrated in two JMI catalogues, one produced for "Musicland" in Bexleyheath, which was run by Paul Jennings, Tom's son.

Opening of the Musicland catalogue showing the AC2 (the only amplifier encompassed).

And in 1962, the AC2 in turn became the AC4, having substantially the same ciruit as the AC6, but with a name reflecting the power output of the amp more accurately

OS/007 - the single speaker AC/30

Schematic OS/007 is labelled "AC.30 Amplifier No.2", and was drawn on the 1st January 1960. Note, however, that JMI began advertising (and presumably selling) the AC/30 from the summer of 1959 - .

The amplifier has an HT of 400-430v, and a pair EL34's in cathode bias, a cathode resistor of 470ohms on each valve. Output is a nominal 30 watts. The AC/30 featured four 12AX7 pre-amp tubes and a 12AU7. It had the full "Vibravox" circuit (parts still in three sealed cans); and provided for 8 and 15ohm speaker outputs. A "Works" schematic - OA/030 - was drawn on 1st March 1960. In adopting cathode bias for the EL34s, it is likely that Denny was trying to capture the brighter tones of the EL84s in the AC15. Single speaker AC/30s were issued in TV Front cabinets.

On the left, a general view of a TV front single speaker AC/30 (serial number 4041). On the right, a detail of a chassis of another amp. Note on its left side, the three sealed canisters masquerading as capacitors but containing vibrato/tremolo circuitry. Other AC/30s are .

OS/008 - the AC10

"AC/10 Amplifier No.3" (OS/008) drawn on 9th September 1960 was a circuit similar to that of "AC/15 Amplifier No.2" (OS/005). The amps built according to this schematic had the smaller transformers of the "AC10 build" mentioned above (in the section on OS/005). HT was reduced to 305v, effectively creating a 10W amp. The separate tone controls of the No.2 circuit were reverted to the common cut control as per the output stage of the AC.1/15 circuit. The plain Tremolo version of the Vibravox was now fully integrated into the chassis as a standard feature, and the Tremolo Deep/Soft switch changed to a 500K Amplitude (Depth) pot. The two channels (Vibrato and Normal) each had a volume control. Therefore, five controls in all. This amplifier was effectively the JMI AC10 and continued in production through the 1960s.

Rear view of a TV Front AC10. The amp retains on front its brass "Jennings" plaque, Vox logo, and "TEN" runner.

OA/031 and 0A/032 - the AC15 and AC30/4 (1960)

The full Wurlizter style 'Vibravox' circuit made its reappearance on the AC15 contempo chassis (OA/031) and the AC30/4 chassis (OA/032) both drawn on 29th April 1960. It was substanially the same Wurlitzer phase shifting circuit as used in the AC.1/15, albeit with the previously 'accidentially' added Tremulant (Vib/Trem) switch, and with an adjustable 500k trimpot (not accessible on the control panel) rather than a fixed resistor to regulate "depth".

The location of this trimpot directly below the Vib/Trem switch suggests an intention to implement a 500k depth pot on the panel, perhaps with a combined push/pull for the Vib/Trem selector. This was never brought into being. The unusual position of the depth trimmer remained for much of AC30/4 production and also AC30/6 Pre LOC production (i.e. before May 1961).

A detail of an AC30/4 from the third or last quarter of 1960 showing the position of the trimpot and the extent of the Vibravox circuit (bounded in red).

The AC15 and AC30/4 also saw the reintroduction of a higher HT voltage, presumably to give the new build the best possible output level. HT in the AC15 was restored to 340v, and the 130ohm cathode resistor retained. The AC30/4 was given a more tempered increase with the HT set at 325v and the cathode resistor at 80ohm (sometimes in practice, 82ohms). For the AC30/6 in late 1963, the value was reduced to 50ohm (in practice often 47ohms), which really pushed the quad of EL84s to their limit.

JMI was now comfortable to have the complete Wurlitzer style circuit fully shown in the schematics of both the AC30/4 and AC15, and all components openly mounted on the tagstrips, this possibly due to the accumulation of minor changes over several years.


The schematic of the contents of the three sealed cans has never been located, but the author has managed to reverse engineer it. This was tested by the owner of a vintage AC.1/15 amplifier that had sealed cans which had failed. He was delighted to report that the Vibravox worked perfectly with the reverse engineered information.

Postscript to Vibravox

As one might expect, there are some anomalies in both the schematics and production (assembly) of the circuit, particularly in relation to the AC30/6. To take the schematic first. The "List of Changes" of 8th May 1961 indicates that the value of C41 "was 0.005mfd". Yet on the schematic, that value (which is probably incorrect for other reasons - that's another story) remains in place.

Detail from OA/026 - list of changes panel. The text is clearest in this copy. The value is 0.005mfd, not 0.003 as has been supposed.

Detail from OS/065. The value given for C41 is 0.005.

One might consider that the change was intended to be written ".05" in the schematic as in almost all builds of the post-LOC AC30/6 one finds a 0.047uf capacitor as C41, "almost all" being key here.

Sometime in late 1963 and continuing through to 1964 at least one subcontractor (Westrex) was making an assembly error in the final stage of the AC30/6 Vibrato/Tremolo section.

C14 and C41 were interchanged, resulting in C41 (0.047uf) being fitted in one leg of the high pass filter network where C14 (0.0047) should have been; and the audio bypass of the R41 (47K) HT supply resistor became 0.0047uf rather than the required 0.047uf.

A Westrex-made chassis from 1964. The arrows indicate the two capacitors that have been wrongly positioned. The left one should be where the right one is, and vice versa.

Enough chassis with this identical error have been observed to confirm that a good number left the factory, passing QA, made in this way. The net result is that audio will still pass and is not hugely affected, but an error it is nevertheless, and another quirk in JMI history.

Glen Lambert - June 2020