Derek Underdown's Ten Watt Amplifier designs from Univox to AC10

1952 - 1967

Glen Lambert

Updated August 2024

If you're an enthusiast of the Vox Amplifier you might be suprised to learn of its humble beginnings, consisting of a series of modest low powered devices cobbled together from electric organs and hifi magazine circuits mainly intended as accessories for organs and accordions. These progressed through some primitive and not so great features, gradually weaving a way to an amp very suitable for electric guitar. 6BW6 output tubes gave way to the brighter and more efficient EL84's. 6BR7 preamp pentodes changed to the quieter EF86. A long dalliance with negative feedback was eventually dropped and a 'long-tail pair' phase inverter introduced. The larger Jennings amps utilised a complex 'phase shifting vibrato', whereas the 10w amps introduced a simpler preamp based tremolo which may have been inspired by the 'Throbbing Vibrato' of "Popular Electronics" magazine, December 1957, and the early 1950s Gibson GA-40 guitar amplifier. Whatever the case, by 1960 ideal forms of Vox amplifier had come into existance for 10, 15 and 30W amps, which coincided perfectly with the steadily growing popularity of electric guitar based popular music in Britain.

Derek Underdown was one of the important early engineers at Jennings Musical Industries and has not often received due credit for his contributions to the company's product heritage both electronic and mechanical, especially during its formative years. Prior to 1952 the company dealt mainly in sales and maintance of accordions and smaller instruments. Existing staff were skilled in repair and refurbishment of those but not in the assembly and servicing of electronic keyboards, which Jennings was keen to move into as a response to Selmer's Clavioline product. There was also a growing interest in amplifier products. Underdown was employed in May 1952 by Tom Jennings to train up existing staff and get things rolling with production of their new Univox product, as well as implementing more products. The Univox was a portable electronic keyboard designed for Jennings by local electronics design consultant Les Hills, and much work was required to bring it to production at Jennings facility. According to his contemporaries Derek Underwood was a capable and efficient man and following his employment by Jennings not only did he get the Univox successfully into production but also a host of new products gradually appeared. These included not only new console and portable organs, but also mechanical and electronic assemblies. For some of these Derek was even granted patents. These products were all marketed by the "Jennings Organ Company".

The earliest known amplifier sold by Jennings was called the 'Univox' (not to be confused with the portable keyboards released soon after it). These are thought to have been made by Weill & Co and were definately a rebadging with the Jennings name. They were a 16" x 11" x 12.5" combo amp covered in imitation lizard skin, advertised as 15 watt output but was probably actually closer to 10 watts and AC /DC powered. The earliest records of these are from December 1951 in an advert published in the "Music Trade Review" (the price: £29 and 15 shillings), and "Melody Maker" magazine, 26th January 1952. Both predate Derek's employment at Jennings. The amp's final appearance was in silhouette images in an advert published in the music trade press in December 1956, after which nothing more is known of it.

Detail from an advert placed by Jennings in "Music Trade Review", December 1951. "It is ideal for accordion, guitar or string bass amplification...".

Following Underdown's arrival at Jennings in 1952 there were an assortment of amplifiers and speaker cabinets originally made as accessories for organs but adjusted to suit accordions and guitars. Very little is known of these but they are mentioned for the sake of completeness. It was in May 1954 that the first official mention of a Jennings made amplifier occured. This was in the "Melody Maker" magazine which described a product as a 30W 'powered speaker'. These were intended as add-on speakers for organs - but the pathway was now open for portable guitar amplifiers as the music scene moved in that direction. Sadly nothing more is known of this first Jennings combo type amplifier.

An amplifier called the E.B.1 designed for the Jennings upright Electro-Bass product was advertised for 40 guineas in three different trade journals in late 1956, with mention of an F.V.2 foot (Foot Volume) pedal to be available soon. We have nothing more on either product, not even an illustration. The F.V.2 may have evolved into the Vox Volume Control Pedal range which was first advertised for sale in December 1957. An overview of Jennings volume pedals .

Derek recalls making at least two 5-6 watt combo amplifiers in 1956. They were designed for any type of high impedance microphone or instrument pickup and thought to have been in TV-style boxes covered in the same tweed as the Univox of that period, however no images exist and nothing more is known of these. They were apparently sold to accordion players.

Left, the Jennings 30 watt powered speaker, May 1954; centre, the portable amplifier supplied with the J48 organ, May 1955; right the Vox Conversion Box for the Univox.

The Vox Conversion Unit - An adapter box which connected directly to a Univox organ speaker using the keyboard's 7-way bus lead. It could be used instead of the keyboard or piggy-backed with the keyboard. The unit included a preamp with three 1/4" jack inputs and controls to allow connection of a microphone, guitar, phono player, accordion or similar high impedance device. It allowed the extra equipment - e.g. guitar or microphone - to replace the keyboard or share amplification with the keyboard. It was unlikely that the unit was suitable for 'any type of electronic keyboard' as claimed in the advert but rather 'any recent Univox keyboard' (as early Univox used a different 5-way bus connector). The Conversion Unit was first advertised in "Melody Maker", 21st January 1956, for 9 guineas and was still listed in the Jennings catalogue of early 1958 at the same price.

Strange to say, out of all these things, the Univox organ ended up being JMI's longest-lived instrument. Mentioned in adverts as early as June 1952, and featuring prominently at Trade Shows of the mid 1950s. Some would say it was superceded by the Vox Continental, one of the most important (and handsome) organs of the 1960s, yet the Univox remained in production through to 1966, perhaps even 1967.

Left, the three models of Univox (J6, J7 and J10) as advertised in spring 1955; right, the telescopic chrome stand, introduced in late 1953.

A Univox J6, c. 1955/1956.

The first proper Jennings 10 watt guitar amplifier

The first 10 watt Jennings amplifier aimed at the guitar market was the G1/10 designed by Derek Undertown. It was first released in a 23" x 13" x 10 1/2" dark brown faux 'gator' bookcloth-covered rectangular cab which narrowed in thickness towards the top and was very much like the Univox portable organ cabinet in appearance. The 'G' prefix is said to signify "Guitar", and hence considered Jennings first purpose-designed and built amplifier for guitar use.

The '57 G1/10 was of similar width and height (but slimmer) to the contemporary Univox organ casing and considerably lighter of course. As it resembles so very closely the brown 'gator' covered Univox of 1956-57, even replicating its single 10" speaker offset to one side, it was likely made using cabinets intended as extension speakers for the Univox. The chassis appears to have been a general purpose box type with multiple hole position options along one side, and was configured by Underdown to suit not only existing organ and accordion applications, but also the rapidly growing electric guitar market. It was a single channel/single input with a volume and tone control, hexagonal power jewel, Arrow SPST power switch, and a paxolin wafer voltage selector. Its black control panel was attached directly to the rear edge of the box-type power chassis, which was mounted on a wooden plinth positioned at the lower right of the cabinet's rear, allowing for a small compartment for cable stowage underneath. Underdown recalled the design as being similar to the Univox keyboard although he was likely referring to the physical appearance.

Detail from the JMI brochure of spring 1958.

Underdown was possibly inspired also by the Jennings J48 organ speaker accessory but it was really quite a standard chassis inside. Research suggests the G1/10 circuit was probably derived from period application diagrams or a magazine article such as the 1954 Guitar Amplifier in "Radio and Hobbies" magazine or the 10 Watt Amplifier in "Radio Constructor Magazine" in October 1955 (volume 9, no. 3). The schematic was drawn later on the 6th March 1958 as 'G1/10 Watt Amplifier'.

The G1/10 used a single 6BR7 pentode pre-amp (similar to the EF86), an ECC83 phase inverter, 5Y3GT rectifier, and a pair of 6BW8 output tubes (glass version of the popular 6v6), with Hi-Fi style negative feedback from the output to reduce distortion. It was listed at 27 guineas in a composite advert published in "Melody Maker" on the 16th November 1957, with an internal Vibravox option for an extra 7 guineas. This would have been quite a customisation as there was no convenient socket for adding the unit inside (as in the AC.2/30), let alone positions for the extra controls on the cab. The G1/10 was offered with two inputs 'suitable for guitar' in the catalogue of spring 1958. An advert from Spring 1958 mentioned the use of Goodmans speakers in the amp although generally the brown Plessey 10" speakers of the Univox appear to have been fitted in the G1/10. This same six spoked hammertone brown Plessey speaker was used in many subsequent 10W amplifiers designed by Underdown.

"Melody Maker" magazine, 16th November 1957.

Les Hills' Univox power amp section - note no negative feedback from output stage as per the G1/10, AC10 Mk Ia, and "Throwback" amps.

A very similar Guitar Amplifier circuit, quite close to the G1/10 (including the feedback loop) appeared in 1954, "Radio & Hobbies Magazine".

In the autumn of 1957 guitarist and electronics 'tinkerer' Dick Denney was formally taken on by Jennings - principally on the strength of the nice-sounding 15W prototype he had been trying to market. His first design for the company appears to have been the external Vibravox unit, its circuit drawn as OS/001 and described (in the title block) as also being for "AC.1 - 2 Amps". Pages on the Vibravox can be found and .

The unit was probably designed in tandem with Denney's 15W amplifier, designated AC.1/15, its circuit later drawn as OS/002. In the same time frame, to cater for larger power requirements, Derek also re-purposed his existing Jennings 30 watt Organ 'Power Pack amplifier' as a 30 watt amplifier designated AC.2/30 (JMI circuit diagram OS/003). The 15W and 30W amps were modular designs which could be ordered as stand-alone chassis with a protective cage, or as a combo, the chassis housed in a distinctive upright narrow single speaker split-front cabinet with a large upper margin area on the front baffle. The AC.2/30 .

From OS/003 we know that Underdown's 30W amp was a fairly conventional design, its starting point being his 30W organ power section (as noted above): an output stage with EL34 outputs, negative feedback, instrument and line level inputs, and an adapter socket for tuner or add-on Vibravox. Derek's 30W version seems to have made it to market first and was advertised for sale with the Vibrovox stand-alone unit and the G1/10 by late 1957. See the "Melody Maker" advert of November 1957 above. The first version of Denney's 15W amp is presumed to have been substantially the same as the prototype he came to the company with (hence its designation as OS/002), and probably close to the AC.1/15 schematic drawn in March 1958. It is unknown how many AC.1/15s were issued in tall upright cabs.

Denney is pictured above with his amplifier at the Embassy Ballroom at Welling in 1958, likely with his 15 watt build. These tall narrow style cabinets were marketed in 1957 and '58. No examples of either type (15W or 30W) are currently known to have survived. A new square TV-style cab covered in diamond embossed ivory coloured levant was introduced for the 15W amp in 1958, the circuit redrawn in March 1958 but retaining the same model designation (AC.1/15). At around the same time production was beginning on a new 10W amplifier based on Derek Underdown's G1/10, but now also also housed in a new square style TV front cabinet.

The Vox G1/10 TV Front

JMI flyer, summer 1959, still the TV front G1/10.

The G1/10 became one of the earliest TV front amplifiers produced. It consisted of Derek Underdown's existing G1/10 chassis mounted in the base of a newly designed small TV cabinet 16" x 16" x 7.5" in size and covered in diamond embossed ivory levant with a white solid handle. The holes for the rear-facing controls were left unused. The controls were remoted via coax flying leads to an early-form control panel placed in an island cutout at the top. The schematic OS/004 was redrawn with the name G1/10 in the title block in August 1958 and was identical to the previous undated sheet except for a change in the value of the cathode resistor on the pre-amp, from 3.3k to 2.2k, indicating a slight increase in pre-amp gain for the new build. It was advertised to have a quality Goodmans 10" speaker and variable Tremolo option. The example shown below illustrates the unused holes that had been punched for fitment of the control panel in the Univox style cab. Early versions of the TV Front G1/10 had much the same small output transformer as the Univox J6, although as production proceeded a larger enclosed type of OT was adopted.

The 10" speaker used in this example is a Goodmans T6/1010.

The choke in the G1/10 with feedback resistor mounted atop. Side of G1/10 TV Front with Bulgin power socket high on the cab's side. Top view of a very early unit with Bulgin metal jack-nuts.

Format

The TV Front G1/10 was a single channel combo amplifier with two parallel inputs and single volume and tone controls. The original G1/10 black fascia panels were re-utilised, but mounted in an island style cutout (11 3/8" x 2 1/3") in the top surface of the cab and the controls linked to the chassis by flying leads. In common with its forerunner, the TV Front version employed a standard circuit consisting of 6BR7 pentode, configured in triode mode, into a 12AX7 'paraphrase' phase splitter driving a push-pull pair of EL84 outputs with negative feedback. The 16" x 16" cab had a simple pressboard rear panel with punched-out cooling slots and serial number stamped into it. Initially, a Plessey brown 10" speaker, as in the Univox organ and earlier G1/10 amps, was fitted, however this was soon standardised to a 10" Goodmans, as indicated in contemporary advertisements.

Component dates on a very early example indicate that chassis may have been in production as early as the final quarter of 1958, though it seems unlikely that many sales were made before early 1959. The model was not officially advertised until June/July of 1959 (at 27 guineas) - possibly to facilitate the clearing of stocks of the previous version. The TV Front version was advertised throughout 1959 as the G1/10 (and also in some instances as the AC/10). At the changover to TV front a new serial number sequence was begun at '3000'. The AC1/15 was started off at '3500' and the single speaker at '4000'. Whether this is an indication of the order in which the various models came into stock is a matter for further consideration.

G1/10 TV Front fascia panel. This example still using leftover G1/10 fascias, evidenced by the legend 'INPUT' not yet centred between the two inputs. It is the same amp shown above with small channel mounted Univox J6 type output transformer. This panel shows the use of newer coloured Rendar type with insulated nuts, which appear on all subsequent examples.

By 1959, the legend on the input jacks now centered. The panel has the same Render jacks with insulated nuts, in this instance two black ones. This is a 'Tremolo' factory optioned amp. No visible difference when viewed from above, but a speed pot was mounted in the rear pressboard panel and a foot switch included for activation. Graphically enhanced for clarity.

The black painted aluminium fascia panel of the TV Front version sprang naturally from that of the previous G1/10, and as already stated, leftover fascias were used on the first batches produced. The knobs adopted were of the larger ivory-coloured splined Radiogram variety (as used on the contemporary '59 AC1/15 and some AC/30 TV front amps), though these tended to obscure the arrow rotation indicators. As had been introduced in the later Univox style cab amps an extra input jack was added by drilling or punching a new opening to the left of the original. The legend was centred between the two on subsequent batches of panels. In terms of "gain", the inputs were equal in level (with 220K dividing/isolating resistors).

During 1959, the legend on the input jacks was centered. The above panel has the same Render jacks with insulated nuts, in this instance two black ones. This example is a later 1959 "tremolo" factory optioned amp. No visible difference when viewed from above, but a speed pot was mounted in the rear pressboard panel and a foot switch included for activation. The Tremolo option (shown in the example below) involved the preamp valve being deleted from lower chassis and a purpose-made tremolo preamp assembly based around two 6BR7 pentodes in a small "L-shaped" chassis being installed in the upper left of the cab (viewed from the rear), with cabling linking the upper and lower sections. The tremolo circuit is likely to have been taken over (and customised) from a pre-existing circuit such as one adopted in the 1952 Gibson GA-40. Similar variants also appeared in contemporary British amps like the Selmer Stadium TV19/T.

A speed control potentiometer was wired on flying leads and simply mounted to the punched pressboard rear panel (not shown). There was no depth control on these amps; it was simply preset in the circuit. There was also a foot pedal/switch added on the usual flying lead arrangment to activate the effect. The above example is a late 1959 build as evidenced by the absence of cutouts in the chassis for rear-facing controls, the corner riveted construction, and the fitment of the larger type output transformer.

The G1/10 had an unusual history, beginning in 1957 and extending as late as 1962, when it final disappears from view. All in all, fewer than 150 units are likely to have been made. At the release of the Denney designed AC1/10 with integrated vibrato at 39gns, which became the main 10 watt model, the TV Front G1/10 became (by September 1959) a budget level 10 watt amp, priced at 29 guineas. Fewer than 100 of these were made with only a handful of surviving examples, two of which were featured in Jim Elyea's Vox Book - one with Tremolo and one without.

With the introduction of the new AC10 Amplifier No.2 in early 1960, the G1/10 TV Front was no longer required to fill the role of budget, however, as the new amp could be supplied without the rear mount plug Tremolo to cover the pricepoint. The G1/10 went into haitus until the release of the AC10 Amplifier No.3 at the end of 1960, where it was surprisingly reintroduced in a new split front fawn cabinet, a forerunner of the type of cab which would soon dominate JMI production. It retained the same cab size and island style control cutout as the TV front version and has been called the "Throwback" AC10. This final type of G1/10 was apparently brought back briefly to serve as budget 10 watt amp as the new AC10 (Amplifier No. 3) had tremolo fully integrated into the chassis. It was described as "10 Watt, 2 input vibrato" and was priced at 35 guineas in a November 1960 advert in Melody Maker. The AC10 Amplifer No.3 was 45gns. It appears that relatively few of this final varient of G1/10 was made as they were never advertised after this, and the few surviving examples all date from the late 1960 to early 1961 period with serial numbers in the 34XX range. More information can be found on this last version G1/10 here - on to the .

The Univox organ continued to use Derek Underdown's G1/10 style power amp, including 6BW6 outputs, small channel mounted 10 watt output transformer and 5Y3 rectifier until the end of its production in 1966/67. The brown Plessey speaker became virtually standard for all AC10 amps from 1960 until early 1961 when the Elac 10N series speakers were introduced.

Detail of the main compartment of a tweed-covered J6 Univox c1954 showing various parts that ended up in later G1/10 amplifers.

To summarise, from 1952 onward Derek Underdown had a major hand in keeping not only Jennings keyboard products but also amplifiers in production. He also had a major hand in the electronics and mechanical design of other amplifiers including such milestones as the design of the "contempo" chassis, and "doubling up" the EL84's to produce the final and best version of the AC30. It appears that most 10 watt amps produced through to the end of the JMI era were primarily Derek Underdown's design.

Derek Underdown, pictured in the later 1960s.

.