115 Dartford Road, photo taken in late 1960, published in May 1961. Amplifiers awaiting packing. At left, seven two-tone AC15s.

Below, the new format chassis and control panel layout. In the preamp, the phase-shifting vibrato circuit was reinstated.

Note the new arrangement of the EL84s and EZ81, and re-positioning of the output transformer and choke. Second circuit chassis were very different - see this page.

AC15s were for the first time supplied in two voicings - "NORMAL", indicated by an "N" on the serial number plate; and "BASS" indicated "B".

A detail of a JMI advert issued in April 1960. The AC15 Bass was 70 guineas, the AC/30 Bass 85 guineas. The AC30 Twin was first announced in May.

Below, the plate of serial number 4208, one of the earliest split-front AC15s, "Normal" voicing. Relatively few "Bass" amps were produced early on.

Serial number 4208N.

Salient points in the production of third circuit AC15s are in overview:

TV Front amplifiers: serial numbers c. 4060 to low 4200s. On the serial number plate, two have a "J" after the number.

Split-front cabinets, black control panel: serial numbers c. 4208 to 4540.

Celestion blue speaker: standard from around serial number 4430.

Variant serial number plates: 4376N, 4476, 4452, 4510B - of the type also used on AC30/6 serial numbers 4680 and 4813 - see this page.

Split-front cabinets, copper panel (late 1961): from around serial number 4540.

BASS models: serial numbers 4510B, 4543B.

The alternate components needed for BASS voicing were marked on the circuit diagram (JMI sheet OA/031) with an asterisk - see the section below.

Chassis were for the most part assembled at Dartford Road throughout 1960 and 1961. Westrex took over production in late 1961 with the advent of the copper panels.

The circuit

Phase-shifting vibrato restored - the superb vib/trem circuit. Below, a copy of OA/031, "VOX AC.15 amplifier circuit No. 3", drawn out on the 29th April 1960 and signed off by Dick Denney. No supplementary sheet for the circuit was produced or required, the Vibravox circuitry having been included as an integral component - much as Derek Underdown had done on a diagram for the second circuit AC15 produced for JMI eyes only. "Build" sheets are likely to have been produced for or by Westrex however when it took over production of the AC15 in late 1961.

As has long been recognised, the new AC15 circuit was not developed on its own but in conjunction with the new Vox AC30/4 and AC30/6 amplifiers - circuit diagrams OA/032 and OA/026, also drawn out on 29th April 1960 and signed off by Dick.

Detail of the info panel of the sheet.

Note of changes, 7th February, 1961.

So far, three amps have been identified that have pre-changes circuits - serial number 4060, and two early chassis. C42 was added later to serial number 4060.

Detail of one of the early chassis. The 1 Meg resistor at far right is R15, the capacitor to its right C17.

Note that the three chassis in view all have a single long lower preamp tagboard. Later, the lower tagboard was formed in two sections - a small board at left for the vibrato footswitch circuitry and a longer board for everything else.

It is perhaps well to say that a number of amps represented on this page have been restored in such a way as to make their position difficult to assess in terms of the changes noted on OA/031.

As for the date - 7th February '61 - that probably records the date when the additions were made to the sheet, not the date of the changes themselves. C42 and C17/R15 may have been adjusted at different points in production.

Since the operation of the circuit has been documented so well over the years, it is only worth outlining general aspects here. In the Normal Channel preamp, V1 is an EF86. The phase inverter (V2) for both Normal and Vibrato Channels is an ECC83, configured as a long-tailed pair - the topology first used in the AC1/15, first circuit.

For the Vibrato Channel preamp, the first stage is an ECC83 (V5), followed by an ECC83 (V7) as vibrato oscillator (V7), and an ECC82 as modulator (V6). The vib/trem circuit (i.e the sections principally around V6 and V7) is switched in and out via the footswitch.

For commentary on the circuit and superb layout diagrams, see Stephen Grosvenor's "A Service Engineer's Guide to the Vox AC30 Valve Amplifier" - details here - along with Glen Lambert's page here.

In the power section, an EZ81 recitifer valve (as in the AC/15 second circuit) and pair of EL84s, their cathode bias resistor (R22 in the diagram) still 130 ohms, the value chosen for the AC15 by Dick Denney and Derek Underdown in late 1957.

The section above will probably be enhanced slightly in due course, but for the time being, it can remain as it stands.

Transformers

The mains and output transformers were commissioned from Haddon, as in earlier years. The choke came from Radiospares. The output transformer was part number "OA 063", the mains "OF 031", and the choke "OA 123".

Late black panel AC15, mains transformer part number "OF 031".

Black panel AC15, output transformer.

The primary taps on the mains transformer diverged fairly significantly from the voltages marked on the voltage selector. The mains transformer above gives: 105v (green wire), 145v (unconnected), 160v (yellow), 205v (brown), and 245v (blue). The presence of a tap for 145v is puzzling. Which country or place had this supply voltage? The question was asked here too of other equipment. The provision of a 145v tap goes through to end of production in 1965.

At the voltage selector on the control panel, the choices are still 110v, 160v, 205v, 225v, and 245v, but JMI connected both the 225v and 245v settings on the selector to the 245v tap on the transformer. A slight reduction in power for Europe. Glen Lambert has kindly raised the point that these amps, along with many other JMI models, run extremely hot in the USA when set to 115v, the wall outlets feeding up to 125v into what is actually a 105v primary tap.

Chassis

Normally assembled for JMI by Westrex in North London. Type I: a relatively narrow lip below the front edge of the control panel, around 1/4" deep. 550 or so of these were made, serial numbers in the low 4000s to mid 4500s, black and early copper control panels, early/mid 1960 into early 1962.

Black panel AC15 from later 1961.

Type II: a larger lip, around 3/4" deep, with cut-outs for cage nuts (for the back panel fixing screws) inset. Serial numbers in the mid 4500s to the end of the run.

Copper panel AC15 from 1963, chassis also assembled by Westrex.

The change from narrow to wider chassis lip took place a little earlier where AC30/6s are concerned - mid 1961, noted on this page. AC30/4s had a narrow lip through to end of production (late 1961).

Separate tag-board for vibrato footswitch

In the earliest third circuit AC15s, the lower preamp tagboard was a single strip. At around the time the new (type 2) control panel was introduced, the decision was made to have two boards instead of one - a small 6-way section at left for the vibrato footswitch circuitry, and a long 33-way board to its right for everything else.

The small vibrato footswitch board at left, part of the main lower board to its right.

OA/031. Detail of the vibrato footswitch circuitry.

Just to note that the gap between the two boards is more pronounced on the larger chassis of the AC30/4 and AC30/6.

Control panels

There were two main types of control panel, both anodised black and gold, much as the early panels for the AC30/4 and AC30/6 in overall effect:

(1) The earliest, gives 110v instead of 115v; has a more generous spacing of letters - see the legend for "TOP CUT" in the pictures below; and puts dots after the "I". and "II." in the TONE compartment.

(2) Gives 115v as the lowest voltage; has a more compressed letter-spacing; and omits dots after the "I" and "II" in the "TONE" section.

Serial number 4177

Two-tone cabinet, serial number unknown.

It is also worth noting that both types of control panel give a somwhat inaccurate picture of the tone controls. The "Brilliance" control was actually for the Normal Channel (Channel II as marked on the control panel), the "Top Cut" for both channels. The error persisted on copper panels, to be corrected in 1964 on the grey. Thanks to Glen Lambert for the info.

Speakers

For the TV front and early split front cabinets, JMI generally fitted Celestion alnico G12s, either silver or "oyster" coloured hammertone CT3757 or T530 - the immediate fore-runners of the "blue". One of the early TV Front amps, cabinet with a cream-coloured diamond-pattern rexine covering, has a hammertone T530, date code "21FE" = 21st June 1960.

The original serial number plate of the amp to which this speaker belongs was never stamped.

Also to note that a small group of amps with serial numbers in the mid 4200s have Goodmans Audiom 60 speakers.

Celestion blues (T530)

An overview of the Celestion blue, produced exclusively for JMI, can be found on this page, which has links to ancillary pages on the cones (RIC and Pulsonic). The earliest AC15 currently known with an original blue is serial number 4318 (Celestion date code for 15th June 1961).

AC15 serial number 4318.

Blues apparently did not become standard in single speaker AC15s until later though. Serial number 4408 for instance has an oyster-coloured alnico G12, probably either a CT3757 or a pre-blue T530.

There may be a number of reasons for the apparently slow adoption in AC15s. (1) blues appear to have been made in relatively small numbers at first; (2) JMI evidently reserved the new speakers at the outset for AC30s. The earliest verifiable example that has come to light so far is from February 1961; (3) there is some evidence to show that AC15s were made up in relatively large batches. Transitions could be irregular (note the reappearance of TV Front cabinets well into the run of new split fronts).

Other amps with T530s of known date are:

4318: 15th June 1961.

4438: 18th July 1961.

4482: June/July 1961.

4567: 19th March 1962.

4633: May/June 1962.

The page on the AC15 in 1962 and 1963, serial numbers 4500 into the 5000s, can be found here.

Split front cabinets

The earliest example that has come to light so far is serial number 4208 - probably late 1960 / very early 1961. No date codes are discernible in the amp however. The lower edges of the upper back panels of the new-style cases have simple straight edges, departing from the cut-away on the upper backs of the TV Front cabinets. This was perhaps in part to save money, but the intention may also have been to close the back a little further to protect the speaker at higher volumes.

Surviving amplifiers

Serial numbers generally fall in the range low 4000s through to 5000s. A good number of preamps have been worked on by repairmen and restorers over the years. Some original black WIMA Tropydur signal caps (normally the first thing to be replaced) remain in place though. Gold-coloured WIMA Tropyfols were apparently not used until very late 1961, perhaps even early 1962. The main 16u-16uf electrolytic capacitor in the power section is generally from Radiospares (plain can, red running man logo); other electrolytics are Hunts (plain can or blue outer sleeve) and TCC (green can) - a TCC particularly as main filter in the preamp. Resistors are white Eries.

If you know of further amps that should be incorporated on this page, do let me know.

TV Front cabinets (mid to late)

Cream-coloured diamond-pattern rexine; two-tone grey and oatmeal.

Serial number 4046 N

Black panel, early third circuit. The vinyl is beige "weave" of the type often used for the lower sections of two-tone TV Front AC15s. The chassis is secured to the upper backboard with three screws (as serial number 4060, below). The RIC cone code of the Celestion is unfortunately not quite readable. Thanks to Roberto for info on the amp and its serial number.

Serial number 4060 J

Formerly owned by Mike Handley. A "J" is stamped after the serial number, as on the plate of serial number 4177, a little way below. The lower preamp tagboard is a single strip. Control panel is of the earliest type, giving 110v as the lowest voltage. The speaker is likely to be a Celestion (pre-blue) T530. Below, older pictures of the amp.

Serial number 4129

Condition good externally; what looks to be the original Celestion hammertone/oyster speaker in place. The meaning of the suffix "O" on the serial number plate is unknown at present. Thanks to Michael fof the photos.

Serial number 4165

Note the stamped "N", indicating the voicing of the amp, "Normal" rather than "Bass". "Normal" and "Bass" versions are only found in third circuit amps.

Chassis only - serial number unknown

A chassis on its own, type of cabinet unknown. The main Hunts filter capacitor has the date code "WHY" = 12th week of 1960; two of the blue Hunts 25uf cathode bypass caps in the preamp have "HTY" = 24th week of 1960; and "HIY" = 23rd week 1960. The lower preamp tagboard is a single strip. A number of black WIMA Tropydur capacitors remain in place. The output transformer is a Radiospares part - presumably there is a cut-out in the chassis for a Haddon, now gone. The control panel is of the earliest type, 110v given as the lowest setting.

Post January 1961

It is likely (no more than that at present) that the amps below were produced after January 1961. The circuit diagram, OA31, indicates that C17 and R15 should be deleted, and they are indeed absent from the amps below.

Also introduced at around this time, the small tagboard for the footswitch circuitry. The terminal block in the preamp for the mains cable, present in the first of the amps below, was soon dispensed with (after the last of the initial runs of chassis had been used up).

Updated

Serial number 4177 J

TV Front. Two-tone vinyl - blueish grey / oatmeal. Both back-boards are oatmeal. Brown diamond grille cloth. Celestion alnico G12 B024 speaker date code "29DE" = 29th April 1960. The lower preamp tagboard is a single strip. No indication of the amp's voicing ("N" or "B") is given on the serial number plate. However, there is a "J" after the number, meaning unknown at present. The control panel is of the earliest type. Two of the TCC capacitors in the preamp have the date code "RJ" = September 1960 for their manufacture.

Chassis only - serial number unknown

Another chassis without cab. The control panel has the 110v setting. The lower preamp board is a single strip.

Serial number 4184

Illustrated by Jim Elyea, p. 355. TV Front. Two-tone vinyl - blueish grey / oatmeal; plain rough woven grille cloth. FIFTEEN runner and bronze Jennings badge.

The new split-front cabinet (early 1961 on)

Carol Elvin, resident at the Star Club, Hamburg, in late 1962. Picture from this page. Her guitar is a Vox Consort.

Serial number 4208 N

The voicing of the amp is "N" = Normal; a "W" follows the serial number. Haddon transformers and Radiospares choke. The lower preamp tagboard has two sections, the 6-way part at left for the footswitch. The preamp probably re-capped in the early 1970s - at least one of the Mullard mustard capacitors has a late 1968 date code (C68).The speaker currently in place is a Celestion alnico B024 made for Selmer - T731.

Serial number 4212

No voicing indicated on the plate, though the "W" is stamped in the upper panel. The speaker is a Celestion CT3757 - - thanks to John for the info.

Updated

Serial number 4229 N

Late 2024: The cabinet now houses the chassis and speaker (date code "20JE" = 20th September 1960) of serial number 4270 (pictured further down this page).

The general views and details of the control panel were taken in 2022, thanks to Steve. The output transformer of the chassis in the cabinet at that time had failed at some point (and had been replaced). The speaker is a CT3757, date code "20JE" = 20th September 1960, cone code "RIC 1MF".

Cabinet of 4229 now with the chassis of serial number 4270 (see below). Note the black smudges on the green TCC cap and the series of capacitors to its left on the board. New holes were made in the aluminium lip of the chassis to match up with the holes in the cabinet of 4229.

Serial number 4230 N

Cabinet recovered. A separate tagboard in the preamp for the vibrato footswitch. The speaker is a Goodmans Audiom 60.

Serial number 4254 N

Currently in the UK. Superb condition. The speaker is a Goodmans Audiom 60, red label, 15 ohms, 15 watts handling.

Serial number 4261 B

Currently in Finland. Superb condition. A late two-tone TV Front amp with a Goodmans Audiom 60 speaker. The knobs are additions. Thanks to Valentin for the pictures.

Serial number 4267 N

Single speaker AC15, split front cabinet, beige vinyl. Chassis assembled by Westrex. No original component date codes known at present. The choke is a "Heavy Duty" Radiospares. The signal capacitors in the preamp were replaced with Mullard mustards at some point after the second quarter of 1968. The speaker is a Celestion G12 T658 - 15ohms, alnico magnet. The pink and yellow wiring is original.

Serial number 4293

Original footswitch and Celestion hammertone speaker still present. Thanks to Eoin for the pictures.

Serial number 4318 N

TV Front, two-tone blue/grey and oatmeal vinyl, as serial number 4261 above. In the preamp, Hunts cathode bypass caps with the date code "TIY" = 43rd week of 1960. The main preamp filter - a green TCC - has "RA" = January 1960 for its manufacture. The speaker is a Celestion blue (T530), date code in June 1961. The wiring and solder connections look to be old. Serial number 4318 is the latest instance to date of an old-style TV front cabinet, and apparently the earliest instance of a Celestion blue in an AC15.

"15FF" = 15th June 1961.

Serial number 4376 N

Sold by Soundgas some time ago. Do not attempt to go to the site - it is currently infected with malicious code. The main Hunts filter capacitor of serial number 4376 has the date code "TTY" = 44th week of 1960. The speaker currently in the cabinet is a Celestion G12 B025 without a "VOX" label, so doubtless not original to the amp.

Serial number 4452

Illustrated by Jim Elyea, p, 356. Note the variant type of serial number plate. In the cabinet, a Celestion blue in place, along with the original pink and yellow wiring.

NEW

Serial number unknown

Serial number 4476

Serial number 4482

Chassis in excellent order; original transformers in place. Visible date codes on two of the Hunts capacitors are "ISY" and "TW" (third letter unreadable), respectively the 35th and 41st weeks of 1960. "TW" can only be "TWY". The chassis has been with the box for a long while, though at this stage in production one would normally expect to find a split front rather than TV front. The pebble vinyl appears to be of type used on some AC30s in 1963; the bagging of the grille cloth suggests some sort of refitting. The Celestion blue has the cone code "RIC 1 YG", which places it in July 1961. Note also that the serial number plate is of the type also found on AC15 serial number 4510 (below) and some black panel AC30/6s (at this point too). Thanks to Sid for the pictures.

Serial number 4499

Still black panel, original footswitch. The cabinet is pictured with an earlier speaker - a hammertone T530 or CT3757, not wired in.

Serial number 4510 B

Good general order; the original mains and output transformers were replaced some time ago, however, so too the control knobs. The yellow-print Mullard EL84s may be the ones installed at factory. The serial number plate is of the type also used on serial number 4482 (above).

Copper panels and the new AC15 TWIN - late autumn 1961

Vox prices at 23rd September, 1961. This is the earliest firmly dated mention of the AC15 Twin

Serial number 4526 N

Currently in the UK, illustrated in a Music Radar article on this page.

On to the page on the AC15 in 1962 and 1963, serial numbers 4500 into the 5000s.

Back to the page on the second version of the AC15 (the "AC15"), early 1960.

A selection of material relevant to the AC15 in the 1950s and early 1960s is being gathered together here.