The Vox AC30/4: black panels, split-front cabinets

mid 1960 - early winter 1961

Cliff Richard and The Shadows, New Musical Express Poll Winners Concert, March 1961, with their new split front AC30/4s in smooth black cloth. Note that Jet Harris still has the slightly lower amplifier stand that had been issued with the Shadows' TV front AC30s earlier in the year.

In terms of what went out for sale, the first runs of the new split-front AC30/4 overlapped with the last of the AC30s in TV front cabinets. Serial number 4414 TV front; number 4401 (below) new split front.

From a practical standpoint the new cabinets were a marked improvement on the old - three carrying handles instead of one, and three vents in the top to allow air to circulate better around the chassis (at least that was the theory). Backboards were stouter. Cosmetics changed too: a new type of beige covering (sometimes called "fawn" or "tan"), and two lines of gold piping around the body of the cabinet.

The circuit and control panel of the AC30/4, however, remained much the same as for the TV front amps. The legend by the fuse continued to have the Greek alpha rather than an "A", and the two tone controls, "Brilliant" and "Bass", held their place.

From mid 1960 production of the AC30/4 went side by side with that of the AC30/6. All preamp chassis were drilled for six inputs, though two were naturally blanked off by the control panel of the AC30/4:

A detail of the chassis of AC30/4 serial number 4715.

"Melody Maker" magazine, 19th November, 1960. The AC30/6 is 89 guineas; the AC30/4 is 85 gns. There is also an AC30/4 Bass model at 89 gns.

On serial number plates "N" stands for "Normal voicing", and "B" for "Bass". Often in the lower compartment of the plate, there is a "4" in the upper panel (to indicate the AC30/4), and a "W" (an upside-down "M"), which may signify something along the lines of "wide frequency", distinguishing these amps from the more constrained tonal response of the early AC2/30 and AC/30.

Further images of AC30/4 sn. 4669 BASS below.

The explicit designation of the amp as a "4" on the plate (something not found on the TV front amps) was a means of making record keeping easier (the registers kept of sales and despatches). Since AC30/4s and AC30/6s had serial numbers in the same sequence (in the range low 4200s to low 4700s), one could not tell simply from the number alone whether the amp was of one type or the other. Black panel AC30/6s (Twins and Super Twins) continued on their own from the low 4700s to mid 4900s.

The speakers chosen for the split-front cabinet were new - and standardised (or at least regularised): the Celestion "blue" - G12 alnico T530, sprayed Jennings blue - as depicted in the page from the brochure below.

Note that the brochure illustrates a control panel for the new circuit of the AC30/6.

By October 1961, Jennings was signalling the phasing out of the AC30/4. See the . Orders for the AC30/4 were honoured though at the old price of 95 guineas (£99 5s). For a limited time, one could purchase the AC30/6 at the special price of 100 guineas (£105).

Components, assembly, etc.

Chassis were assembled at the Vox works at 115 Dartford Road. For pictures of the site and the factory, (on the Vox AC100 website). In Spring 1961, Westrex took over the assembly of AC30/6 chassis, but evidently not the AC30/4. Farming production of the former out will have simplified production considerably.

Notes on how to read component date codes, are .

The transformers for both the AC30/4 and AC30/6 were made by Haddon ("Haddon Transformers Ltd." of South Ruislip), the choke by Radiospares.

Haddon mains transformer of AC30/4 serial number 4401.

The schematic for the AC30/4 is JMI sheet OA/032 (drawn 29th April 1960, and registering a change dated 7th Feb. 1961).

Detail from sheet OA/032. There is another drawing of this schematic, same date, same number. Points at which the circuit is different for the AC30/4 Bass model are indicated with asterisks. For the early AC30/6 schematic, see this page.

The circuit of the AC30/4 Bass differed only in the values adopted for six coupling capacitors (C3, C6, C10, C7, C9, C11). Most are of a higher value to reduce the amount of treble admitted.

Detail from a JMI brochure of mid 1961 prepared for "Musicland" in Bexleyheath - principally guitars, but the AC30/4 Bass is mentioned (even if the wattage is given incorrectly).

In general outline, the preamp of the AC30/4 goes hand in hand with that of the AC15 (schematic OA/31) - an EF86 as V1, three ECC83s (V2, V7 and V9), and an ECC82 (V8). The output section is of course doubled up: four EL84s (V3-V6) instead of the AC15's two.

The EF86 which added so much to the sound of the AC30/4 was unfortunately also its weak point - EF86s can be extremely sensitive to vibration and prone to microphony. The AC30/6 with its complement of ECC83s was much less troublesome.

Jim Elyea's book, "Vox Amplifiers. The JMI Years" (California, 2009) is again a wonderful repository of info on these amps and other AC30s.

"A Service Engineer's Guide to the Vox AC30 Valve Amplifier" by Stephen Grosvenor (2006), gives excellent schemas of the circuit boards and electronic layout. His site .

If anyone knows of further early new-style black panel AC30s that should be incorporated on this page, .

AC30/4 new split-front cabinet: Mid 1960 - Sept./Oct. 1961

Serial number 4250

Belonged to Chas McDevitt. Existence noted by J. Elyea.

Serial number 4290N - currently in the USA

Serial number 4290 also apparently belongs to a TV front amp (circuit said to be AC2/30) in light grey . "N" signifies normal (rather than bass) voicing.

Serial number 4347 B - currently in the USA

Pictures from some time ago (the three smaller ones from 2005). The date code on the TCC capacitor by the choke is "RD" = April 1960. The detail of the plate has been blown up a little beyond its natural size to show the number better. A relatively rare early instance of a bass voicing.

Serial number 4400 - currently in the USA (?)

Existence noted by J. Elyea.

Serial number unknown - currently in the USA

The green TCC filter capacitor in the preamp has the date code "RC" = March 1960. The Hunts Tropydur capacitors have been replaced.

Serial number 4401 - currently in the USA

From the holes along the lip of the preamp it may be that this chassis was originally prepared for a TV front box but actually went out from the factory in a new split-front cabinet. No designation of voicing (normal or bass) on the serial number plate. Sold by in 2016/2017.

Serial number 4430 - currently in the USA (?)

Existence noted by J. Elyea.

Serial number 4495

Existence noted by J. Elyea.

Serial number 4530

Existence noted by J. Elyea.

Serial number 4570

Existence noted by J. Elyea.

Serial number 4586 N - currently in the UK

Sold in the UK in 2018. A nice example externally - no pictures of the electronics unfortunately. On the serial number plate the "N" for normal voicing.

Serial number 4606 N - currently in the USA

"AC30 4W" "4606 N" The "N" for "Normal voicing" was stamped over a "B" for "Bass". The cab is later - from 1964, with rounded corners to the backboards, basketweave vinyl, and black vents.

Date codes "16FF" and "29FF" = 16th and 29th June, 1961.

The amp does not survive, but its serial number plate and speakers, which travelled together, do. Given the date of the speakers, the amp will have been ready for sale in early June 1961. Thanks to Chris for the pictures.

Serial number 4627B - whereabouts unknown

Serial number 4633N - currently in Europe

Said to be in Italy, but it may be elsewhere by now. The pictures, supplied by a potential seller (caveat emptor, as always), actually come from an ebay auction.

Serial number 46x9 N - formerly in the UK

Noted as having original components with date code March 1961. The choke in this amp was made by Radiospares. Mains and output transformers are Haddons. The serial number plate was preserved with the chassis.

Serial number 4669B - currently in the USA

Serial number 4715 B - currently in the UK

Excellent condition. Good shots of the Haddon transformer label, and the RIC cone code of the alnico blues. The choke, which is probably a replacement, has a Drake Transformers code: 292-426.

Serial number unknown - currently in the UK

Updated

Serial number unknown - currently in the USA

Two sets of pictures, one from Plexi Palace days long ago, the others slightly more recent. The output transformer is a Radiospares unit. The shrouds of the choke and mains transformer have been painted blue in place (note the brackets on the mains transformer) shrouds. The speakers are likely to be Celestion CT3757s.

NEW

Serial number unknown - currently in the USA

Distinctive in having one large round black control knob (the first on the panel), and five smaller (normal sized) ones. At some point in or after the mid 1960s, the electronics were overhauled - visible mustard capacitors have 1964 date codes.

Serial number unknown - currently in Australia

Superbly restored, the Celestion B024s from a Hammond Organ. Thanks to Guy for the pictures.

Serial number unknown - currently in the UK (?)

Note the cigarette burns near the outer handles. Grille cloth possibly renewed.

Serial number unknown - currently in the UK (?)

Old, old pictures, but a reasonably intact AC30/4 chassis and its rather battered cab. The drivers are Goodmans Audiom 60s, 15 ohms each. The black covering of the box is probably original - note the finger joints showing through the fabric.

Chassis - currently in the USA

An AC30/4 chassis with "cooker" style knobs in a later AC30 head cabinet. Sold by Cream Street Music. The serial number plate on the cab belongs to the cab not to the amp. Haddon transformers in place; no visible date codes on components.

Chassis - currently in the UK

The aluminium preamp has been drilled for six input jacks (see the last picture, top left corner), the centre pair blanked off by the control panel. The Lemark transformer is a replacement.

NEW

Chassis - currently in the USA

Voltage selector set to 115v. The box, with its black basketweave covering and plastic handles, is later.

Chassis - currently in the USA

Voltage selector set for 115v. The fuseholder is a replacement.

Chassis - currently in the UK

Chassis - currently in the UK

A knocked-down AC30/4 chassis - the "Brilliant" and "Bass" tone control legends just visible - sold on ebay some time ago along with a selection of parts. The panel is interesting. The control legends and outer decorative band look normal; but the inner bands marking off the pairs of control are gone, and instead there are what look like incised markers.

On to .

Copper panels appear in the high 4000s, see this page - .