The Vox AC/30 - summer 1959 to early 1960
Single speaker, 4 inputs, two EL34 valves; cathode bias.
An early advert for the AC/30
The AC30 in a new format: its cabinet and styling borrowed from the AC1/15, which had remained in production throughout 1958 and 1959. Tremolo and Vibrato come built in. There are no longer "standard" and "vibrato" models.
The amplifier chassis comprised of two parts: the preamp, with controls, at the top of the cabinet; the power section fixed at the foot. This was the standard arrangement of the day, employed also by Selmer and other manufacturers.
The AC/30 preamp section, which contained all the valves except the GZ34 rectifier, was initially a modified AC1/15 chassis. See the pictures of surviving amps towards the foot of this page. In early 1960 a dedicated AC/30 preamp was designed and put into production.
During this period, i.e. 1959-1960, Denney evidently worked to turn the AC/30 into a single chassis amplifier, using EL34s at first, then four EL84s. This new design, or rather format, was to become the AC30 Twin.
The schematic of the single speaker dual-chassis AC/30 amplifier is OS/007, dated 1st January 1960. Certain elements changed (or were variable) as production proceeded.
Detail from OS/007.
The power chassis of the new AC/30 was housed at the foot of the cabinet, the preamp at the top. The complement of valves was: POWER SECTION - GZ34 rectifier, and two EL34s in cathode bias. PREAMP - three ECC83s and one ECC81 (the phase inverter).
Although the pair of EL34s were easily capable of delivering more than 30 watts, power had to be constrained - the single speaker was the limiting factor. It was still extremely rare in 1969/1960 for a 12" speaker to handle more than 15 watts. This at any rate was the power handling of the speakers chosen by JMI - either the Goodmans Audiom 60, or the Celestion G12.
The semi-closed back or the cabinet, normally made of brown or black perforated fibreboard (again, as in the case also of early Selmer amps), gave some degree of extra protection to the speaker, increasing safe power handling to over 20 watts.
OS/007 - the cathode bias of the EL34s - 470R resistors bypassed with 50mfd capacitors. 470R was the value of resistor recommended by Mullard in its specimen cathode bias circuit and 375v the recommended HT (see any EL34 data sheet.) It is likely that the AC/30 in actual fact produced less than 30 watts.
On the back of serial number 4052 the legends stencilled in white are: "Made in England, Jennings Musical Industries Ltd., Unity Works – Dartford – Kent – England". At the bottom, centre: “Warning, Disconnect Mains Plug Before Removing Back Cover". On the left side: "For AC Mains Only". Right-hand side: "Check Voltage Setting On Back Of Chassis". At the bottom of the panel on the left-hand side:"Serial No." though no number is supplied.
The details above come from John Teagle's interesting piece published in "Vintage Guitar " magazine, May 1998 - available here. See also the pictures towards the end of this page.
Serial number 4052's speaker is a Rola G12. The original cover, which still survives with the amplifier, is green with tan piping.
The note in the "Accordion Times", June 1959 (below) that "a 30-watt model CAN be supplied..." indicates the amplifier's newness at that point.
The new AC/30 - "Accordion Times" magazine, June 1959.
"Accordion Times" magazine, July 1959. The amplifier is not illustrated but described briefly: "4 inputs, 30 watts output, 70 guineas".
"Melody Maker" magazine, 31st October, 1959 - new guitars, including the Electronica Oval-Basso (the small image showing it in profile). The amplifier priced at 70gns is the new AC/30. This advert first appears on 10th October, 1959 (though lacking the profile representation of the electro-bass).
"Melody Maker" magazine, 14th November, 1959. A growing roster of artists, now including Cliff Richard, Marty Wilde, and the John Barry Seven. The AC30 - 30 watt output, 70 guineas.
"Melody Maker" magazine, 28th November, 1959, a detail from a Boosey and Hawkes advert. Dave Goldberg, noted as a Vox user in the ad. above this one, with what may be an AC/30.
"Accordion Times" magazine, April 1960, the AC/30 (not described explicitly) being the amplifier priced at 70 guineas. Note the new style of Vibravox with an inset black control panel.
"Melody Maker" magazine, 30th April, 1960. The AC/30 now 79 gns. There is also a bass version at 85 gns.
Printed in "Vintage Guitar" magazine, May 1998, in relation to AC/30 serial no. 4052, a JMI pricelist, undated but doubtless mid 1960 given its correspondence with the advert placed in the "Accordion Times" below.
"Accordion Times", May 1960. The AC1/15 at 65 gns, the AC/30 at 79 gns. The amplifier at 100 gns is the new AC30/4 twin - see this page.
Detail from above.
"Banjo, Mandolin and Guitar Magazine", October 1960, still the AC/30 in its single speaker format. Note the logo/emblem lower left on the front grille, as in the illustration at the top of this section. Picture from this great blog.
in early 1960 the price of the AC/30 had gone up to 79 guineas.
Single speaker amplifiers.
At present, three amplifiers with serial numbers are known: 4041, 4052 and 4076, the sequence presumably beginning at 4000 or 4001.
Perhaps around 150 AC2/30s and AC/30s were made in total - ie. running from serial no. 4000 to around 4150. The last AC/30s will presumably have had numbers overlapping those of the new TV Front twins.
Serial number 4041 - currently in the USA
Serial number 4052 - currently in the USA
A great article - well worth reading - by John Teagle in "Vintage Guitar" magazine, May 1993. The amp has its original cover (which was priced separately in 1960) - see this page.
Serial number 4076 - currently in the USA
Serial number unknown - currently in the UK
Recovered in thick black vinyl, orange visible underneath. The speaker is now a modern Celestion greenback. The upper chassis has a "laydown" transformer and a small plinth for the first of the vibrato cans. Underchassis electronics in good condition. One of the green TCC capacitors presents the date code "QM" = December 1959.
Serial number unknown - currently in the UK
Picture from some time ago. The amp still has its "THIRTY" runner lower left on the front.
Serial number unknown - whereabouts unknown
Sold by Ampaholics a good while ago.