AC10 Amplifier No.3 - TV Front and Split Front
From late 1960
Glen Lambert
Detail from the standard version of JMI circuit diagram OS/008, signed off by Derek Underdown, dated 9th September, 1960. Dick Denney ("DD") signs off "ISSUE 3" in September 1964, presumably as a preliminary to its dispatch to Thomas Organ. Early copies of OS/008 survive with no date and no additions.
Serial number ranges: TV front - from around number 325X to 343X. New-style split front - from 343X on.
By the end of 1960 the AC10 Amp No.2 was superceded by the AC10 Amplifier No. 3 - a five control dual channel amp with tremolo integrated for the first time into every chassis and panel on all builds. The chassis was mounted on a sliding plinth (see the photo lower down this page) in a cabinet with a control panel opening extending all the way to the rear. The first AC10 schematic proper - OS/008 - was drawn in September 1960 and designated "AC10 Amplifier No.3". This was effectively the final version of the AC10, containing much of Derek Underdown's earlier work, but updated with some of Dick Denney's to produce a well developed and excellent sounding combo.
AC10 serial number 3337, control panel.
The AC10 Amplifier No.3 amps first appeared at the end of 1960 in a 1x10" TV front cabinet with rounded corners. This style of cab lasted through to mid 1961, after which the new fawn-covered split front cabs took over (there was evidently some period of overlap). Chassis initially had gold-topped knobs, black photo-anodised control panels, and other familiar AC10 trappings. This version of the amp, produced in greater numbers than any earlier TV front AC10, is the type most commonly owned by collectors, currently at least. In common with the AC30, copper and grey control panels were subsequently introduced, along with Twin and Super Twin variants. More on these later AC10s and their offshoots can be found on the AC10 index page.
The circuit of later 1960 (OS/008) carried through to the end of the JMI era c1966/67. As an aside, the G1/10 single channel amp was briefly re-introduced in a non-standard (tall) split front cabinet at around the same time as the AC10 Amp No.3, seemingly to cover the budget 10w pricepoint, which had previously been covered by the AC10 Amp No.2 (version without rear tremolo). During this period JMI was competing with other English makes such as Selmer, Watkins, and Grampian who all had well-priced 10w amps. Jennings appears to have thought it best not to leave an opening for competitors. More about this budget single channel AC10 here. A page on the "Throwback" AC10 can be found here.
"Melody Maker", 12th November, 1960, advertising the AC10 Amplifier No.3 (here called "10 Watt 4, input vibrato") at 45 guineas, the reintroduced G1/10 with tremolo added under the control panel now called "10 Watt, 2 input vibrato", priced at 35 guineas
As mentioned above, the earliest five control AC10s were housed in a TV front cabinet with a control panel cut-out at rear to allow the chassis to slide in (and out when required) on a board. Input jack positions were 'Vibrato' at left and 'Normal' at right, the reverse of the order of those on Amplifier No.2. Below, a general view of a chassis, a rare early one, similar to serial number 3337, with dark brown Pekalit knobs and old style B9A voltage selector, both of which were soon changed to the more familiar pointer knobs and 5-way paxolin wafer selector.
Detail of a chassis, slider board, and footswitch.
AC10 serial number 3303, detail to show the arrangement of the inputs.
A quirk in the design of the AC10 Amplifier No.3 allows the tone in the main (EF86) channel, which has more gain and harmonic content than the Normal channel, to be adjusted via the Amplitude control. When the 'Amplitude' control is set to maximum the screen voltage is increased, which drops the voltage on the plate by more than half, applying a slight increase of g1 bias. Note that this occurs - or rather is achieved - when the footswitch pedal sets the tremolo to "OFF". With the Amplitude control all the way down a more open, louder tone is produced. With Amplitude all the way up a more compressed quieter tone results. This is true for all five control AC10 amps made from late 1960 onward.
The AC10 Amplifier No. 3 proved to be a highly successful design which was retained unchanged by JMI until the end c.1966. It is a favourite of many, and its design is credited to both Dick Denney and Derek Underdown. An overview of surviving AC10 No. 3 amplifiers from late 1960 and early will be presented shortly on a new page.