The material on this page was initially posted on the main updates page at the end of January.

The earliest surviving diagram for the AC2 / AC4 is OS/009, dated December 1961. There is also a further sheet, un-numbered, but identical otherwise.

Between late 1960 (the first AC2s) and late 1964 very few amps were constructed in conformity, perhaps not too surprising though, as these circuit diagrams are not blueprints. They were drawn up and circulated largely for repairmen.

The point to observe initially is the value of the grid resistor between the EL84 and the wiper of volume pot, given as 5.6K.

Detail from the AC2 circuit diagram drawn out by John Bell on the 9th December 1961.

The actual values used in production were:

Late 1960 - late 1961:: 4.7K - AC2 serial numbers 2000 to c. 2200. See this page for pictures.

Late 1961 - late 1964: 6.8K: - AC4s with pink/champagne coloured panels, serial numbers c. 2200 to 4100. Production contracted out to Triumph Electronics in Purley. Also to note that Triumph changed the relative positions of the EZ80, EL84, and main filter capacitor on the chassis. New holes - ie. a new design of chassis - were not required however as the components were of the same diameter - i.e. the socket for EZ80 fitted perfectly into the cut-out used up to that point for the EL84.

Late 1963 - 1965: 6.8K: - Domino Standard/Normal combos. These have the same circuit as the AC4.

Late 1964 through to end of production: 5.6K: - AC4s. Present in serial numbers 4257 and 4761. At last, the value specified in OS/009.

Most of the AC4s in the first batches sent over to the USA will have had a 6.8K value in place - and when Thomas Organ came to redraw the JMI circuit diagram a 6.8K was indeed the value stipulated. In many respects the Thomas diagram is easier to use than JMI's, laid out more clearly, and printed rather than hand-drawn.

Detail from the Thomas circuit diagram for the AC4.

Strangely, JMI never caught up. Circuit diagram OS/051 - "AC4 Amplifier and Domino Normal" - drawn first in March 1964 and redrawn in September has the 5.6K resistor.

Detail from OS/051, drawn by Albert Hogben.

The change in value of the on/off/tone pot from 50K (in the AC2 diagram) to 1M (OS/051 and Thomas) is more difficult to chart at present. Perhaps more later.

Where other models of Domino are concerned, the "Domino Bass" was effectively an AC4 voiced differently and without the ECC83 tremolo circuit; and the "Domino Reverb" and "Domino Super Reverb" species of AC10 Super Reverbs, as mentioned below (entry for 16th January).

On to the AC2 / AC4 in 1962 and 1963.