Schematic of the Mark 1 AC50 - OS/044

Early 1964, single channel, valve rectified, two input and four input models

Around 500 amps were built to this circuit, January to July 1964.

The original schematic is undated (ie. bears no date). Above, the circuit redrawn (and checked) from OS/044. Click to enlarge (and to download the full-sized image). A number of amps had a slightly modified bias circuit, as noted below.

A note on the bias circuit

Click for larger images

Above, parts of the two earliest AC50 schematics that survive - OS/044 for the single channel amps (two-input and diamond-input); and OS/053 for the twin channel amps (see below for a full version, redrawn).

Very few, if any AC50s seem to have been built exactly to OS/044. Generally there is a second 220K resistor parallelled across the 220K specified in the schematic, bringing the total resistance at that point to 110K - pretty much in line with OS/053.

The second 220K resistor is piggy-backed across the larger white one at the top of the board

Second, as has been kindly pointed out to me, there is sometimes a third 220K underneath, and in parallel with, the 25K trim pot, limiting the effective value of the pot to 22.5K.

The purpose of these changes was evidently to increase the scope for bias adjustment in the two-input and diamond-input amps - most likely an initiative undertaken at Triumph.

The bias voltages indicated in OS/044, -32 to -35v, will therefore have been brought closer to the values later expressed in OS/053, -32v to -40v. Presumably the range -32v to -35v was felt to be too limiting, running less strong EL34s too hard.

The little neon lamp was a current and voltage regulating device, acting much as the larger lamp limiters (placed in series with mains positive line) that old radio men used to use for testing potentially defective devices.

Schematic of the Mark 2 AC50 - OS/053

Two channel, valve rectified, four inputs (small and large boxes)

Around 1500 amps were built to this circuit, mid 1964 - early 1965

Schematic originally drawn up on 16th March 1964. Above, the most accessible version, drawn by Chris Devine. The copy embodied in the Thomas Organ Vox service manual of 1967 is extremely blurry. See below however for the AC50 parts list that Thomas Organ compiled.

Schematic of the wiring of the XLR speaker connectors - OS/054

Drawn up for new large box AC50s

Schematic undated. Note that small box AC50s do not have an impedance selector on their back panels.

Schematic of the Mark 3 AC50 - OS/072

1965-1967, two channel, solid state rectified, four inputs

Around 5000 amps were built to this circuit.

Schematic drawn on 7th January 1965. 14th October 1965 addition of the brimistor noted.

Schematic for the Vox add on reverb unit - OS/075

Schematic drawn on 15th Jan. 1965 primarily for Vox AC30s, but the unit was certainly fitted to a small number of AC50s too.

Parts list for the solid state rectified AC50 issued by Thomas Organ in 1967


Older versions of this list probably survive in older versions of the service manual, though the roster of items should hardly differ. Note that provision is made for the servicing of valve rectified amps too: the GZ34 and the neon bulb (employed in the bias circuit).