TRANSFORMERS FOR VALVE RECTIFIED AC50s
Over the years, the two main assemblers of AC50s for Vox - Triumph Electronics and Burndept Electronics - used a variety of transformers.
The main types can be seen lower down this page - those used in valve rectified amps (the earliest sort of AC50), as well as those in solid state amps (from early 1965).
Mains transformers of valve rectified amps require an extra feed for the GZ34 rectifier valve - a 5v supply (which is not needed in amps with solid state rectifiers).
Graham Huggett, who worked at Triumph from 1963 to early 1966 as "chassis beater", related of early amps:
Control panels (bezels) were sent from JMI for the finished amps. Runs of around 50 chassis were made up on receipt of orders from Dartford. Assuming we had received the transformers from our manufacturer (took some weeks) the staff would easily wire up 15 chassis in a week."
One can often tell which contractor made an AC50 simply by looking at its transformers.
Units in Triumph-made amps have plain metal tops with a squarish profile. The leads are often tightly and neatly twisted in the underchassis.
Burndept initially used Wodens - stamped with Woden's name and date code - as it did for the AC80/100s it assembled. Later (in 1966) a black-shrouded type, sometimes wrongly said to be made by Albion, was adopted. The manufacturer that made these units also supplied Burndept with transformers for the Vox solid state line of amps (the Conqueror, Defiant and Supreme, et al.).
Majestic Transformers in Poole (Dorset) have original specification sheets for several AC50 transformers. The sheets were evidently drawn up by someone at Vox - presumably at Dartford Road - either on the basis of sheets provided by the transformer suppliers, or by breaking down specimen units.
Two of the transformers are described as being complex in structure, having silver shrouds, and large stacks. One is wound normally; the other reverse phase wound.
Effectively these two correspond to the transformers used in early small-box AC50s.
Above, the winding styles of two output transformers: above, simple; underneath, complex. Image from the wonderful Lenard Audio website. The complex form corresponds to the early AC50 large-stack transformers - pairs of wires for each side of the primary - P1 and P3 on one side, and P2 and P4 on the other.
The second of the two early transformers on file at Majestic is reverse phase wound - effectively the wire of two of the windings (P2 and P4) was wound in the opposite direction from the wire of the other two windings.
The result of reverse phase winding is said to produce an output transformer capable of reproducing a greater amount of bass. It is possible (no more than that) that these transformers were reserved for AC50 bass amps - ie. ones destined to be matched with bass cabs.
Majestic also has the schema for the later black-shrouded transformer (the type registered at the foot of this page).
TRANSFORMERS IN VALVE RECTIFIED AMPS
1964-1965. The AC50 mark 3 (mark III)
Transformer types are numbered simply for convenience. When an existing type reappears in a later species of amp, the number is repeated.
SMALL BOX AMPS
TWO INPUT AC50s
1.Large stack - Triumph-made amps. Straight lead. Red, black (and blue?) primaries. Double leads for secondaries. 1964. This type of transformer also reappears in amps made in late 1964 and early 1965. Thanks to Glen Lambert for the info.
DIAMOND INPUT AMPS
2. Small stack - Triumph-made amps, twisted leads. Red and black primaries. Single leads for secondaries. 1964. Note that the vertical slots in the chassis upright enabled both large and small units to be fitted.
1. Large stack - Triumph-made amps, straight (untwisted) leads. Red, black and blue primaries. Double leads for secondaries. 1964.
3. Large stack - Triumph-made amps, straight (untwisted) leads. Red and yellow primaries. Single leads for secondaries. 1964.
LARGE BOX AMPS (still valve rectified)
4. AC50 assembled by Burndept. Woden transformers, silver shroud, analagous to the plain-shrouded Wodens used in later AC80/100s, late 1964/early 1965. Mains = code 73691; Output = code 73692. Woden date code "JV" = September 1964.
5. Similar in style to (1) above, but red, black and brown primaries; single leads for secondaries. Tightly twisted leads. Triumph made amps. Early 1965.
6. A different amp from the one above, no pic. of shroud. Blue, black and pink primaries. Tightly twisted leads. Triumph-made. Early 1965.
For reference, a Triumph-made AC100 - mid 1965 - red, yellow and orange primaries. At around this date, Triumph were using a similar-looking transformer for the AC100s they were producing on a fairly fitful basis.
TRANSFORMERS FOR SOLID STATE RECTIFIED AC50s
1965-1967. The AC50 mark 3 (mark III)
7. Similar to (5) but red and black primaries. Made by Triumph. An early solid state rectified amp - no brimistor.
8.Silver shrouds, but solder tags on underside. The unused tag presumably the 4ohm tap. Note the presence of the brimistor.
9.Burndept-made AC50s. Code 66523. AC100 version, code 66776. Black shrouds, solder tags on underside. The unused tag, as in the case of (8) presumably the 4ohm tap. Below, images of the transformer (dead unfortunately) from AC50 serial no. 5369. The end bell went onto the replacement OT.