240V/6.3V toroid power transformer
Dual 120V/240V primaries; ideal for preamps and very low output power amps
This tiny toroid power transformer is made in England by a traditional and long-standing transformer winding company. Being toroidal in design, it has a major benefit over the conventional EI-style transformers – it radiates a significantly reduced electro-magnetic field, and that helps keep hum levels very low.
It’s designed for use in a 2-3 valve preamp and is small enough to be fit in a 1U rackmount case. It has dual 120V/240V primary windings and two secondary windings. The high voltage winding is 0-240V rated at 30mA. In a typical circuit, this will produce a B+ of around 320V or so after smoothing. Of course, you can drop more voltage if necessary with zener diodes or simple voltage dropping resistors in your power supply rail.
The heater supply secondary is a non-CT 6.3V rated at 1A. This makes it perfect for an amp using two or three 12AX7s (or equivalent. I have also tested it with a single ECC83 and a 12AU7/ECC99 configured as a push-pull amp for 1-2W output together with the 4W output transformer. (Note that with an ECC99, the total heater supply runs slightly over-spec at 1.1A, but the transformer has a 15VA core and handles it well enough.)
Dimensions & info
It’s 65mm in diameter and just 29mm tall. It has a resin core, drilled with a central 5mm mounting hole for mounting to your chassis. Note that – as is always the case with toroidal transformers – you must make sure that only one end of this mounting screw is attached to the chassis. The other end should have no connection back to the chassis (because this would create a magnetic short that would destroy the transformer). The wiring diagram is also shown (right). It has 20cm flying leads for all connections. Shown with a 100nF Orange Drop cap for scale, this toroid combines a tiny footprint and a super low hum field The TX has dual primaries to suit European 220-240V mains and North American 110-120V mains