0-190-275V 20W power transformer
Choice of output voltages; create an amp with switchable B+!
I specced this power transformer to be very flexible, able to run medium-power single-ended amps and low-power push-pull amps. The first step was to choose high voltage windings to suit a range of projects. I looked at popular amp projects and chose the following:
- The 0-190V winding produces around 260V when rectified – great for EL84-based amps. There’s the full 160mA available, so you can run a pair in push-pull, parallel SE or even in stereo.
- The 0-275V winding gives typically 360V when rectified – designed for octal power valves (EL34, 6L6, etc) in more powerful SE amps (15W or so). This also has 160mA available, so it’s also perfect for push-pull amps with lower-power valves, such as the EL84 and 6V6.
- The 190-275V winding is also available, and gives around 115V when rectified. This is a ‘freebie’ – a side effect of the two main values I chose – but it adds a really useful option to use in amp projects… …Switchable B+!
With these asymmetrical windings, there are three combinations available to run into the rectifier bridge. This means you can use a 3-position rotary switch to get a switchable B+ supply in your amp. With the ratios given you get:
- Full power from the 0-275V winding
- Half-power from the 0-190V winding
- 1/10th power from the 190-275V winding
It’s like an ultra-simple version of Power Scaling (see Kevin O’Connor’s London Power Website) or VVR for the cost of a switch! With 3A at 6.3V available for heaters, this is a very flexible transformer for amps up to 18W or so in output.
Dimensions & info
It’s 78mm wide, 70mm deep and 64mm tall, with standard solder lugs for all primary and secondary connections. It’s available in a lay-down style, to drop through a rectangular cut-out in the chassis (such as that on AC04 small and AC02 medium chassis). The rectangular cut-out should be 54x49mm, and the four 4mm mounting bolts are on 64x51mm centres.
There’s also a frame-style version (currently on sale at a discount) that mounts without the need for a rectangular cutout. Both are available in 120/240V versions or 220/230/240V versions.