Transformers

The most expensive part of the amp build is the transformers.

Amps all have a power transformer and an output transformer. The GA-40 also has a choke. A choke is used to filter the audio signal.

From the information we obtained relating to the valve specifictaions and form the schematic, we can start to build up a picture of the type of transformers we need to use.

Finding transformers online is difficult her ein the UK for some reason. The best source I have found is Bluebell Audio who sell transformers made by the Canadian manufacturer Hammond.

The power transformer needs to have three secondary windings:

1. high voltage 300-0-300VAC. This winding has to power all of the plates of all of the valves. So you need to figure out how much plate current in total will be drawn by the valves. That is why I collected the plate current figures for each valve in hte previous post. I make the total about 113 mA. So we need the high voltage winding to produce at leats that much current.

2. Heater 6.3V CT

3. Heater 5V CT

Looking at the Hammond website, I found that the Hammond 372fx provides all three of these secondary windings and the high voltage winding is a 150 mA. It is £57.

The output transformer needs to be capable of coping with 14-16 watts and provide a matcning inductance of about 8000 ohms. Philip at Bluebell advised me to get the Hammond 1650E which gives 15 watts and 8000 ohm inductance. It costs £49.00.

The choke is a 3H from the schematic and the rectifier outputs 125 mA. So the choke needs to be a 3 Henry choke @ 125 mA. The closest I have found is 3.5H @ 150 mA in the Hammond 157Q for £10.25.

A total of £106.25 which is pretty reasonable I think.