Turtlerock Forum

The power supply schematic for Gyraf's G9 valve pre (http://www.gyraf.dk/gy_pd/g9/g9pd.htm) has 2 power transformers, each 120+120:15+15.

You connect the 240V AC power supply to one, which then gives you 2x15V AC supplies - one is used for the +48V phantom power, and the other for a 12V valve heating circuit. So far so good.

The heater circuit 15V supply is also fed back into the second transformer to "regenerate" the original 240V power supply, which is then rectified, buffered and regulated to provide a +245V HT supply for the valve circuit.

Can anyone tell me the purpose of using a second power tranny to regenerate the HT power? What not have the 240V supply go to just the one tranny only and also to the HT circuit, rather than being dropped to 15V and then brought back up to 240V?

Hi Chris, It's a common trick used these days when you want a high voltage transformer, but don't want to pay the enormous expense for a Hammond or mercury magnetics one or any other (insert brand).
By putting 2 inexpensive transformers back to back(connect the low voltage winding of one back into the low voltage winding of the other) you achieve isolation (You must have isolation, directly rectifying mains is very dangerous) as you would using 1 proper transformer, at a much lower cost but at the expense of the extra weight and space of a second transformer. Torroidal transformers are great for this as they're compact and efficient. You may have to be careful with placement/orientation as you now have additional magnetic fields, but again toroidals minimize this effect. If the Stray field is a problem you can always build the power supply in a separate box.
I've used this a few times for experimentation along with voltage multipliers to get a good HT supply on the cheap.
Another way of doing it is using an oscillator and high speed transistor on an inductor to create a charge pump then rectifying this for HT, These are great for Pre-amp tubes and things that don't need much current as they are very compact and allow you to generate a 300V HT line at about 10mA from a plugpack.Effectivly a simple Switch-mode power supply.

Thanks Alastair - you deserve every one of those valves under your name!

Isolation makes sense - I was thinking that there had to be some safety rationale behind it.