Reducing HT

[izzy wizzy]

I have a question for the panel.

I need to increase the HT to the front end of my power amp so I made it cap input with a 6u8 cap after a FW valve rectifier. It got me going but isn't enough HT. I'm experimenting by DC coupling the first and second stages.

I want to go back to choke input (to rule out that change to cap input) but a bridge would give me too much HT; about 450V when I want about 370.

I have some transformers lying about that I could do one of two things. I could bolster the secondary by adding windings in series from the other transfomers or I could buck the primary to reduce the voltage. I've never done either and not sure of the issues. My preference would be to buck the primary if that's the best/better way?

Ultimately I will buy a new transformer when all this mucking about is over.

What does the panel think?

cheers,
Stephen

[Nick]

So if I understand, you have 450 and want 370? So that's 82% reduction you need. So assuming the primary voltage is 240v, you want to change that to 197v, so a reduction of 43v. So I would get something like a 300v 18v+18v torrid. 20v would probably be better, but you could always add some extra turns to it as needed. Then wire all the windings up in series in phase, apply 24v across the lot, and you should be getting something like the voltage you want from the 240v part of the windings, Apply that to the primary of your existing transformer and see what you get.

If it needs tweeking a little. Wind ten turns of insulated wire around the torrid (why its worth getting one without a filled center). Then measure the voltage that is developed in those ten turns, divide that voltage by 10 and that gives you the volts per turn for the transformer. Armed with that, you can then work out how many extra turns you need to add either in phase (to reduce the voltage into the original transformer, as you are reducing the ratio of the torrid primary to the other turns) or out of phase (as you are increasing the ratio).

That's what I would do anyway. All without killing yourself of course

[izzy wizzy]

Thanks Nick. That explains it real well and I should be able to use a multiwinding tx I have lying around.