Saturday, 28 November 2009

HT Supply for Valve Projects

Disclaimer: This blog mentions dangerously high voltages. Bear in mind that you might get frightened or worse if you are incompetent, careless or unlucky - consider yourself warned!


I spent last week on a business trip to HB-Land. Took along my Eton E5, hoping to listen to some CW as "background music" but both 80 and 40m were silent. Don't know if my hotel was working as a Faraday Cage or if Swiss, French, Italian and other hams were on strike!

Instead of listening, thoughts turned to planning what I could build on returning home...

The next item on my list for the "Valve receiver" Project is the HT supply. I have a conventional power supply for my Paraset (made of an old Tektronic 'scope transformer) - but this kicks out nearly 400V with no load and I wanted something tamer.

I had also experimented with an inverter (after the circuit published by Harry sm0vpo and others). This worked well enough, but not with all transformers tried. I guess the transformer has to present inductance in the right "ball park" to get the astable to run in such a way as to make the entire system efficient.

The original inverter is shown below, along with its "favourite" transformer...

Whilst sniffing around the 'net, I found a circuit for an inverter in which the transformer is not a part of the timing of the astable - it should make the circuit less fussy about the transformer it is used with!

My circuit is seen below (I claim no novelty - all I've done is added the inverted stages to sharpen up the voltages driving the power transistors switching the transformer secondaries).

The (very) ugly version, made using "Dremel PCB" methods on a pre-owned piece of copper-clad, is shown here...

It works - with any transformer in my junk box - so now it is time to play with the audio stage... I wonder if I can find an EL84.

...-.- de m0xpd

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