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Steerpike 12th November 2008 08:40 PM

How best to use the PSU xformer I have
I have a 300VA transformer with 400mA secondary, with following taps:
0-10-165-220V (and several other separate low current windings)
I need to get around 450Vdc B+ for a stereo EL35 fixed-Vg3 Push Pull (that's 4 pentodes in all, 2 per channel).

It seems my best option is 165Vac - through full-wave doubling Si rectifier - gives around 460Vdc as required.

Two worries though: the doubling rectifier doesn't give as good regulation as a full-wave bridge, and I'm wasting a good deal of the secondary winding.

However, if I use the whole secondary with a bridge rectifier, I get 220*1.41 = 310V; not really enough.

Also, 400mA at 165V is only 66 VA - I think thats bordering on the limit of what 2 x 2 EL34s as pentudes will consume. The transformer has many other windings that I will not be using at all, so can I stress the 165V tap for a bit more than 400mA - intermittently? (its a class AB PP, so long term average power will be lowish)

Anyone got any great suggestions please ?

Tom Bavis 12th November 2008 10:27 PM

What are primary and secondary DC resistance? I would guess that you can get considerably more than 400 mA, depending on the winding resistance, but it still may not be enough. Figure transformer current to be around 3X DC current...

Regulation of a full-wave doubler is essentially the same as a full-wave bridge, assuming truly equivalent transformers (for the same amount of copper, the winding with half the voltage would have one quarter of the DC resistance).

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