PSU Question

[XZur]

What's the trick to sqeeze 500v of B+ from a trafo with secondary 300-0-300v? Un-Possible

[Eli Duttman]

Tie the center tap off. Use a hybrid bridge of 2X UF4007s and a 5AR4. Use choke I/P filtration. The B+ will be slightly above 500 V.

BTW, a "proper" choke I/P filter is LCLC.

[Giaime]

Eli,

will the mA rating will affected? Can I squeeze out the same current or am I wrong? In theory your'e increasing voltage at the expense of avabile current...

[Eli Duttman]

Giaime,

You're absolute correct about TANSTAFFL applying. A rectifier winding is rated for some number of VA and that number is NOT to be exceeded.

The saving grace in the situation is the fact that when choke I/P filtration is employed, the full RMS current capability of the rectifier winding is available as DC. When cap. I/P filtration is employed, approx. 1/2 of the max. AC RMS current is available as DC.

Choke I/P filters are voltage poor and current rich. Cap. I/P filters are voltage rich and current poor.

[Giaime]

Yes, thanks, I understand.

But with bridge rect. and choke I/P, will I have more current avabile than full wave rect. and cap I/P or it will be the same, only the voltage will be different?

I ask this because I'm making plans for a tube power amp, and I'm restricted with a salvaged 300-0-300 transformer, so I can't decide if I can go to SE of big tubes (aka 6L6GC, 6550, KT88) or PP of small ones (6BQ5, 6MB8, 6V6 etc etc...)

[Tweeker]

Choke input will allow similar wattage as cap, if not somewhat more. The high current peaks are mitigated. So yes, more current, lower voltage.

Edit- this post assumed the same rectification in both cases (bridge).

[Tom Bavis]

You can get more DC watts with a full-wave bridge than a 2-dode full-wave, for the same AC VA. Think of it this way...all the copper is carrying current on both halves of the AC, not just one half.

You can estimate the AC current capacity from the winding resistance. For a big transformer, 3 watts of heating in that winding will be ok. So max current is sqrt(3/R). You can use PSU Designer from duncanamps.com to model the RMS current with a given DC current draw.

High voltage is NOT good for single-ended operation - class A usually is best at a lower voltage. at 500-600V, EL34 and 6550 are not in a very linear region - better for class AB push-pull.

[Giaime]

Originally posted by Tom Bavis
High voltage is NOT good for single-ended operation - class A usually is best at a lower voltage. at 500-600V, EL34 and 6550 are not in a very linear region - better for class AB push-pull.

Hi Tom,

so I assume it's better to use that PT for an EL84PP, like the amp where it comes from.

Thank you very much

[XZur]

Thank you all for the inputs.