terry wrote:
Understand that inside a typical computer switching power supply there
is a higher multi kilohertz voltage (350v RMS????) produced by the
switching action of the rectified 115 or 230 volt AC input?

This high freq AC through various usually toroidal step down
transforner windings is then rectified to provided the plus and minus
5, 12 volt outputs etc. Correct?

We recently modified such a power supply to get a single 12 volt DC
output at about 20 amps for a particular, amateur radio application.
In order to do so we had to load one of the 5 volt ouputs with a
couple of amps in order to get the unit to work; but that's normal.

Question: Is it possible to get at that higher AC voltage inside and
directly rectify it as a B+ supply for tube equipment? While also
possibly tiddling one or more of the 5 volt DC outputs closer to 6.3
volts for tube heaters?

Rectified 10 or 20 kilohertz wouldn't need much filtering compared to
60 or 120 DC ripple of a conventiaonl power supply? Recall building a
number of conventional 50 and 60 hertz power supplies many years ago
with heavy chokes and large capacitors. But now have a number of
slightly older ex computer power supplies of various wattages around.

Or is the idea completly off base?

The "higher voltage" is usually on the line side of the isolation
transformer. You would create a safety hazard by accessing that voltage.

A thought might be to use the non-rectified low voltage side to drive a
small high frequency step up transformer or use a diode voltage
multiplier circuit or a combination of transformer and voltage multiplier.

There is a company that builds high voltage switchers for amateur radio
amplifiers, and there have been construction articles to build these.
(Check QEX magazine).

Building from a surplus PC power supply would have a cost advantage.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P