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RE: Was: Re: PAS Transformer Replacement

166.250.0.62

Posted by sbalfour on March 22, 2011 at 10:04:59

In Reply to: RE: Was: Re: PAS Transformer Replacement posted by xaudiomanx on March 22, 2011 at 02:34:10:

The line and phono sections are in parallel, otherwise we'd need voltage
of 48V, not 24v. The triodes of each tube are in series; the tubes in
each section are also in series. From the 12ax7 spec, series wiring the
tube draws 150ma@12.6v (~10.5v, after the selenium rectifier). Series
divides the voltage; parallel divides the current. At 24V from the
voltage doubler, each section sees 24v divided between two tubes, so
each tube sees 12v. Each tube draws 150ma@12V, so each section draws
150ma@24V. The two sections in parallel draw 300ma.

The 12x4 draws 300ma@12v. The #57 12v 5W bulb draws ~400ma, less as it
ages. Let's count 300ma.

So I get:

150ma phono
150ma line
300ma rectifier
300ma pilot bulb
-----
900ma

That's not the whole story, because we have I^2*R losses in the voltage
doubler and selenium rectifier as well as the rest of the circuit. The
ESR in those old 2000uF filament filter caps could be a few ohms, so
we're losing 50-100ma right there.

Since heater current doesn't vary with load (we hope), it looks to me
like we're over spec @800ma on a constant basis.

See Norm Korens figures for this at
http://www.classicvalve.ca/docs/PAS-K_phonoboard_docs.pdf. (Scroll down
to near the bottom). He's an old hand at this stuff, too.

The HV winding isn't much better off: each section of 12ax7 (from the
spec) "typically" operates at a draw of 1.2ma. There are 4 tubes, 8
triodes, so the "typical" draw is 9.6ma. I think I miscounted in my
original post, and said it was only 4.8ma, oops! But maximum dissipation
at the plate is 1.0W@300V = 3.3ma (B+ isn't 300V, but more like 265-280V
at the plate for PAS). There are 8 sections, so under full load (250mv
square wave input at phono, at maximum gain), the draw /could/ be 26.4ma.
Of course,the voltage would sag badly, and the dissipation drop sharply
if you did that for longer than a transient (the PS caps will support
the voltage for brief transients). The HV winding is rated at 10ma reportedly.

The secondary windings aren't independent - they're on the same piece of
iron, and share the magnetic flux. If you overdraw one, it'll suck juice
out of the other(s). If you overload the HV winding, it'll draw down the
heaters. And that is where we're at, with the PA211.
Stuart

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