Not exactly a stompbox question. More of a power xformer question

[caprotesta]

Hello all,

I realize that there are probably better forums to post this question, but I get such good responses from you guys I thought ask here.  And I'm still pretty new at this, so bear with me if the answer to this question is obvious.

I'm thinking of building an AX84 P1 guitar amp:

http://ax84.com/p1.html (schematic on p. 2 of AX84_P1_101004.pdf)

The power xformer in the schematic appears to be stepping up the primary AC voltage from 120 to 214v, with each line of the secondary being rectified through a single forward-facing UF4007 diode to arrive at a B+ of 266VDC. I can compute the rest of the voltage changes in the schematic, but I can't figure out how 214VAC becomes 266VDC.

Any/all help appreciated.

[R.G.]

Neither can I. You'd expect more like 300Vdc. Maybe it's got enough ripple in it to sag to 266 by meter. Maybe the power trans has a high internal resistance.

Generally, if you rectify with a solid state diode, you get the peak of the AC waveform minus diode drops and I-R losses. The peak to rms ratio for a sine is 1.414, so you'd expect 214 * 1.414 = 302.6 before you start subtracting off diode drops and I*R losses, as well as a ripple voltage loss of I*dt/C peak to peak for a load current I.

But ripple and I*R losses are highly situation-specific.

And I make lots of typos, so I have to allow other people to make them too.    Could be that.

[amptramp]

Transformers are rated by RMS voltage, just like the 120 volt AC line (or 220 if you are in the UK and other such places).  RMS voltage is the voltage that would give the equivalent dissipation in a resistor connected to the AC line as would be given by a DC input.  THe 120 volt AC line goes up to a peak of 169.7 volts (root 2 times the root-mean-square (RMS) value of 120 volts).  A rectifier-capacitor combination usually works close to the peak voltage, so the DC is usually higher than RMS AC.  If the voltage declined significantly during each half-cycle, there would be a lot of buzz at double the line frequency because the capacitor would be brought up to the peak voltage twice each cycle.

Root 2 = 1.4142135

[PRR]

> if the answer to this question is obvious.

Look-up the specs on the 269EX ?

269EX - 380V C.T. @ 71ma

So _190V_ each side when run near full 71mA load.

Then as R.G. and Ron say, wall-juice is Sine and this rectifier is Peak-catching, there's a 1.414 difference. 268.66V DC.

We must deduct ~~1V for diode drop.

In audio we expect ripple-drop to be more than 2% but rarely over 10%. The computed 268V/266V is less than 1% drop. But the big-bottle pulls 52mA, the little one less than 2mA, the bleeder just over 1mA; we are pulling more like 56mA than the rated 71mA. I suspect that at 56mA the PT delivers more than rated 190V, perhaps more like 195V.

So where does the "214VAC" come from?? This is very likely measured NO-load, in early testing, big bottle removed, or in Standby. We expect 10%-20% regulation, change of voltage from no-load to full rated load. 214/190 is 13% regulation, a fine figure.

[caprotesta]

Ahhh. That makes sense. Like R.G., I was coming up with the 302V figure. Thanks to all for taking time to reply to this post!