PT and PS design question

[krair]

Fair enough. I had been doubling the diodes anyways on the FWCT. I realize there are lots of other ways to get bias, zener I assume? But at the moment I am trying to stay with the mantra of the fewer components the better. I want this one to be as simple as possible.

[R.G.]

I fully agree with JM on the windings. Use FWB modules. The lowered voltage probably lets you use integrated bridges instead of four discrete devices. You'll need to snub each diode in each bridge or they'll squawk with little RF blips at 120Hz and make you think you have hum from your wiring.

In terms of labor, any tap will be more expensive (and complicated to wire up!) than a separate winding. I'd go with two power windings and one bias winding. I like to make bias windings independent and extra robust because of their critical effect on what happens in the amp. Bias supplies are no place to go cheap. The nice thing about bridges is that the windings supplying them have half the turns and no taps, so you pay the same for weight of copper, but less for labor.

So three high voltage windings: 275Vac @ 800ma *OF DC*, 125Vac @800ma of DC, 60Vac @ nothing for bias. One to three heater windings if you put heaters into your main power transformer.

Notice that the heating effect of a full wave bridge on the winding copper is higher than the DC output by a factor of 1.6 to 1.8 depending on how big your filter caps are. 800ma of DC out of those windings means the transformer has to be wound with copper capable of 1.3 to 1.44A RMS in the windings. So the transformer is supplying

275*1.4 + 125*1.4 ~= 560VA, before heater power is figured in. It's a big transformer. Make sure your transformer winder understands that you intend to get 800ma of DC out, not 800ma of AC RMS from the winding. Otherwise, you're back into being too hot again.

[krair]

Interesting. Thanks for the insight. I will be sure that he understands the ratings in RMS vs. DC. And this is for a 200W all tube bass guitar amp So I do expect a pretty big tranny considering the power I am demanding for the tubes!