Darlington or Mosfet?

[airboss]

Greetings:
I've stumbled over here from the Valve/Tube neighborhood because I can hardly wire a transistor to save my life.

Anyway, I have power transformers made for me for guitar amp projects with 12v filament windings. 4 amps. There are times when I would just as soon go DC on the filaments. So, with a cap input filter you get 17 volts. I would like to regulate that down to 12.6. Four amp regulators are ten or twelve bucks; when you can find them. So what's a good Darlington transistor that I can source 12V with a zener and multiply the current and not loose my 4.4 volts of headroom? Like this:


The font is too small, I used paint to make this. Anyway, the base to emitter reverse diode is to protect the fragile E/B junction. I threw the other one in for spikes in the line. RS I have found, in spite of manufacturerers recommendations is usually arrived at experimentally. It's there to turn the 12V source zener on.

Anybody got a good number for one like this? Or am I going to have to go with Shotkey types with all the extra parts and doodads. I originally designed this for screen regulation and it works spiffy up in the 200VDC-300VDC range with the right transistor. I don't even know if they make 450V Darlingtons anymore. For screen supplies I used an NTE 2317

You know what? I just noticed this won't work unless one filament lead is grounded. I need a series device. Hmmmmmm. Lemme think. Maybe it will, just terminate the anode of the zener to the other filament line and lose the diode on the right? Like I say, I ain't too swift on these little gems.

[smartx21]

Hi!
Take a look here: Permanent Magnet DC Regulator
I used this configuration once in order to lower supply voltages in an AIWA power amplifier, with great success.
The attached one is to be used in series with positive rail. A slightly different configuration (with PNP BJT) can be used for negative supply.
But I can't see why use Darlington. Only a single NPN BJT will do the job. Be careful with heat, providing necessary sink.
Cheers,
Max.

[airboss]

Thanks Max, I'll have to look at some data sheets and see what the losses amount to. I could probably just put a couple of series diodes in the circuit to lose the extra volts. Filaments don't require regulation. They are sensitive to overvoltage.