hustist1
Member
- Messages
- 132
Okay, I've gotten to the point where I think I have a basic understanding of the pre- and power-amp stages. I've been looking at the power supply now, and this doesn't seem like rocket science, but as usual I seem to be missing something fundamental.
If I look at the power supply, I see the following components:
- a transformer to convert line voltage to high voltage (let's ignore the filaments for now)
- a rectifier to convert ac to something dc-like
- an initial capacitor to smooth the dc
- one or more resistor-capacitor filters to drop voltage and smooth out the dc some more.
So if I have a mains transformer with a high-voltage winding which is 300-0-300, and I use a solid state full wave rectifier, I theoretically have 300 x (sq. root of 2) = 424 volts after the rectifier, correct?
Then I want to drop this down to 250 volts for the plates. This is done with a resistor, whose value I calculate using Ohm's Law, R = V/I. V as above, I is ... what? sum of the plate current draw of the tubes? I read the Glass Audio articles on power supply design, and it's not clear what current I use to calculate the resistor values -> makes me suspect it is obvious and I am missing something very basic. If the circuit consists of a single ECL86 tube, with 250 V on both plates, the data sheet says the triode will have 1.2mA and the pentode 36mA, for a total of 37.2 mA. Then I would need R = (424 - 250) / (37.2 / 1000) = 4.7K ohm.
It would be potentially life-saving if some kind soul would put me straight here before I do something stupid like pick up a soldering iron.
Also, would it make sense to drop the voltage over two (or more) resistor-capacitor filters instead of one, to better smooth out the DC?
Thanks in advance from someone who is probably venturing out into waters best left untread ...
Steve
If I look at the power supply, I see the following components:
- a transformer to convert line voltage to high voltage (let's ignore the filaments for now)
- a rectifier to convert ac to something dc-like
- an initial capacitor to smooth the dc
- one or more resistor-capacitor filters to drop voltage and smooth out the dc some more.
So if I have a mains transformer with a high-voltage winding which is 300-0-300, and I use a solid state full wave rectifier, I theoretically have 300 x (sq. root of 2) = 424 volts after the rectifier, correct?
Then I want to drop this down to 250 volts for the plates. This is done with a resistor, whose value I calculate using Ohm's Law, R = V/I. V as above, I is ... what? sum of the plate current draw of the tubes? I read the Glass Audio articles on power supply design, and it's not clear what current I use to calculate the resistor values -> makes me suspect it is obvious and I am missing something very basic. If the circuit consists of a single ECL86 tube, with 250 V on both plates, the data sheet says the triode will have 1.2mA and the pentode 36mA, for a total of 37.2 mA. Then I would need R = (424 - 250) / (37.2 / 1000) = 4.7K ohm.
It would be potentially life-saving if some kind soul would put me straight here before I do something stupid like pick up a soldering iron.
Also, would it make sense to drop the voltage over two (or more) resistor-capacitor filters instead of one, to better smooth out the DC?
Thanks in advance from someone who is probably venturing out into waters best left untread ...
Steve