basic power supply question

[hustist1]

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

 

[TheAmpNerd]

Hey,

These chapters should answer some of your questions.

http://www.pacificrecone.com/JackDarrBook.html

enjoy : )

 

[doctord02]

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.
========================================

If you want to drop it that much voltage, then you want a smaller transformer... something like 220-0-220 would be much more suitable.

 

[John Phillips]

That is all basically correct, yes. Don't forget to calculate the power dissipation necessary for voltage-drop resistors as well... you'll need at least 6.5W (10W for a good safety margin) with those values.

I agree with doctord02 to an extent, that is a big voltage drop (and a waste of power), but OTOH many amps do exactly that sort of thing between the power stage supply and the preamp - all you're really doing is building a power supply with no conventional "power stage", and it will work OK. You may be better running the pentode at higher voltage though - it will take it, despite the design ratings. Marshall ran them at about 320V in the 'Popular' model (going from memory, I could be wrong).

You can also achieve quite a significant voltage drop by using a rectifier tube (eg EZ80) instead of a solid-state rectifier.

 

[hustist1]

Thanks to all for the responses, it's good to know that I'm not completely on the wrong track. I was beginning to think my questions were so stupid, you had all decided to let natural selection take its course, in the hope I hadn't passed on my worthless genes yet.

Yes, I know I should use a smaller transformer, but the one I have laying around is 300V, so I might as well use it.

I don't want to use a tube rectifier, because I have a "concept" here -- 1 tube. The original idea was to put it in a Pignose box, but I don't think there's enough space.

I also don't want to run higher voltage on the plates -- the data sheet includes curves for 300V -- because I want a low-gain, not-loud amp. The real goal is to design and build the simplest possible amplifier, so I can figure out what is happening. That's why I didn't build another Champ clone.

And if I succeed in building this thing without permanently damaging myself (I am very, er, respectful of European line voltage, having met it once unexpectedly while installing a light in the bathroom) then I'll probably re-use the transformer for the next project, which will be a class AB amp using 2 el95s I have.