basic power supply question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hustist1, Jun 16, 2005.

  1. hustist1

    hustist1 Member

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    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
     
  2. TheAmpNerd

    TheAmpNerd Member

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  3. doctord02

    doctord02 Member

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    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.
     
  4. John Phillips

    John Phillips Member

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    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.
     
  5. hustist1

    hustist1 Member

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    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.
     

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