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What are these sections of this power supply?

Discussion in 'Power Electronics' started by j4cobgarby, Sep 18, 2018.

  1. j4cobgarby

    j4cobgarby

    38
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    Sep 18, 2018
    First of all, apologies for the multiple-part question. I don't know if that's allowed. Anyway: Power-Supply-Schematic-SE-6L6-5881.png

    This is the power supply for the following amplifier circuit:[​IMG]



    I've circled all of the parts which I'm confused about, which, unfortunately is most of it. Is such a complicated power supply really necessary? My assumption is that the green section is the power supply for the tube heaters, and the blue section is for supplying power to the tube cathodes?

    The first part of the question:

    Look at the red section. Is that a 3-way transformer? What does that mean? Is some voltage being taken off at one of the secondary coils, and a different amount taken off at the other secondary? Are they both called secondary here? How would I make such a transformer?

    The second part:

    Look at the blue section. What on earth could all these passive components be doing? Why not just have one capacitor? They're for filtering the signal, right? To get a flatter DC signal from the AC input?

    The third part:

    How's that LM338K working? It just looks like a jumble of components scattered around it.

    The last part:

    I want to make a tube amp, I'm not interested in making a power supply. Is there any way to buy a component which essentially is one of these power supplies?
     
  2. Minder

    Minder

    2,707
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    Apr 24, 2015
    One confusing detail is that the P.S. shows the power common referenced to chassis, and the amp referenced to Earth GND.
    Looking at the site it appears to come from, I am wondering if these are originally from two different sources?
    What is it supposed to be?
    M.
     
    j4cobgarby likes this.
  3. Minder

    Minder

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    Apr 24, 2015
    The tube cathode heaters are permanently connected in order for instant on when the B+ is connected.
    From the dual secondary transformer.
    The four valve filaments are shown bottom right.
    M.
     
    j4cobgarby likes this.
  4. j4cobgarby

    j4cobgarby

    38
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    Sep 18, 2018
    If you mean what is the diagram supposed to be as a whole, it's a tube amp and a power supply.
     
  5. Minder

    Minder

    2,707
    563
    Apr 24, 2015
    Yes I realize that but they have two power supply Common reference symbols.
    M.
     
  6. Ylli

    Ylli

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    Jun 19, 2018
    Most of the complication is an attempt to reduce any possible 'hum' from the amplifier.

    The transformer would need to have two secondaries, first 250 VAC @ 150 mA . Second winding 6.3 VAC @ 3 amps. If you are going to build only one channel, then the 6.3 VAC winding can be @1.5 amps. (A 6L6 filament draws 0.9 amps, a 6SJ7 draws 0.3 amps). You *could* use two transformers.

    I'd connect the transformer 6.3 VAC winding directly to the tube filaments. Forget the bridge rectifier and regulator circuitry.

    Using multiple caps in the HV supply has the effect of reducing the ESR (Effective series resistance) of the resulting effective capacitor. You can get by with one. But you still do need to include the RC parts between B+ and A+.
     
  7. kellys_eye

    kellys_eye

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    Jun 25, 2010
    T1 is specc'd at 9V for the filament winding hence the regulator and DC drive (also presumed to reduce 'hum').

    What 'gets me' is that the secondary amperage is stated to be 2.5A and after Bridge Rectification into a capacitive smoothing circuit the maximum current will be around 1.7A which isn't enough for a dual channel version of that amplifier if the tube spec's for heater current are as stated......

    Download the LM338 datasheet to see how/why the associated components are used - basically two resistors set the output voltage (R4/VR1).
     
    Last edited: Sep 18, 2018
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yeah, and the heaters require 2.4A, so the secondary must be rated at 3.4A or more
     
  9. j4cobgarby

    j4cobgarby

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    Sep 18, 2018
    So this transformer with two secondaries -- would I have to create this myself? Or is there somewhere I could buy such a transformer?
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You would typically buy one.

    However these days, transformers like that are not all that common.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Here are some transformers that have almost exactly half the current required. You could purchase two of them.
     
  12. ramussons

    ramussons

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    Jun 10, 2014
  13. j4cobgarby

    j4cobgarby

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    Sep 18, 2018
    I see one which has an output current of 2A. Why would I need double that? Also, I had no idea transformers were that expensive!
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Because you need a minimum of 3.4A for the heater winding.
     
  15. VenomBallistics

    VenomBallistics

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    Aug 30, 2018
    This isnt that hard ... the top portion of the supply is a fairly common tube amp arrangement. Front end goes through a power resister to help reduce noise, meanwhile caps are paralleled to save cost
    the low side is simply the 6V high current line to fire your heaters. which are the pointy thingies in the schemy.
    The caps just level out the ripple to prevent possible interference with the rest of the tube.
    Red outline is your transformer ... probably a hammond.

    Seems to be a lot of engineering to go through for a single ended tube though.
    You might want to review a few commercial schematics as well. Marshall "Plexi", Peavey 5150, Fender twin reverb and compare with some of the audiophile designs like McIntosh.
    You'll see similarities between all of them in the power amp sections. The main difference is how the screen bias is accomplished.
    Guitar amps tend to have fixed bias to facilitate overloading of the output tubes for that hard rockin thunderous sound of heavy metal glory.
    Audiophile amps tend to have bias taps off the output transformer to vary with demand to help avoid saturation and enhance clarity.

    There used to be a tube model add on for LT Spice circuit sim out there.
    Toob luv is a good thing, but it can get expensive in a big fat hurry when your flying blind
     
  16. kellys_eye

    kellys_eye

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    Jun 25, 2010
    You could wire the tube heaters in series and use a 24V 1A secondary transformer (separately from the 250V transformer).
     
  17. Ylli

    Ylli

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    Jun 19, 2018
    Careful, the 6L6 filaments draw 0.9 amps, but the 6SJ7's only draw 0.3 amps. You *could* put the 6L6's in series, and the 6SJ7's in series - then parallel these and connect to a 12 VAC 1.5 Amp transformer.

    Just to add, for the B+ side, you could use a transformer designed to convert from a 220 volt line to a 110 volt line.
     
    kellys_eye likes this.
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Two of the tubes require 900mA, and the other two require 300mA for the heater. You'd be better off wiring the 900mA ones in series, and the 300mA ones in series and powering both of these pairs from 12V 1.5A transformer.

    *SNAP*
     
  19. VenomBallistics

    VenomBallistics

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    Aug 30, 2018
    yes, welcome to audio.
    be it tube, or solid state, power is arguably the most critical portion.
    it is better to have a 300W power supply driving a 100W amp than it is to try to run 100/100.
    it only works in theory as the amps demand turns the power rails into a washboard road.
    you always want some surplus on the power side of any project. It saves effort and heat in expensive components.
    Given the apparent sudden realization that transformers are expensive, I have to assume that this is your first foray into the "hollow state" world.
    Thus I must pose the question "What are you trying to achieve?"
    Amps are not created equal. Music reproduction and guitar amps employ distinctly different strategies. Which road do you intend to travel?
     
  20. j4cobgarby

    j4cobgarby

    38
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    Sep 18, 2018
    I'm trying to achieve a guitar amp.

    Thanks for the comment, I think I understand what you're talking about.
     
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