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PT secondaries question.

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  • #1

    PT secondaries question.

    How do you figure the rectified DC voltage output of your transformer from a spec sheet?

    For example: my Classic Tone Trainwreck PT specs 300vdc from each secondary wire. But after a full bridge rectifier, my B+ is about 400vdc with 3-12AX7's and 2-EL34's at idle.

    So I'll ask again: If I was shopping for PT's, how would I figure the B+ voltage at idle by looking at a spec sheet?
    Tags: None
  • #2
    It has to do with a few things. What type of rectifier you plan to use and the current of the circuit having the greatest influence. After that there's rail resistance as it relates to current at each node, etc. I use Duncan PSU (download here PSUD2) and usually get within +/- ten volts of my results. It's important to factor in all the info as accurately as you can.

    Sorry I can't offer some pat formulas. There are just so many variables with a power supply.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #3
      Basically capacitors charge to peak AC value, or 1.41 X the AC voltage.
      In your case, 300 * 1.41=423V .
      That's the no load voltage.
      When your amp starts pulling current from that main capacitor, voltage will lower somewhat, because the transformer windings do have some resistance, which causes a loss of energy, think some 10% in a properly designed supply, so you'll measure around 423V*0.9= 380V or thereabouts

      Duncan PSU calculator is excellent but it's important that you can guesstimate reasonably close values by yourself.
      Juan Manuel Fahey

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      • #4
        Originally posted by J M Fahey View Post
        Duncan PSU calculator is excellent but it's important that you can guesstimate reasonably close values by yourself.
        Yes. The actual winding resistance of a transformer isn't always included in the sales info. Duncan PSU2 does offer this parameter but you won't usually know it. Other than that it's pretty easy to know what current the tubes will draw. Once one learns to use the program it's really quite adequate for knowing where things will end up.

        I think leadfoot might be asking about how to solve for the answers without software. For that I can only suggest reading up on Ohm's law. It won't get you the actual ripple figures and time constants like the software, but it'll get you reasonable voltage estimates.

        EDIT: Thanks Juan for the specifics on the rectifier type. There are also differences in rated current (which can affect loading) with different power supply designs. So, as implied by Juan, look into rectifier designs and how they affect the circuits also.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment

        • #5
          Originally posted by leadfootdriver View Post
          How do you figure the rectified DC voltage output of your transformer from a spec sheet?

          For example: my Classic Tone Trainwreck PT specs 300vdc from each secondary wire. But after a full bridge rectifier, my B+ is about 400vdc with 3-12AX7's and 2-EL34's at idle.
          A transformer puts out AC volts, not DC. The amount of DC voltage obtainable from a given AC voltage depends on the configuration of the rectifier(s) and the filtering. Most tube guitar amplifiers use what is called a Full Wave or Full Wave Center Tap rectifier configuration, not a Full Wave Bridge. A full wave bridge would put out over 800 VDC if used with a capacitor input filter.

          This pdf does not discuss the difference between AC and DC Volts, but it does cover rectifier configurations. http://www.hammondmfg.com/pdf/5c007.pdf
          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
          REMEMBER: Everybody knows that smokin' ain't allowed in school !

          Comment

          • #6
            Originally posted by Chuck H View Post
            It has to do with a few things. What type of rectifier you plan to use and the current of the circuit having the greatest influence. After that there's rail resistance as it relates to current at each node, etc. I use Duncan PSU (download here PSUD2) and usually get within +/- ten volts of my results. It's important to factor in all the info as accurately as you can.

            Sorry I can't offer some pat formulas. There are just so many variables with a power supply.
            THnaks. The Duncan programs are pretty cool.

            Comment

            • #7
              Originally posted by J M Fahey View Post
              Basically capacitors charge to peak AC value, or 1.41 X the AC voltage.
              In your case, 300 * 1.41=423V .
              That's the no load voltage.
              When your amp starts pulling current from that main capacitor, voltage will lower somewhat, because the transformer windings do have some resistance, which causes a loss of energy, think some 10% in a properly designed supply, so you'll measure around 423V*0.9= 380V or thereabouts

              Duncan PSU calculator is excellent but it's important that you can guesstimate reasonably close values by yourself.
              YOur 1st formula is pretty accurate. I have a Bias Probe from Eurotubes that shows the plate voltage and current separately. I was rolling some tubes, the other day and flipped on the power. THe PV was about 423 until the cathodes started flowing current, then it dropped to about 400.

              Comment

              • #8
                Originally posted by loudthud View Post
                A transformer puts out AC volts, not DC. The amount of DC voltage obtainable from a given AC voltage depends on the configuration of the rectifier(s) and the filtering. Most tube guitar amplifiers use what is called a Full Wave or Full Wave Center Tap rectifier configuration, not a Full Wave Bridge. A full wave bridge would put out over 800 VDC if used with a capacitor input filter.

                This pdf does not discuss the difference between AC and DC Volts, but it does cover rectifier configurations. http://www.hammondmfg.com/pdf/5c007.pdf
                Good catch. THis is why I ask questions!

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