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Voltage divider or just a parallel resistor to bring down a tap voltage?

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  • Voltage divider or just a parallel resistor to bring down a tap voltage?

    If I have a transformer that has a 100v tap, say, and I want to use it in a circuit that needs 40v dc. 100 VAC RMS through rectification = 50v rms ac, x 1.414 = about 71 vdc.

    The question is whether to use a parallel resistor, just one, or a voltage divider. I don't know how to look up the load impedance/resistance (control grid to ground?) But if it ws much larger than the dropping resistors, at least the voltage divider equation is a lot simpler.

    Is there a way to look up what the dc bias current is, or is this something you have to measure?
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  • #2
    Is this for an output stage bias supply? A bias supply doesn't have to provide any current. You can just use a voltage divider to set the desired voltage.

    Comment


    • #3
      Hi Dave, its for a bias circuit for a potential "from scraps" amp. The transformer has a 100v ac tap, but the circuit needs -40v dc range. Thanks that's what I was trying to figure out: the current that the bias ckt needs to feed. So, cool, the voltage divider eq will work as is! Thanks!
      The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

      Comment


      • #4
        Originally posted by mikepukmel View Post
        If I have a transformer that has a 100v tap, say, and I want to use it in a circuit that needs 40v dc. 100 VAC RMS through rectification = 50v rms ac, x 1.414 = about 71 vdc.

        The question is whether to use a parallel resistor, just one, or a voltage divider. I don't know how to look up the load impedance/resistance (control grid to ground?) But if it ws much larger than the dropping resistors, at least the voltage divider equation is a lot simpler.

        Is there a way to look up what the dc bias current is, or is this something you have to measure?
        The first cap after the rectifier will charge to the peak of the transformer voltage i.e. 141V. Half wave rectification does reduce the rms voltage but the peak is unchanged.

        IMHO it's better to add a resistor in series with the diode to drop the voltage as it helps to filter the waveform more for the same value of filter cap. It also gives a measure of transformer protection for that tap under fault conditions for free. You''ll need a smaller value than you might expect due to the impulsive nature of the current.
        Experience is something you get, just after you really needed it.

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        • #5
          +1

          Pretty much every bias circuit uses a voltage divider and most use a series resistor to feed the the divider.

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          • #6
            This should do it.

            Click image for larger version

Name:	Bias -40V.png
Views:	1
Size:	14.8 KB
ID:	848040

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            • #7
              And a tube grid, ideally, doesn't draw significant current. So the resistance values provided by Dave serve to keep from unnecessarily stressing the winding wile still maintaining an appropriate grid circuit impedance Of course, it's also nice to be able to adjust bias for new (but always somewhat different) tubes without heating an iron. The solution would be to add a potentiometer to the circuit allowing you to change the voltage division higher or lower than your ideal. Like this:
              Attached Files
              "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


              • #8
                And to touch on why a "parallel resistor, just one" is not a good idea, it is adding an unnecessary load to the PT, reducing current available from that tap, and creating unwanted heat.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


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                • #9
                  Please draw your transformer secondaries, including your 100V tap.

                  We may be overlooking something.
                  Juan Manuel Fahey

                  Comment


                  • #10
                    Originally posted by Chuck H View Post
                    Of course, it's also nice to be able to adjust bias for new (but always somewhat different) tubes without heating an iron. The solution would be to add a potentiometer to the circuit allowing you to change the voltage division higher or lower than your ideal. Like this:
                    For safety reasons be sure to use a high quality locking pot, and design the circuit so the pot doesn't have an overly wide range of adjustment -- just what you need to compensate for variability in tubes and nothing more.

                    Is this the time where we talk about the relative safety of Marshall vs. Fender style pot wiring in bias circuits, should the pot fail?

                    Personally I think the M style circuit is more tolerant to pots going bad, the F only managing never to self destruct because F used such high quality pots.
                    "Stand back, I'm holding a calculator." - chinrest

                    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

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                    • #11
                      Thanks everyone! Chuck, what software do you use? I will measure and draw up the windings.
                      Thanks!
                      The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                      Comment


                      • #12
                        Originally posted by mikepukmel View Post
                        Thanks everyone! Chuck, what software do you use?
                        I modified Dave's image with mspaint But I also have ltspice. I draw all my schems with paint too. I have a master image with all the components, symbols, letters and numbers all drawn up (by me at some time) so now it's just copy, past drag, etc.
                        "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


                        • #13
                          Thereīs a great guy, SM0VPO, ex RAF Electronics Technician who worked for ages in Saudi Arabia mainainingbthe siploads ow weapons systems they bought from UK, and is a very smart Hobbyist.

                          Focused on Radio equipment mostly but does some audio and is ingredibly resourceful, you canīt exactly download "anything" into Saudy Air Force computes, nor easily order from Mouser or Farnell, etc. , so he became very resourceful and had to work "with what he had available".

                          Among lots of other things he made a system of useful, easy to cut/paste blocks for plain vanilla Paint, which enabled him to draw both schematics and very compact PCBs, on par with best dedicated Software:

                          DRAWINGS & PCBs by Harry Lythall

                          this is an example of what can be done:

                          4W no (unavailable) IC but discrete amplifier:
                          same Paint box of tricks allows:
                          * schematic:


                          * PCB:
                          Click image for larger version

Name:	afamp4w-1b.png
Views:	1
Size:	4.6 KB
ID:	848047

                          * layout:
                          Click image for larger version

Name:	afamp4w-1c.png
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ID:	848048

                          EDIT: hbad to convert Paintīs native .BMP to .PNG to allow Forum software to display it.
                          But no big deal, I guess modern Paint can straight save in .gif or .png.

                          The general purpose set of cut/paste images is in:
                          http://213.114.131.21/_download/pcb1.zip
                          Last edited by J M Fahey; 12-25-2017, 12:01 AM.
                          Juan Manuel Fahey

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                          • #14
                            PCB:
                            Click image for larger version

Name:	afamp4w-1b.bmp
Views:	1
Size:	53.0 KB
ID:	848046
                            Juan Manuel Fahey

                            Comment


                            • #15
                              Yep. After a recent pooter crash I had to download a lot of my old schematics FROM HERE in jpg or png. No big deal as I only wanted them for reference and no longer need them in bmp. Some stuff I did have saved. Like my master component/symbols/alphanumerical stuff. Phwew... Who wants to draw all those transformers again? Worst was all the letters and numbers. I didn't like any of the fonts because they all use shading as part of their character. I had to invent single color characters in two sizes. BUT it's done now and drawing schems is pretty easy.

                              This is one of my mspaint schems saved to png:
                              Attached Files
                              "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

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