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  • Parallel Voltages and Current.

    OK, time for me to go to school, as I've only learned informally and have big holes in my knowledge of electronics.

    If two voltages are coupled together, how does the voltage change / react ? I have a Hi-Fi amp that runs the power tube screens from two voltage sources, one is around 290vdc, the other around 340vdc. Not sure what is going on there. The schematic I have does not show this, so I am inserting a pic of the same EL84 hi-fi amp currently for sale on Ebay. If you look closely at the bottom right EL84 socket (there are 4 of them in this stereo amplifier), you will see a large 640 ohm cement resistor connected to the screen from a node coming from a high voltage. But there is also an Orange colored wire that goes straight to another cap in the can capacitor. That connection is a lower voltage.

    Can anyone explain why this is setup that way ? How would the two different voltages interact ? More voltage stability ? Less ripple ? How about current ? Not sure of the design or the reasons why.

    Thanks for any help !

    Click image for larger version

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    Click image for larger version

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    " Things change, not always for the better. " - Leo_Gnardo

  • #2
    We don't know what your schematic shows, how about you post it.

    Looking at your photo, all teh EL84s have screns connected to some B+ node at a can cap pin. That node is fed from some other node by the 680 ohm resistor. Typically the same node the output transformer gets is B+.

    The voltages are not coupled together. There is a resistor between them.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      two voltage sources
      What makes you think so? If the PT only has one HT winding, there is only one source of power. DC power always flows downstream from higher voltage to lower voltage.

      you will see a large 640 ohm cement resistor connected to the screen from a node coming from a high voltage. But there is also an Orange colored wire that goes straight to another cap in the can capacitor. That connection is a lower voltage.
      Such arrangement is not unusual. Looks like a typical RC screen filter. Just lift one end of the 640R resistor to see if you still get screen voltage by a "second voltage source".
      - Own Opinions Only -

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      • #4
        Originally posted by Enzo View Post
        We don't know what your schematic shows, how about you post it.

        Looking at your photo, all teh EL84s have screns connected to some B+ node at a can cap pin. That node is fed from some other node by the 680 ohm resistor. Typically the same node the output transformer gets is B+.

        The voltages are not coupled together. There is a resistor between them.
        Ok, good point, here's the schematic I have : Click image for larger version

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        It may be an early or later one, as my 7D31 amp has 12AX7 tubes, not the 6FQ7 on the schematic. A couple of the other component values were different as well, but the basics are there.
        " Things change, not always for the better. " - Leo_Gnardo

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        • #5
          Originally posted by Helmholtz View Post
          What makes you think so? If the PT only has one HT winding, there is only one source of power. DC power always flows downstream from higher voltage to lower voltage.



          Such arrangement is not unusual. Looks like a typical RC screen filter. Just lift one end of the 640R resistor to see if you still get screen voltage by a "second voltage source".
          Voltage source is a poor choice of words perhaps. I will measure the voltages again a bit later today and see what it is on both sides of the big cement resistor as well. Something I don't understand about this one, as all the amps I worked on before were very simple guitar amps, and maybe it's the same but the layout just has be confused.

          By the way I just finished reading Huxley's Brave New World. Your an important Alpha in that plot it seems.

          I will let you guys know what I find this time around.
          " Things change, not always for the better. " - Leo_Gnardo

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          • #6
            I agree with Enzo- please upload a schematic. It's hard to tell for sure what's going on there from the pictures. That said, I THINK what's there is this. The orange wires go to a filter cap. But, the cap is only a filter for the screen supply. I don't see another supply connection to the cap. So, there are not two separate supplies to the screens.

            Edit: Sorry, I was typing during your last 2 posts. After looking at the schematic: Yes, the orange wire just connects the filter cap to the circuit. It's not a second supply.
            Last edited by The Dude; 05-08-2020, 12:06 AM.
            "Yeah, well, you know, that's just, like, your opinion, man."

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            • #7
              Look at your schematic, lower right. See the 5U4? Pin 2 has the B+ for the power tube plates (through the output transformers) There is a filter cap there as well. Now follow to the right, from that cap. Is that not your 680 ohm resistor? and right after it, another filter cap. That looks to be the 310v B+, and where does the 310v go? to the screens. And that looks like what is in your amp chassis.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                Yes, I think what you are thinking of as 'two ins' are actually an in and an out.
                Sometimes voltage dropping resistors are mounted on the board, but they can also be mounted on a tube socket, or a cap can.
                The resistor is separating two different nodes of the supply.
                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                • #9
                  Originally posted by g1 View Post
                  Yes, I think what you are thinking of as 'two ins' are actually an in and an out.
                  Sometimes voltage dropping resistors are mounted on the board, but they can also be mounted on a tube socket, or a cap can.
                  The resistor is separating two different nodes of the supply.
                  Yes, you are correct. Not sure what the heck I was thinking about, I must have been tired the last time I layed my hands on it ! Just measured the voltage on the pin side of the resistor and is all makes sense now, as the run from the capacitor (not from the B+) is the same, no mystery anymore.

                  But hey, similar hypothetical question, if I took two separate voltages from the same power supply, let's say one was roughly B+ around 350vdc, and the other had been dropped by a resistor to let's say 250vdc what would I get if I connected them together ? would the higher voltage just "Win out" ? I've never seen that but I wonder what would be the result. Also if current were drawn from that connection would it be stable, as in different than either the line with the resistor or the one without ?

                  It would be nice if I got a book on this stuff ! any recommendations would be welcomed. I now have time to study more than before.

                  Thanks for everyone's help so far !


                  P.S. one last thing, I keep seeing ads for "2019 hottest Bikinis" coming up on this website, is this a feature of being involved in electronics, or something related to the question of voltage perhaps ?
                  " Things change, not always for the better. " - Leo_Gnardo

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                  • #10
                    Won out? No, by connecting them together, all you really are doing is sorting across the resistor. SO the lower voltage would not even exist, as ther would be no dropping resistor.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      I can maybe see what HaroldBrooks is getting at - It takes me back to my college days and hacking through Kirchoff's law which is used to analyze circuits where there may be multiple interconnected voltage sources at different potentials.

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                      • #12
                        But that is a common ploy in those jungle gym networks they give you to figure out. You have to keep track of the circuit to find paths that short across resistors making them irrelevant. That of course is only at the level of a pop quiz. If we get onto the real physics of it, the wire has resistance, so we have a 680 ohm resistor in parallel with a 0.005 ohm resistor. But that is for physics class.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #13
                          I spent a good deal of time using Kirchoff's, but I can't ever think of a single instance where I've had to use the principles in a practical situation. I'm always reminded of how valuable a teacher is in imparting knowledge. I really struggled with Kirchoff - I just didn't get it. The lecturer just kept repeating the same thing to me with increasing frustration, tension and raised voice - like an Englishman shouting at a foreigner. Another lecturer overheard the conversation and afterward suggested I go and see him for an explanation. I did, and in half an hour had no more problems. It just came down to the way it was explained.

                          The same thing happened later on in life - I did my Prince 2 project management practitioner's course and exam. I was pretty excited when I found out the guy taking the course was Ken Bradley - the man behind the methodology and who'd published definitive books on the subject. I don't know what it was, but I found that nothing was getting imparted - I just saw an expert in his field dictating the rules and processes without imparting knowledge. First time round I failed but after some coaching form another practitioner passed easily.

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                          • #14
                            Originally posted by HaroldBrooks View Post
                            But hey, similar hypothetical question, if I took two separate voltages from the same power supply, let's say one was roughly B+ around 350vdc, and the other had been dropped by a resistor to let's say 250vdc what would I get if I connected them together ? would the higher voltage just "Win out" ? I've never seen that but I wonder what would be the result. Also if current were drawn from that connection would it be stable, as in different than either the line with the resistor or the one without ?

                            P.S. one last thing, I keep seeing ads for "2019 hottest Bikinis" coming up on this website, is this a feature of being involved in electronics, or something related to the question of voltage perhaps ?
                            All voltage sources have output resistance even if it's not obvious. If you connect two voltage sources together the resulting voltage will depend on their respective voltages and output resistances. For example say you have a 10V source and a 20V source both with 2k output resistance. If they are connected together the resulting voltage will be 15V with 1k output resistance (2k//2k). It won't be stable because of its 1k output resistance, draw 1mA from it and its voltage drops by 1V.

                            P.S. I think the ads you see depend on your browsing history
                            Last edited by Dave H; 05-08-2020, 12:57 PM.

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                            • #15
                              Originally posted by Mick Bailey View Post
                              I spent a good deal of time using Kirchoff's, but I can't ever think of a single instance where I've had to use the principles in a practical situation.
                              When working out the voltages and currents in guitar amp circuits you'll be using Kirchoff's all the time without realising.

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