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  • Help with PA Conversion

    I'm converting a Muzak PA amp, basically just redoing the power supply and adding a dual triode 12ax7 preamp and vol/tone controls ahead of the 7199 -> 6V6PP power amp. It's a cool looking PA - pics are here.

    I haven't been able to figure out the purpose of the 400ohm 20-wall resistor is between the PT's center tap and ground? If it's too small to read, The schematic says "Connect Jumper for 8 Watts" next to a dotted line in parallel with the resistor. I've searched a number of forums and can't figure out what it's doing there.

    Schematic is below. Thanks.

    [IMG][/IMG]

  • #2
    It is reducing the B+ voltage available to the circuit. If I had to guess 8W was the higher power option so the amp could drive more speakers. With the resistor in the circuit, maybe it produces say 4 or 5 watts.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      It looks to me like two voltages are specified on the HV rail. I'm not sure what the purpose of the resistor is but I can tell you what it does. A resistor in the CT like that drops voltage and adds sag. Just bypass it. You already have a rectifier tube that will sag some and 355V (the voltage when the resistor is bypassed) is a more typical guitar amp voltage.

      It looks like you already have a 12ax7 in the amp that could be rewired in series ahead of the phase inverter for plenty of gain.
      "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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      • #4
        Does bypassing it with a jumper as indicated on the schematic have
        a different effect than simply removing it..? In the course of researching
        and learning to complete several builds, I haven't noticed anyone placing
        resistance between the PT-CT and ground this way..

        Thanks.

        Comment


        • #5
          Originally posted by Jeru View Post
          Does bypassing it with a jumper as indicated on the schematic have
          a different effect than simply removing it..? In the course of researching
          and learning to complete several builds, I haven't noticed anyone placing
          resistance between the PT-CT and ground this way..

          Thanks.
          If by removing it, you mean replacing it with a straight conductor, then it works the same as the jumper, and the resistor has no effect. If you mean just cut it out of the circuit and leave the CT disconnected, then the difference will be that the amp won't work like that

          edit: ...and yes, I've never seen a resistor used like that either. There are some other 'higher-tech' methods of power reduction I've seen on the forum, but not this.
          If it still won't get loud enough, it's probably broken. - Steve Conner
          If the thing works, stop fixing it. - Enzo
          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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          • #6
            Of course -- I meant removing the resistor and securely grounding the PT's HV
            center tap. I actually don't like where/how the builders grounded it anyway..
            Thanks.

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            • #7
              Originally posted by eschertron View Post

              edit: ...and yes, I've never seen a resistor used like that either. There are some other 'higher-tech' methods of power reduction I've seen on the forum, but not this.
              HA - I've been looking into that too, to do with a different build/project. A topic for another thread, to be sure.

              Comment


              • #8
                Originally posted by Jeru View Post
                Of course -- I meant removing the resistor and securely grounding the PT's HV
                center tap. I actually don't like where/how the builders grounded it anyway..
                Thanks.
                The prevailing wisdom I've seen on this forum suggests the best place to land the CT is right on the negative leg of the reservoir cap (I think C13A on the drawing?) to minimize loop currents. Not sure if your layout allows that.
                If it still won't get loud enough, it's probably broken. - Steve Conner
                If the thing works, stop fixing it. - Enzo
                We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                Comment


                • #9
                  Huh - will look into that too. I'd always thought that one of the PT bolts was the best
                  place for it. I do get a whole lot out of being wrong, though -- it's how I learn.
                  Thanks.

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                  • #10
                    Originally posted by Jeru View Post
                    Huh - will look into that too. I'd always thought that one of the PT bolts was the best
                    place for it. I do get a whole lot out of being wrong, though -- it's how I learn.
                    Thanks.
                    Well, certainly don't take my word for it. I'm just repeating what I read on the internet

                    But seriously - there are people here (with the engineering chops I lack) that have shared their ideas and the reasoning behind them. When the reasoning seems solid, I tend to accept the ideas presented. You'll discover who the reasoned thinkers are here, fairly quickly I wager.
                    If it still won't get loud enough, it's probably broken. - Steve Conner
                    If the thing works, stop fixing it. - Enzo
                    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                    Comment


                    • #11
                      Originally posted by Jeru View Post
                      Huh - will look into that too. I'd always thought that one of the PT bolts was the best
                      place for it.
                      Don't connect the PT CT to chassis. It's better to connect it directly to the reservoir cap -ve as stated above. The rectifier current pulses travel between CT and cap negative and it's best not to have that current running through the chassis or other ground connections. There should be only one wire from PT CT to cap negative with no other connections to the CT.

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                      • #12
                        Thank you all for your time and assistance -- this is why I absolutely love places like this.

                        Comment


                        • #13
                          Presumably the resistor is grounded where they wanted it to be grounded. Placing a jumper across the resistor wouldn't change where it was grounded. This is an amp intended for background music use, and not designed with high gain in mind. Make your conversion first, then worry about moving grounds around later.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #14
                            Originally posted by Enzo View Post
                            Make your conversion first, then worry about moving grounds around later.
                            Yes, it's not a new build so no need to fix it until you have broken it

                            Comment


                            • #15
                              Originally posted by Dave H View Post
                              Yes, don't try and fix it until you have broken it
                              Not to worry -- I have every confidence I'll do both.

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