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Help power supply for JCM800 like preamp

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  • Help power supply for JCM800 like preamp

    Hello!

    Im thinking in building a JCM800 like preamp only and i need some hellp for the b+ plate voltages power supply part.

    How it will going to stay the power supply to the plate voltages? will it stay just like the attached schematic ? will i dont need the parts of the power supply inside marked in the attached file?

    Click image for larger version

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    Best regards.
    Last edited by Rod; 04-13-2020, 01:32 AM.

  • #2
    You don't need the part that includes R30 and D1. You don't really need the part that includes C20 and C21, except that your power supply will need some kind of filter, just not as big as the one needed for a 50 or 100 Watt amp.

    You can't really copy what you find in that schematic if all you want to build is just the preamp. For B+, you need something around 350V at about 10mA. You'll need something in place of C24 and C25. With the right setup, these don't need to be 500V chassis mount caps but smaller axial caps rated 450V. Those can be mounted on an eyelet or turret board.

    The requirements for a preamp power supply are so low, it's actually hard to find a power transformer that small. Some projects that build a preamp into a pedal box actually use a filament transformer in reverse to generate B+. So, what kind of chassis are you planning to use and what country are you in ?
    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 !

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    • #3
      Hi! Loudthud and thanks for your reply.

      Well i want to have the nearst clone possible of the real deal, so i want to have plate voltages of 450v, and the voltages in each gain stage like the jcm800 reissue.
      I want to use a 19" rack chassis, and im from Europe.

      Thanks.

      Comment


      • #4
        I just fired up my 2203 clone and the Voltage at pin 1 (the schematic you posted) of V2 was 285V. That's in the ballpark of what you find on most of the Marshall Voltage charts. Highest I found was 310V.
        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


        • #5
          One consideration, while you do not need the higher voltages of the power amp, in the whole amp, the power amp drags on the power supply causing sag. That part of the tone will be missing from your preamp-only unit.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Originally posted by Enzo View Post
            One consideration, while you do not need the higher voltages of the power amp, in the whole amp, the power amp drags on the power supply causing sag. That part of the tone will be missing from your preamp-only unit.
            Yes, the poweramp in this amp and in every amp is very importante to the overall tone, this is the reason i think a preamp will never sound like real amp.

            Before doing a preamp , i thougth in doing a 4xEl84 amp, because 4 powertubes brings a lot of balls and "oomph" to the sound, 4xel84 tubes also because the want gets smaller and less heavy weigth, and i never liked a amp with only 2 power tubes.

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            • #7
              Yes, the poweramp in this amp and in every amp is very importante to the overall tone, this is the reason i think a preamp will never sound like real amp.
              But it may make sense not to use the higher preamp supply voltages from the complete amp at idle, but rather use lower voltages that simulate the sag at high output.
              - Own Opinions Only -

              Comment


              • #8
                Originally posted by Helmholtz View Post
                But it may make sense not to use the higher preamp supply voltages from the complete amp at idle, but rather use lower voltages that simulate the sag at high output.
                OK, i like the idea :-), so what kind of changes would have to be made to the power supply in the attached schematic bellow?
                Maybe someone of you amp gurus can simulate this in a CAE software.

                Click image for larger version

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                • #9
                  I consider the importance of sag not in a lower voltage, but in the dynamics of it. The changing B+ as the note attacks and decays.
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment


                  • #10
                    Ive been Reading about sag and it seems that this effect comes from a drop in power supply voltage induced when the power transformer and or rectiifer is hited hard.

                    "A master knob set to the 12 o’clock position generally puts you in the ballpark.
                    As you ease the knob towards the right, the output stage starts distorting. Guitarists generally perceive this as very musical compression, with notes blooming rather than just sounding.
                    Many guitarists report that this inspires them to play better.
                    This effect is compounded by the sag – that is, a drop in power supply voltage – induced when the power transformer and/or rectifier is hit hard"

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                    • #11
                      Originally posted by Enzo View Post
                      I consider the importance of sag not in a lower voltage, but in the dynamics of it. The changing B+ as the note attacks and decays.
                      I thought so, but that kind of interaction is hardly achievable with a separate preamp.

                      Also I think that most of the sag induced dynamics is produced in the power amp itself by screen compression.
                      Because of the large filter time constants involved, the preamp will "see" the sag only after a delay of maybe a second or more. Apart from that, a supply sag of 10% or 15% shouldn't change much in a triode stage.
                      - Own Opinions Only -

                      Comment


                      • #12
                        Turn on your car headlights. Note how bright they are. NOw turn the key to start the car. Note how the lights dim some - get less bright - until the motor fires up. That is sag. The battery voltage drops from the heavy load of the starter. WHen you play something loud, the power driving the speakers comes from your power supply, and its voltage drops for a moment.
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment


                        • #13
                          I thought so, but that kind of interaction is hardly achievable with a separate preamp.
                          I certainly agree with that, I just didn't think reducing B+ in the preamp would emulate that.

                          And my overal message to the OP is that a preamp from a whole amp when used alone will not sound the same as that same circuit when it is in a whole amp.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #14
                            I just didn't think reducing B+ in the preamp would emulate that.
                            Oh, I didn't mean to imply that. The idea behind my proposal is to supply the preamp with voltages that resemble average values in a cranked amp. The lower voltages will somewhat reduce clean headroom and thus slightly increase preamp distortion. The influence on gain is very small.
                            - Own Opinions Only -

                            Comment


                            • #15
                              I don't think we are really disagreeing in anything, I think we are just trying to settle on words.
                              Education is what you're left with after you have forgotten what you have learned.

                              Comment

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