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  • Amp mash-up build

    Hello, I have some questions about building an amp with sections from a few different amps. Would It be possible to take a solid state pre amp, add a graphic eq and then power it with an output section from a 100 watt tube amp? These are the sections I have and want to string together, I would think I’d have to get the correct voltages from the b line on the power section and string them to the eq and the preamp, and then make sure that the signal chain is running through all the sections correctly. What else would I have to do to get it to work other than creating the pcb’s and putting it together? Thanks for the help and ideas! Much appreciated.

  • #2
    This did not go well last time.
    nosaj

    I am in the process of learning about amps and circuitry etc, my question is this if I was to make several extra circuits for example an equalizer circuit an extra gain circuit a prescience circuit How would I tie them into a build that I’m building from a schematic of an amp I have found online? thanks so much!
    soldering stuff that's broken, breaking stuff that works, Yeah!

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    • #3
      Originally posted by Shrimpfarmer89 View Post
      What else would I have to do to get it to work other than creating the pcb’s and putting it together? Thanks for the help and ideas! Much appreciated.
      Other than building a chassis and obtaining all the parts, you'll have to find some way to power the solid state circuits. The easiest thing to do is use the bias tap from the tube power transformer. A small circuit will rectify and drop the available Voltage to appropriate levels for the preamp, but it would be super helpful to know exactly how much current the preamp requires. So, build the power amp and preamp, get it running using a lab power supply or even batteries, then measure the Voltage and current the preamp needs.

      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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      • #4
        Originally posted by loudthud View Post

        Other than building a chassis and obtaining all the parts, you'll have to find some way to power the solid state circuits. The easiest thing to do is use the bias tap from the tube power transformer. A small circuit will rectify and drop the available Voltage to appropriate levels for the preamp, but it would be super helpful to know exactly how much current the preamp requires. So, build the power amp and preamp, get it running using a lab power supply or even batteries, then measure the Voltage and current the preamp needs.
        lt
        Couldn't have said it better matter of fact i couldn't have.
        Let us know when you have the power supply built.
        nosaj

        soldering stuff that's broken, breaking stuff that works, Yeah!

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        • #5
          The FET (or JFET) preamp can run on single ended supply, though it looks like the MXR Graphic runs on bipolar supplies....I didn't try to enlarge the schematic to verify that. The 10 band MXR graphic I had decades ago ran on AC mains current, bipolar supplies. If the MXR Graphic you have has it's own power supply to run from AC mains, then that problem is solved. Just need to scale the voltage needed for the preamp circuits.
          Logic is an organized way of going wrong with confidence

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          • #6
            Originally posted by Shrimpfarmer89 View Post
            Hello, I have some questions about building an amp with sections from a few different amps. Would It be possible to take a solid state pre amp, add a graphic eq and then power it with an output section from a 100 watt tube amp? These are the sections I have and want to string together, I would think I’d have to get the correct voltages from the b line on the power section and string them to the eq and the preamp, and then make sure that the signal chain is running through all the sections correctly. What else would I have to do to get it to work other than creating the pcb’s and putting it together? Thanks for the help and ideas! Much appreciated.
            So a Randall Fet preamp > MXR Graphic EQ > Fenderish (Tweed) power amp?

            Basically yes, you will probably need to add a simple 10X or so gain stage (easy) between the SS area and the Tweed power amp, simply they are somewhat hard to drive and typically require some 2V RMS (or higher) to drive well, while the preamp you show will put out around 500mV RMS or so.

            But easy to boost up.

            IF you have a 50V or so bias tap, you can get all voltages from there, even +/-V 15V for GEQ , or anything else (loop) ; if not a small cheap 12+12VAC will do.

            Your GEQ schematic is unreadable, post a larger one.
            Last edited by J M Fahey; 11-10-2021, 04:37 AM.
            Juan Manuel Fahey

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            • #7
              Here is a better quality photo of the eq. I can try and get it a bit larger in a bit,

              but yeah my idea was to take a line off the b line and reverse the polarities and use a few voltage drops to power the sections of the solid state preamp and the eq. Would I need a gain stage in that case still? The power amp section could power those two sections I would think I don’t need to re design the power output stage to power everything. It’s a soldano output stage also I don’t think that really changes anything.

              I cut the solid state pre amp picture right at the point where al the lines converge and would have gone to a single signal path, so i figured that would filter into the input of the eq, then run the output of the eq to the single line signal chain part of the power section. Seems logical enough. Than just power each section with the b line from the power section after it’s been reversed in polarity and add some voltage drops

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              • #8
                Originally posted by nevetslab View Post
                The FET (or JFET) preamp can run on single ended supply, though it looks like the MXR Graphic runs on bipolar supplies....
                I think both the preamp and EQ would run OK from a single +30V supply. From the voltages on the Green channel of the preamp it looks like its supply is +30V. The EQ has a center "bias" voltage for what would be the 0V of a +/-15V bipolar supply.

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                • #9
                  Originally posted by Dave H View Post

                  I think both the preamp and EQ would run OK from a single +30V supply. From the voltages on the Green channel of the preamp it looks like its supply is +30V. The EQ has a center "bias" voltage for what would be the 0V of a +/-15V bipolar supply.

                  the bottom right hand side of the eq has a rating of 18 volts for power, which I could get from a tapped b line

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                  • #10
                    Originally posted by loudthud View Post

                    Other than building a chassis and obtaining all the parts, you'll have to find some way to power the solid state circuits. The easiest thing to do is use the bias tap from the tube power transformer. A small circuit will rectify and drop the available Voltage to appropriate levels for the preamp, but it would be super helpful to know exactly how much current the preamp requires. So, build the power amp and preamp, get it running using a lab power supply or even batteries, then measure the Voltage and current the preamp needs.
                    The pre amp runs on + 24 VDC, it states it in the original schematic for that amplifier.

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                    • #11
                      Originally posted by Shrimpfarmer89 View Post


                      the bottom right hand side of the eq has a rating of 18 volts for power, which I could get from a tapped b line
                      Right, I can see 18V now on the better schematic.

                      It would be better to derive the low voltages from the 50V bias supply rather than 450V B+ It's as the Irish man said to the tourist when asked for directions "If I were you sir I wouldn't start from here" i.e. You don't start from 450V if you only need 18.

                      I'm sure the preamp would run on 24V but I can't square that with the voltages on the schematic.

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                      • #12
                        Originally posted by Dave H View Post

                        Right, I can see 18V now on the better schematic.

                        It would be better to derive the low voltages from the 50V bias supply rather than 450V B+ It's as the Irish man said to the tourist when asked for directions "If I were you sir I wouldn't start from here" i.e. You don't start from 450V if you only need 18.

                        I'm sure the preamp would run on 24V but I can't square that with the voltages on the schematic.

                        this is the power section of the other Randall amp, the e lines are the power lines that run to the preamp at 24 volts.

                        and you are definitely correct, bias is the correct way to go. No need to wrangle with 450 volts. Thank you for the help

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                        • #13
                          loudthud was asking more about the current requirement than the voltage.
                          "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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                          • #14
                            Here is the original schematic of the preamp, for the voltages, not sure if this helps. Haven’t tested the lines on the amp for voltages

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                            • #15
                              So lets clarify something Is this amp built or not?

                              nosaj
                              soldering stuff that's broken, breaking stuff that works, Yeah!

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