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  • #1

    building and testing new amp in stages

    Hi all,

    I'm building my first non-kit amp (previously built two kits). I'd like to build in stages and do whatever testing I can before proceeding to the next stage.

    what I DO have: books by KOC, Merlin, and others; digital multimeter (no capacitance), tools, parts, etc.

    What I DON'T have: At the moment I don't have a variac, an oscilloscope, or a dummy load. I could potentially get an oscilloscope before proceeding.

    What I have done so far: I have built the power section and measured the B+ at the different nodes. I will add some fuses in the HT arms and the heater supply, but other than that, I think it is time to proceed to the next steps.

    Questions: Is it possible to wire up the power tubes and OT and test without the preamp in place? If so, should I have a speaker load connected to the output jack? Is it possible to test without any preamp stages feeding into the power amp, and would I need to add something temporarily on the input side of the power tubes (EL84s, push-pull) so that the input impedance is correct?

    -- John
    Tags: None
  • #2
    It's possible to test the power tubes for bias condition correct socket wiring with voltage readings. Obviously you can't test performance without an input signal. It would be smart to include the leads for the jump to the next tube in the filament circuit. Then just cap them off for the test. That way you don't have to make alterations after testing. Do include a proper speaker load when testing so that any hum can reaveal itself.

    You also can't test for is stability. Just because the power tubes are wired correctly doesn't mean your layout is correct. And it's impossible to know how things will interact on all levels without everything in place.
    "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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    • #3
      Thanks for the advice, Chuck.

      I forgot to mention that I wired all of the heaters right off the bat. I plan to cathode-bias the EL84s for starters, so I'll add in a DC standoff using the cathode.
      Should I add anything to the input of the power tubes since the driver is not in place (like a temporary grid leak or something)?

      -- John

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      • #4
        I would suggest that you do not turn on the output section without a load hooked up to the OPT.
        A dummy load (ie: high power/ low resistance element) or a speaker.
        Something.

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        • #5
          You do need the 0V reference resistors in place for the power tube grid circuits. If the grids have no 0V reference the tubes will "run away" and overheat. With no 0V grid in cathode bias the grid is basically out of the picture and the tube acts like a diode.
          "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

          • #6
            Thanks for the advice, guys.

            After some initial testing of the power supply, and output section, I finished the preamp (4 stages), built a light-bulb current limiter, and fired up the amp. Everything checked out fine, so I plugged it.

            I soon realized that I had grounded a shielded cable on both ends, from the treble wiper to the MV. This resulted in a partial tone-stack bypass (the mid and bass pots didn't work). I snipped the shield on one end, and the amp sounded great.

            Comment

            • #7
              Originally posted by stormbringer View Post
              I soon realized that I had grounded a shielded cable on both ends, from the treble wiper to the MV. This resulted in a partial tone-stack bypass (the mid and bass pots didn't work). I snipped the shield on one end, and the amp sounded great.
              That actually doesn't jive the way it reads. But the shield should only be grounded at one end anyway.
              "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
                Correction: the shielded cable was not grounded on the treble wiper - it was grounded on the leftmost terminal (when looking at the pot from behind, with terminals facing up) - the same terminal from which a jumper wire connects to the bass pot.

                In addition to the non-functioning mid and bass pots, I noticed that turning the treble pot clockwise increased volume and mids, as if it were acting like a tone-stack bypass. So I think that instead of going to ground through the mid pot, the signal went to ground via the master volume (hence bypassing the bass and mid controls).



                Originally posted by Chuck H View Post
                That actually doesn't jive the way it reads. But the shield should only be grounded at one end anyway.

                Comment

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