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Too Much Gain with ax84 amps build

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  • #1

    Too Much Gain with ax84 amps build

    I need help..
    I never build high gain amps before, and finally i started my projectw 2 weeks ago
    Here links for the schematic http://www.ax84.com/october/AX84_Oct...lub_090519.pdf
    I like the October sound when i saw in youtube so decided to built one. It's just project not for serious build.
    But when it's finished i thought it just too much gain. Sound terriblle.. hahahaha.. sorie for ax84 members.
    When i played my guitars, specially in low strings in more produce annoying sounds.
    Maybe anyone can give me good advice to solve this problem.
    Thanks..
    Tags: None
  • #2
    I'm sure the circuit can be modified for less gain - not sure how that is accomplished though. Doesn't ax84 have a forum? One simple possibility is to replace the first 12ax7 (V6 a & b) with a 5751 or even a 12at7 - either will be lower gain tubes.

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    • #3
      I'm going to go out on a limb. I'm no authority, but It looks like a voltage divider would work to dump some signal. I would stick one in just before V5B, something like a series 220k followed directly by a 220k to ground. I believe that will dump half the gain. If it dumps to much gain raise the value of the resistor to ground until the gain is where You want it. You can play with the values. 220k then a 500k to ground will get Ya about 75% of the gain You have now. The bigger the series resistor the more top end that is lost, as well as gain.

      Again, I'm no authority. I would appreciate if someone would confirm if I'm right, or correct Me if I'm wrong.

      FWIW C. Smith

      Comment

      • #4
        What Manic said. Experiment with changing the voltage dividers at the grid of each stage to dump some of the gain (by either upping the series resistor or lowering the leg resistor or both. Experiment with various values one at a time through the signal chain. You can also bypass the series resistor with a small (1000pf or so) high voltage cap, to allow more highs to pass.

        You can also experiment with coupling caps values (making them smaller helps get the bass out of it). Also try experimenting with smaller cathode bypass caps.
        Building a better world (one tube amp at a time)

        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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        • #5
          There is alot that can be done. A voltage divider is a good suggestion. But since you chose this amp I'm guessing you wanted a high gain tone. So perhaps the amp can be adjusted only where needed to make it sound the way you want. I see that your english is not perfect, but can you better describe what sounds bad now? Is the bass too loose? Is the top too fizzy?

          Lots of ways to tweak this amp.

          Chuck
          "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
            Originally posted by captntasty View Post
            I'm sure the circuit can be modified for less gain - not sure how that is accomplished though. Doesn't ax84 have a forum? One simple possibility is to replace the first 12ax7 (V6 a & b) with a 5751 or even a 12at7 - either will be lower gain tubes.
            The 5751 and the 12ay7 are both good choices, but having put the numbers through calculators and having done tons of tests- including a blind experiment where I had removed the writing off some tubes (brake clean!) and gave them to a golden eared friend- I would like to posit this:

            the 12at7, a common sub for the ax7, actually yields similar or more gain in common use, due to the vastly lower plate impedance- about 12k compared to 68k. Both of these calculators will show that, for common application: 100k plate res, bypassed 1.5k cathode, the gain difference between these two tubes is negligible:

            Adam's Amplifiers: Triode Calculator
            Tube Circuit Gain Calculator

            but the results are somewhat contradictory, maintaining some suspension of judgement... commentary?

            Also, I'd like to put some ideas forth on that schematic- It's got a lot going on in those first stages, lots of bass and gain, and not as much in the midamp... the cathode follower isn't adding any gain, but it IS reducing the loss through the tone stack. I'm not sure that by rewiring it to a plate coupled stage it will actually reduce net gain, though, but really just make it more compressed and crispy and reduce the effectiveness of the EQ... I'd suggest dropping the bypass caps in the first stage and implementing a marshall style change to the midamp- adding a 470k grid load resistor, perhaps with a bypass cap (250 pf?) if you're in love with the parallelism of the preamp to shed some gain. Grid resistors in front of the first stage could help make it more livable too by cutting down potential super-high frequencies which will make some harshness. Adding more neg. feedback could help if you think the driver and the power stage are breaking up too much; a local feedback section around v5b could also be interesting, and that could cut gain by a lot while (I think) getting some compression out of the deal- it could be switchable, too.

            Check out randall aiken's pages ("Designing Single-Stage Inverting Feedback Amplifiers") for some insight into design, or simply steal a page from Leo and copy the AB568 Bassman- although that amp puts it's coupling cap at the grid of that stage... You need those caps because the stages run at different voltages... so your design looks like the last ones on the page, which puts the feedback coupling cap at the grid...

            Sounds like a fun addition to the project, and it's easy to switch (after the cap with 10m+ across the switch for no pop I bet), thus Footswitch, if it's a real improvement to the variety of tones the thing can make. Have fun!

            Comment

            • #7
              Thanks everyone for the help
              @Chuck H : Yes. The bass is loosy and like mixed with treble. So it's produce annoying sounds. When i start play rhythm, combination all strings make sounds horrible.
              Already try voltage divider (220k) before V5b and just reduce gain but not make low strings sounds goods.
              Will try another suggestion here and will report soon.
              Thanks alot guys

              Comment

              • #8
                Too much gain from an October? Are you sure there are no bugs in your build? I built a bigger version of the October Stage and find there is plenty of clean tone if you know how to dial it in.

                Try these settings:
                bright gain: 3
                dark gain: 1
                treble: 6
                middle: 2
                bass: 6
                master: __
                presence: 4
                NFB: 5

                If that's not "fender clean", I think you should do some voltage measurements on your build...

                Comment

                • #9
                  I think I agree with Andrew. This actually looks like a very bright amp to me. How close to the schematic is your build?

                  It may be that there is a mistake in the wiring or voltages causing an incorrect bias on one or more tube stages. That could sound like you describe with this amp.

                  Voltage readings wuld be helpful at this point.

                  Chuck
                  "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

                  • #10
                    I think I agree with Andrew. This actually looks like a very bright amp to me. How close to the schematic is your build?

                    It may be that there is a mistake in the wiring or voltages causing an incorrect bias on one or more tube stages. That could sound like you describe with this amp.

                    Voltage readings would be helpful at this point.

                    Chuck
                    "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

                    • #11
                      @andrew_k and @Chuck H thanks.
                      You're right about it's bright. My serious project before this, is fender amps. Maybe my ears already fenderish..
                      Also notice from me, it's okay when i played clean channel. But when is started crank bright and dark volume, my low string starting loose.
                      Starting rhytm and more "brmm brmm" sounds with low strings. With high strings it's good to me.
                      So i think maybe is just too much gain ( or i'm wrong )
                      Already check the build and seem it's all correct
                      Almost forget, i change MV from pre PI to Post PI. Using JJ 6L6GC, 370volt on plate biasing 52ma it's about 19 watt output.
                      Speaker using weber 2 x 10A125, rate at 25 watt each..
                      Thanks again guys

                      Comment

                      • #12
                        Originally posted by djembutz View Post
                        @andrew_k and @Chuck H thanks.
                        Almost forget, i change MV from pre PI to Post PI. Using JJ 6L6GC, 370volt
                        Thanks again guys
                        Could you perhaps be over-driving the phase inverter?
                        An experiment worth trying would be to clip in a pre-PI master and turn up the post-PI master

                        Comment

                        • #13
                          Hey guys, i already re-examine the whole circuit and "damn" i had found 3 bad caps labeled C20a (22nf), C20b (2n2nf) and C16 (10nf).
                          Change all bad caps and .. viola, the sounds is what i expected.
                          3 bad caps also make me 3 bad days..
                          Thank you guys for the help, and when someone ask me; why not to "that place for looking the answer", so why i had to go to another place if here already have alot friends to help..
                          Peace,,

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