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Traynor YBA-2 (EL84 Version) bad sounding asymmetric distortion... what's up?

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  • Traynor YBA-2 (EL84 Version) bad sounding asymmetric distortion... what's up?

    So a friend recently brought over his Traynor YBA-2 (EL84 - volume, bass, treble knobs) and we both agreed it sounded way worse than my YBA-2 (cathode biased 6V6 - volume, tone). There was a crappy buzzing type octave distortion that fizzled out as the notes died down. (The amps are so different that it is very confusing that they even share the same name, but I wasn't expecting his to sound straight-up "Bad" when overdriven.


    Here's the schematic:
    Click image for larger version

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    I decided to throw it on my bench and probe around to try and figure out why this thing sounds so bad. Immediately I noticed that the distortion is very asymmetrical before it even hits the tone-stack and volume knob. Unlike my 6L6 YBA-2, this one has two gain stages back to back before the tone-stack and volume control.

    Here are some scope shots (VOLUME @ 4, BASS+TREBLE @ 5:

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    1: Signal Generator input. 1Vpp sine.
    2. Output from first gain stage after C1. 12Vpp nice sine.
    3. Output from second gain stage after C3. 90Vpp hot mess.
    4. Speaker output.

    Click image for larger version

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    1. Signal before PI before C7. 2Vpp hot mess.
    2+3. Outputs of Phase Inverter. 40Vpp
    4. Speaker output.


    So you can see after the second gain stage, the distortion is very asymmetric and then only gets worse after the output section as the crossover distortion is added. This certainly explains the octave sounding buzzy distortion that faded with the notes decay.. This is definitely not how my YBA-2 looks in the scope. I desoldered and tested the resistance of everything associated with the first two gain stages and they all match the schematic, no DC passing through the coupling capacitors.

    - Is this the expected behavior of this schematic?

  • #2
    In the meantime, I've replaced the 1M resistor R6 between the first two gain stages with a 1M potentiometer to dial back the signal that distorts the second gain stage and it definitely makes the amp sound better.

    Comment


    • #3
      Well... EL84's have some gain of their own. So this isn't going to be like your 6V6 version. Even in the first shot you can see a crossover artifact on the output. Part of the problem you may be having is that this amp is designed for a guitar and not as a reference amplifier. A full volt of input is entirely too much and will overtax the gain structure of the amp way before the PI or power tubes. A guitar is more like 100mV nominal with very brief attack episodes reaching half a volt. Maybe back off the input to a tenth of what you were using and see what sort of clean wattage you can dial in.?.
      "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


      • #4
        Originally posted by waspclothes View Post
        - Is this the expected behavior of this schematic?
        Yes, it's to be expected with that input signal level. The first gain stage is not overdriven but 12V pp is far too much for the input of the second stage to handle and it's cold bias causes it to clip asymmetrically (top first). The PI output at 40V pp is also far too much for EL84s resulting in hard clipping and crossover distortion.
        Last edited by Dave H; 01-17-2020, 08:35 AM. Reason: Changed PI output to 40V

        Comment


        • #5
          Both schematics can be compared, pg.4 & 5 of this pdf
          Attached Files
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          Comment


          • #6
            Originally posted by Dave H View Post
            Yes, it's to be expected with that input signal level. The first gain stage is not overdriven but 12V pp is far too much for the input of the second stage to handle and it's cold bias causes it to clip asymmetrically (top first). The PI output at 90V pp is also far too much for EL84s resulting in hard clipping and crossover distortion.
            Thank you, I will try it with a lower input level. I've always used 1Vpp as a test into guitar amps because my guitar peaks at 1Vpp with humbuckers and a reasonably hard strum.

            Comment


            • #7
              Consider that that 1v pp is a peak number for your pickup - the hard attack. if it clips, it only clips that attack. if you input a sine wave at that level, it is as if your guitar was permanently peaking. A clip goes on forever.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Originally posted by waspclothes View Post
                Thank you, I will try it with a lower input level. I've always used 1Vpp as a test into guitar amps because my guitar peaks at 1Vpp with humbuckers and a reasonably hard strum.
                1Vpp should measure .5V on your meter. Which is the peak I indicated in post #3 regarding too much input signal in your test. But if it's Dave that gets the credit that's alright too He certainly deserves the respect. Just so long as the issue gets resolved.
                "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


                • #9
                  For sine signals 1Vpp corresponds to 0.35Vrms and this is what an AC voltmeter will show.
                  - Own Opinions Only -

                  Comment


                  • #10
                    Originally posted by Helmholtz View Post
                    For sine signals 1Vpp corresponds to 0.35Vrms and this is what an AC voltmeter will show.
                    Always happy for the accurate details. I did not account for the RMS conversion when I said "meter". I should have said scope.
                    "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
                      Originally posted by Enzo View Post
                      Consider that that 1v pp is a peak number for your pickup - the hard attack. if it clips, it only clips that attack. if you input a sine wave at that level, it is as if your guitar was permanently peaking. A clip goes on forever.
                      Hi Enzo, yes.. but I'm trying to diagnose a distortion that only sounds bad when being strummed very loudly and that goes away as the notes decay. So I figured driving the amp at that initial attack level would show the problem. If I back my function generator off to say 200mVpp, the distortion after the second stage once again turns symmetrical - looking like every other amp I've put on the scope.

                      I understand all of the responses say that my input signal is too high, but I'm trying to show the asymmetric distortion that's happening during hard attacks because of the configuration of the amp's first two gain stages.

                      Comment


                      • #12
                        Originally posted by waspclothes View Post
                        Hi Enzo, yes.. but I'm trying to diagnose a distortion that only sounds bad when being strummed very loudly and that goes away as the notes decay. So I figured driving the amp at that initial attack level would show the problem. If I back my function generator off to say 200mVpp, the distortion after the second stage once again turns symmetrical - looking like every other amp I've put on the scope.

                        I understand all of the responses say that my input signal is too high, but I'm trying to show the asymmetric distortion that's happening during hard attacks because of the configuration of the amp's first two gain stages.
                        So what happens to the bias on the second stage when being driven by that high signal? I haven't looked at the schematic, just thinking blocking distortion from your description of the sound.
                        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


                        • #13
                          Originally posted by eschertron View Post
                          So what happens to the bias on the second stage when being driven by that high signal? I haven't looked at the schematic, just thinking blocking distortion from your description of the sound.
                          Can you tell blocking distortion from the scope shots I posted? I can measure what the bias on the second stage does, is that just the DC voltage that's on the cathode?

                          Comment


                          • #14
                            Ok... I went ahead and modeled this amp (because I can, thanks Dave ) and if I plug in .5VAC for an input signal, which I assume for the voltage "component" in spice is 1Vpp???, I get results entirely consistent with what you report and your scope observations. Some things to consider:

                            Tubes are very graceful regarding performance on peaks. That is, you can't just run the signal generator continuous at the guitars peak voltage and expect to see what the tube does on peaks. Because peaks are temporary and tubes are better at short peaks than they are at sustained high conduction. So information garnered by running the amp sustained at what would be a transient in real life is not going to give you an accurate measure audibly or visually on the scope of what's actually happening on peaks. JM2C on that.

                            Also, asymmetrical clipping is NOT a "hot mess" A great many guitar amps employ asymmetrical clipping on the first clipping stage and are revered for their tone. But...

                            That said, it's obvious to me (and probably anyone that's done any mods or design) that this amp IS NOT designed with any intention of good clipped tone. It's designed to take a low level signal and make it loud in a linear way. I wouldn't expect graceful clipping from this design and since that (clearly) wasn't the intention of the designer I suppose that's alright.?. But I'm calling a spade a spade here. I wouldn't expect any good clipping from the stock design.

                            I think an audio clip of the problem would be more helpful at this point.
                            Last edited by Chuck H; 01-18-2020, 12:06 PM.
                            "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


                            • #15
                              Originally posted by waspclothes View Post
                              Can you tell blocking distortion from the scope shots I posted? I can measure what the bias on the second stage does, is that just the DC voltage that's on the cathode?
                              OK. So i took a brief look at the scope traces. Which of those traces look most like what you see at the speaker? What's the earliest trace that look most like what you see at the speaker? Whatever you're looking for happens before that.

                              As Chuck mentioned, a sin wave (cranked or not) doesn't behave the same way a complex signal does.
                              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

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