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  • Diode clipping mystery

    Hi,

    i built a new amp, based on a little prototype i've made with diode clipping. The proto worked fine, so i decided to build a more versatile amp.
    So here is the schematic. Just look at the second channel with diode clipping. I put four diodes on 2 switches to be able to switch between four possibilities.
    Basically it's a fender twin channel with a 2 gain stages in front, and diode clipping between these 2 triodes, to simulate a distorsion pedal before a fender twin.
    I used a 1 knob tone control and a volume also. So except the switching between diodes, the proto is the same and works fine. In the proto i can switch between 2 diodes OR no diode at all.

    In the new amp, when i move the 2 switches, nothing changes. Same tone and same volume. Strange! After some time of debug i found out that the diodes don't clip nothing actually, if i disconnect them with a switch, the tone is the same.

    Only difference, the 2 switches are relays for layout concerns...

    So i really don't anderstand.
    Attached Files

  • #2
    At least the 2 center contacts of the switches need to be connected to provide anti-series wiring of 2 zeners and to allow for a current (to ground).
    - Own Opinions Only -

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    • #3
      Originally posted by Helmholtz View Post
      At least the 2 center contacts of the switches need to be connected to provide anti-series wiring of 2 zeners and to allow for a current (to ground).
      Yes they are connected, just my schematics, sorry
      Attached Files

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      • #4
        Actually, it works, but i had to take diodes with lower values, like 6V or 3V and a little red led. At the second stage, i don't have 20v, even 13v...
        So now i can play with my toy...

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        • #5
          Originally posted by tepsamps View Post
          Actually, it works, but i had to take diodes with lower values, like 6V or 3V and a little red led. At the second stage, i don't have 20v, even 13v...
          So now i can play with my toy...
          Makes sense.
          I was actually going to ask how you determined the (high) zener voltages.
          - Own Opinions Only -

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          • #6
            And I was going to ask what the signal level was at that point.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Safe to assume that the signal level wasn't strong enough to clip at 18v?
              "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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              • #8
                About zener voltages, if you look at this schematic, first top channel (ch3 actually), between first and second gain stage, there are 2 16V Zener diodes to ground, intended (i think) to lower the signal before the second gain stage and add distorsion. Have we got 16V at this point ?
                Attached Files

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                • #9
                  Originally posted by tepsamps View Post
                  About zener voltages, if you look at this schematic, first top channel (ch3 actually), between first and second gain stage, there are 2 16V Zener diodes to ground, intended (i think) to lower the signal before the second gain stage and add distorsion. Have we got 16V at this point ?
                  As far as I can tell the Zeners will only conduct on hard picked peaks with humbuckers. Slamming chords that include the E and A strings. Not at all with single coils and only maybe on single string attack with a hot humbucker on the E and A strings.
                  "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

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                  • #10
                    Originally posted by tepsamps View Post
                    About zener voltages, if you look at this schematic, first top channel (ch3 actually), between first and second gain stage, there are 2 16V Zener diodes to ground, intended (i think) to lower the signal before the second gain stage and add distorsion. Have we got 16V at this point ?
                    From the test signal voltages in the schematic the gain of input stage (V1-A) calculates as G=57. So with gain control fully open it needs a peak input voltage of more than 280mV to produce a signal peak of 16V at the zeners.
                    Such high guitar signals are untypical and might only occur during the first milliseconds of the attack. So I exclude that the purpose of the zeners is to produce distortion. Maybe they are required to protect the Mute FET.
                    High input (hum interference) signals of several volts can be produced when the instrument cable is plugged into the amp first and then connected to the instrument or if the player touches the tip of the plug.
                    - Own Opinions Only -

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                    • #11
                      Originally posted by Helmholtz View Post
                      From the test signal voltages in the schematic the gain of input stage (V1-A) calculates as G=57. So with gain control fully open it needs a peak input voltage of more than 280mV to produce a signal peak of 16V at the zeners.
                      Such high guitar signals are untypical and might only occur during the first milliseconds of the attack. So I exclude that the purpose of the zeners is to produce distortion. Maybe they are required to protect the Mute FET.
                      High input (hum interference) signals of several volts can be produced when the instrument cable is plugged into the amp first and then connected to the instrument or if the player touches the tip of the plug.
                      Here's something I found regarding guitar output voltage. It's consistent with tests I've done (but never made records for).

                      http://tomsguitarprojects.blogspot.c...ge-levels.html

                      I remember DiMarzio put out a silly looking humbucker in the late 80's that had huge black hex head slugs for pole pieces. It was reported to put out a full volt on peaks.

                      So while those Zeners may be there to protect the mute circuit they will clip the signal under some circumstances. But it's probable in the Fender amp that this happens well after the signal is clipping in later preamp stages.
                      Last edited by Chuck H; 11-27-2019, 01:59 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

                      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                      • #12
                        Dimarzio claims some humbuckers have an output of 500mV, the Tone Zone i think...
                        A few years back, i tried to clone this channel (6 gain stages, wow) without the zeners, and i ended with a jam full of larsens, untamable, and now i thought it was because of the no zeners in my layout. But i must be wrong...

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                        • #13
                          Originally posted by tepsamps View Post
                          Dimarzio claims some humbuckers have an output of 500mV, the Tone Zone i think...
                          A few years back, i tried to clone this channel (6 gain stages, wow) without the zeners, and i ended with a jam full of larsens, untamable, and now i thought it was because of the no zeners in my layout. But i must be wrong...
                          That high output claimed for those pickups would be SLAMMING AN OPEN E CHORD. And then only for a few milliseconds, immediately decaying to a much lower level. Look at the link I posted. It's pretty accurate in my observation. The sort of clipping those diodes in the Fender EVH amp would create isn't very useful musically. So they must be there for another reason. Certainly the designers didn't believe that only the hottest humbuckers would ever be plugged into their amp and that payers would only want hard chords to distort.
                          "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

                          "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                          • #14
                            It was reported to put out a full volt on peaks.
                            Zollner has published histograms (statistical distribution) of PU peak voltages repeatedly slamming chords on a R9 LP. They show that very few peaks of even 1.5V were produced. (BTW, input amplitudes above around 1V produce grid conduction clipping in the first tube.)

                            Problem with PU output measurements is that there is no standardized method and the results depend on a number of influences (PU height, PU position, string gauge, pick, player etc.). So values are not comparable.

                            High peak signals only occur for a few periods after the attack and I doubt that clipping them will be perceived as distortion, as the following hundred(s) of periods will pass unclipped. Especially as clipping by the following stages is very likely to mask such short term effects.

                            Also if the zeners were intended to produce some distortion it would not make sense to choose that high zener voltages which will leave lower output PUs mostly unaffected even at full gain.
                            Last edited by Helmholtz; 11-27-2019, 03:26 PM.
                            - Own Opinions Only -

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                            • #15
                              When i measure the output of the most wound PU i 've made, i get no more than 300mV that i can read on my multimeter. And it's pretty hot yet!

                              About the zeners, this first gain stage is pretty hot also, except the 470pF coupling cap, there is no signal attenuation before the second gain stage, anyway it sounds great...
                              On youtube...

                              Diodes clip the signal, but don't clip the noise, so when i switch between different clipping, the noise level is the same, but the output level vary.

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