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Fuzz pedal (blocking distortion) makes one EL34 shows early signs of red plating in 50 watt

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  • #31
    Originally posted by Gtr0 View Post
    here it is... sorry for the messy writing, it was on my lap looking at the circuit...
    Click image for larger version Name: IMG_2579.JPG Views: 165 Size: 1.13 MB ID: 917799
    That can't work properly. You need the extra 4.7k resistor between the 680nF and ground as in the 2203.
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    • #32
      Excellent. I thought from first turn on that those voltages seemed odd and I even had a 4k7 running from that same junction to the ground bus at the pots. But I think I still had the red plating even with that in place. But I will soon know for sure. I will place in the 4k7, check voltages and play..... I will report back.

      Thank you, so very much!
      "'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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      • #33
        okay, so now, voltages are normal

        grid at 25 and 27 VDC
        cathode at 42 VDC
        plates at 242 and 234 VDC

        awesome... going to see if we are red plating still!

        Thank you!!!

        "'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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        • #34
          Originally posted by Gtr0 View Post
          Excellent. I thought from first turn on that those voltages seemed odd and I even had a 4k7 running from that same junction to the ground bus at the pots. But I think I still had the red plating even with that in place. But I will soon know for sure. I will place in the 4k7, check voltages and play..... I will report back.

          Thank you, so very much!
          It will result in better grid drive. But most probably not change the redplating
          As written earlier, look after the socket issue.
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          • #35
            Originally posted by Helmholtz View Post

            It will result in better grid drive. But most probably not change the redplating
            As written earlier, look after the socket issue.
            you mean by replacing the socket?

            it is still red plating on the same side, v4... but the lights have to be off to see it.

            Thanks!!
            "'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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            • #36
              Originally posted by Gtr0 View Post
              you mean by replacing the socket?
              Not necessarily. But if you get higher idle current in one of the sockets, there must be a reason.

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              • #37
                Originally posted by Helmholtz View Post

                Not necessarily. But if you get higher idle current in one of the sockets, there must be a reason.
                totally agree - at idle all voltages are okay, but I will recheck bias now that the tubes have been swapped. if it is still far off as before I will replace it. not sure what else it could be...

                Helmholtz - thank you so much for your attentive help!!!! It is so appreciated.

                PDF and Jon as well! Thank you guys!
                "'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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                • #38
                  okay, right now they are biased in the 50% area...

                  plates @ 458 VDC
                  bias voltage @ -40 VDC

                  CT:
                  v4 to CT 34.1Ω
                  v5 to CT 34.6Ω

                  drop:
                  v4 @ -.980 VDC
                  v5 @ -.900 VDC

                  v4 @ .028A
                  v5 @ .026A

                  no bias pot changes made
                  "'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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                  • #39
                    Originally posted by Gtr0 View Post
                    okay, right now they are biased in the 50% area...

                    plates @ 458 VDC
                    bias voltage @ -40 VDC

                    CT:
                    v4 to CT 34.1Ω
                    v5 to CT 34.6Ω

                    drop:
                    v4 @ -.980 VDC
                    v5 @ -.900 VDC

                    v4 @ .028A
                    v5 @ .026A

                    no bias pot changes made
                    Ok, good. But your earlier measurements were not consistent. Maybe you didn't allow for sufficient warm-up time before measurement.
                    As said, a little redplating at full clipping is not unusual. And as the grid drives are not fully symmetrical, one tube will redplate more.
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                    • #40
                      The power tube grid stoppers suggested by Jon may still be beneficial; whatever, they can't do any harm.
                      My band:- http://www.youtube.com/user/RedwingBand

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                      • #41
                        I would consider using 1k screen stoppers.
                        Full clipping operation requires some extra protection.

                        Edit: Just noticed, the amp already seems to have them (another deviation from the Marshall schematic.).
                        Last edited by Helmholtz; 11-10-2020, 06:00 PM.
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                        • #42
                          Quite possible that I did not allow enough warm up time....

                          @ Jon... right now I am using 5k6 grid stoppers... I believe he said 10k, I will double check and give it a try before moving on.

                          Thanks!
                          "'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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                          • #43
                            bummer... no change, red plating with 10kΩ as well :-(
                            "'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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                            • #44
                              Originally posted by Helmholtz View Post
                              I would consider using 1k screen stoppers.
                              Full clipping operation requires some extra protection.

                              Edit: Just noticed, the amp already seems to have them (another deviation from the Marshall schematic.).
                              yikes... not in the schem? oops... I usually use 1k for el34 and 470 for 6L6 automatically
                              "'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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                              • #45
                                Grid stoppers only make a difference if oscillation is involved. Higher values (in the 100k range) can help to smooth out blocking distortion.

                                If redplating is not excessive I consider your amp fixed.
                                If only one of the tubes redplates you might try a different PI tube and/or experiment with different PI plate resistors to achieve better grid drive symmetry.
                                Last edited by Helmholtz; 11-10-2020, 06:33 PM.
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