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5F6A Bassman total custom build - super low output

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  • #31
    Modified original post, added layout photos.

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    • #32
      1 ohm current sensing resistors in the power tube cathode returns would be very handy here.
      My band:- http://www.youtube.com/user/RedwingBand

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      • #33
        I just looked at the added layout drawings, and have a comment about the heater supply wiring. You can short out either of the 6.3VAC heater windings with that hum balance pot, since you have made a center-tap from the two windings, AND connected the wiper of that 500 ohm Hum Balance pot to that C/T. Removing the C/T Ground connection, leaving the two power tube heaters to run on each winding, then have the hum balance pot's wiper going to ground, it may work for the preamp tubes, but not for the power tubes. I think you need to re-think all that again.
        Logic is an organized way of going wrong with confidence

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        • #34
          ^^^^^^ THAT! You have a potential transformer killer there.
          "Yeah, well, you know, that's just, like, your opinion, man."

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          • #35
            With the preamp tubes in series I don't see the potential for improving hum with the balance control because there's no differential (I think that was conceded). And with the power tubes wired each to ground the 500R pot isn't a false CT, but a variable shunt that, at 250R per side, would probably need a gross adjustment to sink enough current to make a difference. Did I get this wrong?

            And what was said above. It should be changed if for no other reason than it shouldn't be possible to dial in a dangerous failure.
            Last edited by Chuck H; 01-18-2018, 12:43 AM.
            "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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            • #36
              Trying to update my schematics. The schematic is wrong. I soldered the 2 xfmr leads together at the xfmr, never brought out the center tap. It really is wired as a fake center tap at the pot. Also, V1 V2 V3 are not in series. They are parallel from a true 12vac source.

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              • #37
                Thanks for all the great thought provoking posts. This is literally driving me insane. I am actually having serious problems sleeping.

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                • #38
                  The other thing I noticed is that you have one of the PI outputs going to the screen grid instead of control grid on your layout of the output section. Drawing error or build error?
                  Last edited by The Dude; 01-18-2018, 11:41 PM.
                  "Yeah, well, you know, that's just, like, your opinion, man."

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                  • #39
                    Originally posted by The Dude View Post
                    The other thing I noticed is that you have one of the PI outputs going to the screen grid instead of control grid on your layout of the output section. Drawing error or build error?
                    Not at home right now. That could be it! Will check in about an hour.

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                    • #40
                      Originally posted by tomgilmartin View Post
                      Trying to update my schematics. The schematic is wrong. I soldered the 2 xfmr leads together at the xfmr, never brought out the center tap. It really is wired as a fake center tap at the pot. Also, V1 V2 V3 are not in series. They are parallel from a true 12vac source.
                      Sorry, I was unclear. My fault. I meant the two triodes of each tube were in series. Not that the three individual tubes were in series.
                      "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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                      • #41
                        Originally posted by tomgilmartin View Post
                        Not at home right now. That could be it! Will check in about an hour.
                        You nailed it. Son of a bitch. So desperate to make things symmetrical...
                        Waiting for cap discharge.

                        EDIT - I was agonizing over the layout of the bias circuit. At first I wanted to use the unused pin 6 on the power tubes, so while drawing the layout I kept moving the wires around. Got crossed up on pins 4 and 5. Drew it wrong, then wired it wrong. Hope I didn't destroy anything.
                        Last edited by tomgilmartin; 01-18-2018, 01:38 AM.

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                        • #42
                          Power circuit early in the build.
                          Click image for larger version

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                          • #43
                            Originally posted by tomgilmartin View Post
                            Power circuit early in the build.
                            [ATTACH=CONFIG]46611[/ATTACH]
                            I admit it looks nice, in a 'Frankenstein meets Willy Wonka' sort of way. I hope your amp sounds good soon.

                            Oh, and don't worry about the insomnia. Just a side effect of creative process.
                            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

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                            • #44
                              Originally posted by Pedro Vecino View Post
                              Is the secondary of the output transformer connected to ground?
                              Originally posted by tomgilmartin View Post
                              Yes, at the feedback connection point near the presence pot.
                              Just want to verify this. The feedback connection point is not ground. I hope I'm misreading and you mean the ground connection of the presence pot.
                              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                              • #45
                                That was a critical mistake. SOOO loud, even with the soak resistor back inline. Sure can get to grunge sounds. Thanks all!!!!!

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