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Updated Champ amp heater circuit

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  • #1

    Updated Champ amp heater circuit

    Ok, guys, here's a real low-tech question:

    I'm updating my workhorse and very reliable, around the house SF 70's Fender Champ amp. I"m gonna put in a twisted pair heater circuit w/ 2x 100 ohm ground reference resistors. Do I want to attach these resistors to the bias supply (approx 28 Vdc)? Sometimes, I've just grounded the ground reference resistors and other times, I've 'elevated' them via the bias supply. I've never really made a study of what yields the quietest amp but I've have had very good luck, noise-wise (or lack thereof) with just grounding these resistors in the past.

    Any and all on-point comments are welcome.

    Bob M.
    Tags: None
  • #2
    If you REALLY want to know, try it both ways. It's one wire. If there doesn't seem to be any advantage to elevating the false CT at the cathode resistor just ground it. The last two I did were elevated, but only because I didn't try it both ways so I can't say what difference it may have made.
    "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

    • #3
      And my '79 was dead quiet with just the standard heater wiring as it left the factory...
      So, they all can work. But if it ain't broke...

      Jusrin
      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

      Comment

      • #4
        Ok... Let's take it next level. Since the twisted pair is only effectively reducing hum in the single preamp tube (the tremolo tube isn't in the signal path and the power tube has one filament) you could consider a DC circuit. The winding would probably handle it (though I haven't researched it for certain) since that PT was also used in the Princeton Reverb which draws A LOT more filament current.

        EDIT: Rethinking this... Short of DC filaments, a long as the cathodes of the first preamp tube are fully bypassed (the first triode is, the second virtually), as it IS in the stock circuit then the stock circuit should work fine. Now, if you've done something like a partial bypass of the first triode then you may benefit from a different filament winding.
        "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

        • #5
          > Any and all on-point comments are welcome.

          oops.

          Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
          "Stand back, I'm holding a calculator." - chinrest

          "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

          Comment

          • #6
            Even if you can't hear any difference, put your true rms meter on the speaker leads(or dummy load), you will measure millivolts on the AC scale. Rewire with twised pair, remeasure, elevate, remeasure again.

            Comment

            • #7
              Originally posted by Justin Thomas View Post
              And my '79 was dead quiet with just the standard heater wiring as it left the factory...
              So, they all can work. But if it ain't broke...

              Jusrin
              My SFVC is dead quiet too. If it got noisy all of a sudden, I'd be thinking about a bad cap or a bad tube.
              "Stand back, I'm holding a calculator." - chinrest

              "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

              Comment

              • #8
                Jusrin () and Bob,

                Do your VC's have the first triode fully bypassed like the stock circuit? Because I REALLY think that's the key to quiet operation with the stock filament wiring.
                "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
                  Yeah, mine does. Cuz I'm not clown shit crazy.

                  Jusrin
                  "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                  "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                  "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                  Comment

                  • #10
                    absolutely stock.
                    "Stand back, I'm holding a calculator." - chinrest

                    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

                    Comment

                    • #11
                      Originally posted by bob p View Post
                      > Any and all on-point comments are welcome.

                      oops.

                      Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
                      Bob, it's a good thing your here to enforce nomenclature compliance. If it weren't for you, I'd still be calling it "dampening".
                      If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                      Comment

                      • #12
                        Originally posted by bob p View Post
                        > Any and all on-point comments are welcome.

                        oops.

                        Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on.
                        Well... Both an actual CT and the 100R balance resistors act as a 0V reference with very little current aspect. What would YOU call it? Not being snarky, it just seems like a fair question. I didn't make up "virtual center tap" but I'll use the term for lack of a better one. With 0V reference grid bias resistors we call them "grid load". Maybe "filament load"? Except it's not really a load WRT the filament circuit impedance where a grid load IS a load.
                        "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

                        • #13
                          Originally posted by bob p View Post
                          > Any and all on-point comments are welcome.

                          oops.

                          Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on. No offense intended, Bob and Chuck. I know that you know that it's not really a center tap and that the purpose of the resistors is just to provide a DC offset to the heater supply -- I'm just wondering where that "virtual center tap" terminology came from, because even though it's wrong it sure seems to have become sticky.
                          An actual center tap creates out of phase currents at 3.15V with respect to ground (or to an elevation voltage), so that with tightly twisted wires the electrostatic field due to one wire is cancelled by the field due to the other wire. The two 'virtual center tap' resistors perform the same function. Perhaps 'artificial center tap' would be a more accurate description.

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                          • #14
                            Originally posted by the fatch View Post
                            Perhaps 'artificial center tap' would be a more accurate description.
                            The Valve Wizard

                            This person agrees with you.

                            Comment

                            • #15
                              "Fake Center Tap" is easier to say and takes up less interweb space...

                              Justin
                              "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                              "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                              "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                              Comment

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