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Hot Rod deville Bias??

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  • Hot Rod deville Bias??

    I have a Hot Rod Deville That is redplating . The mV reading is 262 at the bias test point is this even possible?

    Rotating the bias pot will move that value up and down but nowhere under 200 mV..
    Suggestions?

    thanks
    nosaj
    soldering stuff that's broken, breaking stuff that works, Yeah!

  • #2
    What is your bias voltage? What is voltage at the output tube grid pins?

    Edit: I'd pull the output tubes while checking to prevent damage.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      Yep. Pull the power tubes for diagnosis.

      The usual suspects? Bad pot, tubes, Ecaps, coupling caps.
      "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
        I would suspect C26, 27 & C37 firstly.
        Remove the output valves and test for -54v across C38, then -50 or soon pin 5 of the 6L6s.
        If those voltages are correct, fit new balanced 6L6s.
        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
        If you can't fix it, I probably can.

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        • #5
          Here's what i got no tubes installed.
          plate 509
          screen 507
          control grid -24.69

          voltage across c38 -25

          Sorry for delay this notification stuff cat wait to get fixed.
          nosaj
          soldering stuff that's broken, breaking stuff that works, Yeah!

          Comment


          • #6
            Heres what else i got cp15-cp17 57.8 Vac
            cr10 shows .097 Vdc
            CR15 one side -35.75 other side 3.5Vdc

            So based on these I presume is either a bad solder joint or a bad cap I really should remember to check them before i solder them in.. So another fun job of removing the pcb.
            nosaj
            soldering stuff that's broken, breaking stuff that works, Yeah!

            Comment


            • #7
              Read the service manual diagrammes and cross reference the voltages. Bear in mind, the voltages are just a ball park number.
              The Cathode voltage on CR15, (CR10 side), is meaningless as it is swamped with chopped AC from the capacitor and CR10. It is a voltage doubler circuit. Click image for larger version

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              Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
              If you can't fix it, I probably can.

              Comment


              • #8
                cr15 anode side is -35.75 so that means c37 is not pumping enough enough electrons for voltage doubling?
                nosaj
                soldering stuff that's broken, breaking stuff that works, Yeah!

                Comment


                • #9
                  Is there a reason for not posting the complete schematic?
                  - Own Opinions Only -

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                  • #10
                    I am following along on this thread. Here is the schematic: hotrod_deville_schematic.pdf

                    Comment


                    • #11
                      Originally posted by nosaj View Post
                      cr15 anode side is -35.75 so that means c37 is not pumping enough enough electrons for voltage doubling?
                      nosaj
                      Does anyone concur or am I barking up the wrong tree?
                      nosaj
                      soldering stuff that's broken, breaking stuff that works, Yeah!

                      Comment


                      • #12
                        Originally posted by nosaj View Post

                        Does anyone concur
                        Yes, try replacing C37.

                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #13
                          Originally posted by nosaj View Post
                          Does anyone concur or am I barking up the wrong tree?
                          nosaj
                          "Pumping" doesn't make sense to me here.
                          Check C37 and C43 for capacitance and leakage resistance.
                          Last edited by Helmholtz; 11-18-2023, 09:46 PM.
                          - Own Opinions Only -

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                          • #14
                            Originally posted by Helmholtz View Post

                            "Pumping" doesn't make sense to me here.
                            Check C37 and C43 for capacitance and leakage resistance.
                            ???
                            As in C37 isn't holding and releasing charge as it should (pumping?). I had no trouble with the adjectives. C37 isn't performing. (<period) That said, I always appreciate corrected terminologies so I might not be misunderstood by the engineers and techs here. So, thanks. But go easy please..
                            "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
                              I believe voltage doublers are often referred to as an example of 'charge pump' type circuits.
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              Comment

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