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  • #16
    Originally posted by Helmholtz View Post
    Results?
    They do not differ from the other B15N head I have here, nothing suspicious but maybe I'm missing something.

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    • #17
      Originally posted by boroman View Post
      They do not differ from the other B15N head I have here, nothing suspicious but maybe I'm missing something.
      THe question is if they (especially screen voltage) ramp up slowly as well.

      Also monitor how the bias voltage at point B changes in the first 30 seconds.
      Last edited by Helmholtz; 03-30-2020, 03:35 PM.
      - Own Opinions Only -

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      • #18
        Originally posted by Helmholtz View Post
        THe question is if they (especially screen voltage) ramp up slowly as well.
        Also monitor how the bias voltage at point B changes in the first 30 seconds.
        After powering screen and plate goes up to around
        - 185v screens pin4
        - 190v plate pin3,

        After I hit standby, they go to around 489v (screen) and 496 (plate) volts. It takes around 30 seconds to get to those readings (the same for bias)


        What is "point B" if I may ask?

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        • #19
          It's your negative bias supply. Left side of posted schematic, 2/3 down from top.

          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 -

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          • #20
            What is "point B" if I may ask?
            Look at the schematic.

            After I hit standby, they go to around 489v (screen) and 496 (plate) volts. It takes around 30 seconds to get to those readings (the same for bias)
            Ok, here is your real problem. Full idle current can only develop when supply voltages are fully up. I guess the voltages in your other amp ramp-up faster? Are standby switches wired identical?

            I know you recapped the amp but still suspect filter caps. New caps are not necessarily good caps.
            - Own Opinions Only -

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            • #21
              Originally posted by Helmholtz View Post
              Ok, here is your real problem. Full idle current can only develop when supply voltages are fully up. I guess the voltages in your other amp ramp-up faster? Are standby switches wired identical?
              I know you recapped the amp but still suspect filter caps. New caps are not necessarily good caps.
              I have tested those caps before I put them in the amp, but ok, they could went bad somewhere during installation.
              Question is: when the amp is warmed up and all the voltages are up, it sounds great - no hint of hum, full power.

              Should I still blame those caps then?

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              • #22
                Please answer my questions.

                And yes, new ecaps can have excessive leakage current at higher voltages which might not show with low voltage testing. After some operating time leakage reduces and voltage increases.
                Last edited by Helmholtz; 03-30-2020, 11:24 PM.
                - Own Opinions Only -

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                • #23
                  Answering that questions, both amps are recapped and both amps have identical circuit and standby switch wired the same way.

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                  • #24
                    It takes around 30 seconds to get to those readings (the same for bias)
                    Same with other amp (I already asked)?
                    - Own Opinions Only -

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                    • #25
                      Double check that you have the bias cap wired with positive to ground. Also, check your voltages are stable at the PSU caps rather than at the other end at the tube.

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                      • #26
                        Double check that you have the bias cap wired with positive to ground.
                        I also considered that possibilty but a reverse wired bias cap would develop high leakage current. That would cause less negative bias voltage and high idle currents.
                        Last edited by Helmholtz; 03-31-2020, 06:08 PM.
                        - Own Opinions Only -

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                        • #27
                          Originally posted by Helmholtz View Post
                          I also considered that possibilty but a reverse wired bias cap would develop high leakage current. That would cause less negative bias voltage and high idle currents.
                          It's in correct position (positive to ground). I'll post pic later.

                          By the way. I have changed value of this cap to lower and bias is rising way quicker now. How low could I get here and be stable? the bias cap is 100uf/100v. On previous years it was 50uf, and later years - 2x10 uf (quite different approach).

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                          • #28
                            By the way. I have changed value of this cap to lower and bias is rising way quicker now.
                            A lower value bias filter cap will accelerate the buildup of bias VOLTAGE. But it might even somewhat slow down the ramp-up of tube idle CURRENT.
                            - Own Opinions Only -

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                            • #29
                              Your other Ampegs from that era, do they have + HV rectifier with tube or solid state.
                              Recommendation for + HV rectifier with tube
                              https://ampeg.com/pdf/R12R212R.pdf
                              16. Power switch turns the main power on and off. Always turn this switch on first, off last.
                              Turn the Standby switch (#15) on at least 30 seconds after turning on the Power switch.
                              The assumption is that in your '69 Ampeg B15NF output tubes (2x6L6GC) and rectifier (5AR4) are new, not "as new"
                              Old power tubes, dried electrolytic capacitors cause longer warming up time.
                              Question
                              When you turn on the amp, for which time (sec), voltages stabilize and reach the rated value (+430 and -50V).

                              Click image for larger version

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                              https://www.vintage-blue.com/assets/b15nf_service_manual.pdf
                              Who does not know and knows that he does not know - teach him Confucius)
                              Who knows and does not know that he knows - wake him Confucius)

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                              • #30
                                Originally posted by Helmholtz View Post
                                A lower value bias filter cap will accelerate the buildup of bias VOLTAGE. But it might even somewhat slow down the ramp-up of tube idle CURRENT.
                                I think I have found out the issue.
                                As I have two identical amps on my bench, one was working fine few years with me on the road and in the studio. And it's EXACTLY wired the same, recapped with the same components and connections - that lead me to overall confusion. BUT I have found that they are not quite wired the same way! But it's not my fault, it's a factory issue.

                                It all comes to wiring the rectifier socket. The "GOOD" amp has PT red wires connected to pins 4 and 7 (factory solder joints). The "BAD" has them on 4 and 6 (also factory solder joints, red-pen marked). NOW - the schematic calls for 4+6, the rectifier tube does not have even any pin on 7th slot and the 7th pin on the socket is not wired to anything.

                                So what I've done, I've resoleded the 6th pin -> 7th in the "bad" amp and VIOLA! It works great. No more 200+DC voltages on caps and anodes when powering the amp (now close to zero), and after I hit standby it catches bias voltage right away within seconds. So performing excellent. Now, WHY??? Can someone explain it to me?

                                Picture proof: Here's a shot of "GOOD" amp on my bench that was main amp for years. It's FACTORY soldered to pin7. No signs of solder or any other work on pin6. It even has factory red markings on solder joints there. When powered ON, I have 0VAC on red pin4, and 720VAC on red (unused) pin7. When amp is in playing mode, both pins have equal 360VAC. What's ever more odd, I looked at photos I have of my older (cathode biased) and younger (fixed bias but different PCBs) B15Ns and they all have connection on pins 4+6 and they all have absolutely no issues and problems. WTF is going on

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