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Low Volume 5150

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  • Low Volume 5150

    early model 5150 head..very low volume..sounds like about 5 watts output...not 120w. if i run it fx loop send into a different power amp , i have full volume. I found a screen resistor open on a power tube socket ---replaced it .

    Swapped all tubes , cleaned sockets , cleaned fx loop jacks , all socket voltages / bias are there , poked and prodded , moved wires ...

    heres 1 thing - the standby LED does not light up . Ive checked R 70 / 72 / 73 . I see that Q6 is right in the path of that Led. I do have 8vdc at 1 leg of the standby Led.

    i remember reading that there might be a bad "muting" ? transistor that might cause this ? If the standby led is bad , can it kill signal ??

    so for transistors on the pcb, we have Q6 / Q7 /Q3 and CR23

    I know Ive read about this volume problem before , somewhere...... here is the scem and layout
    Attached Files

  • #2
    What is the voltage across the LED- not referenced to ground? Check the LED with your DVM. Do you have screen voltage at the output tube sockets?
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      8.5 vdc

      or -8.5 vdc reversing leads

      yes all voltages are at sockets

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      • #4
        If you have 8.5V across an LED and it's the correct polarity, the LED is open.
        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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        • #5
          Originally posted by The Dude View Post
          If you have 8.5V across an LED and it's the correct polarity, the LED is open.
          ok--looking at the scem--could that cause my volume issue?

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          • #6
            Check the Vac at the output tube grids (100Mv/ 1K input, all controls maxed)
            A 50 watt amp should be seeing about 50Vac at the grids. (double that for a 100W)

            If you have that, then the OT is suspect.

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            • #7
              Originally posted by Valvehead View Post
              ok--looking at the scem--could that cause my volume issue?
              Nope, not if you have screen voltage. When screen voltage comes up (standby flipped), it turns on Q6 giving the LED a ground path. IMO, your volume issue is unrelated,....... again assuming you have screen voltage and you said you did.

              Edit: You could also have cracked/cold solder on the LED, which is why I asked to test the LED with a DVM.
              Last edited by The Dude; 09-06-2019, 05:04 AM.
              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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              • #8
                Originally posted by Jazz P Bass View Post
                Check the Vac at the output tube grids (100Mv/ 1K input, all controls maxed)
                A 50 watt amp should be seeing about 50Vac at the grids. (double that for a 100W)

                If you have that, then the OT is suspect.
                so volt meter from pin4 to pin 5 (6l6 ) ac ?

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                • #9
                  A 50 watt amp should be seeing about 50Vac at the grids. (double that for a 100W)
                  The power tubes' grid signal voltage for full/max output is solely determined by the bias voltage (Vbias) - independent of number of tubes or rated output power.
                  E.g., with a bias of 40V, required grid signal voltage is Vbias x 0.707 = 28Vrms or 80Vpp. No difference between 50W or 100W versions as long as bias is the same.
                  - Own Opinions Only -

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                  • #10
                    power tubes-
                    plates are 476vdc
                    pin 4 476 vdc
                    pin 5 -54 vdc

                    from pin 4 to 5 i have 0 vac


                    both channels have low volume and all controls function normal
                    Last edited by Valvehead; 09-06-2019, 08:06 PM.

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                    • #11
                      Oh, the infamous 5150 volume drop. Check the ribbon cable attaching the power tube board to the main board.

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                      • #12
                        Originally posted by DFuller View Post
                        Oh, the infamous 5150 volume drop. Check the ribbon cable attaching the power tube board to the main board.
                        Checked , cable is good , checked continuity

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                        • #13
                          pulled the main pcb--looks great--all joints ok , pulled the riveted in power tube pcb---all ok there .

                          pulled all ribbon cables -- they look good and test good

                          tested the OT primary , i have 28 / 60 / 32 ohms , and no shorts to the secondary

                          I have bias and all normal voltages at all tube sockets

                          speaker jacks are good , as is the imp. switch ........cleaned all jacks , pots

                          could the OT be on its death bed ?? I just cant find anything wrong

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                          • #14
                            EDIT: Actually, a thought. These amps have Flyback prevention diodes. I wonder if they've gone conductive?

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                            • #15
                              Easy enough to check. But usually if those fail, it blows fuses.


                              So did you ever remove Q7? That is my main go-to for this symptom.
                              Education is what you're left with after you have forgotten what you have learned.

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