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Peavy Heritage VTX what is this output circuit?

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  • Peavy Heritage VTX what is this output circuit?

    This is a solid state amp right up to the four 6L6s output stage, and even then there are solid state shenanigans I do not grasp. The complaint is when switched out of standby, there is a loud hum and one pair of 6L6s turns blue. The ones turning blue have recently been replaced, so something is going on with that side. I measure plate -532v, screen = 532v, and grid 15.4v evenly across all four with tubes out.

    On the cathodes I get 0.013v on the side that is not turning blue, and 0.003 on the side that is turning blue.

    Nothing looks off visually, and the problem stays with the sockets when swapping tubes sides. I really don't know how to procede with this. What is going on at the cathodes? And why is there a big 100R on only one screen of each pair. And most of all why is there 15v on the grids, and no signal? I have never encountered a circuit such as this.


    https://www.thetubestore.com/lib/the...-Schematic.pdf
    Last edited by Randall; 07-24-2019, 08:53 PM.
    It's weird, because it WAS working fine.....

  • #2
    Are you talking about R3 and R6? 100ohms, 5w? Don't you have 1 resistor for both V1 and V2 and another for V3 and V4?

    And what about the voltages for R155 and R159? There are notes on the right side of the schematic.

    I'm not an expert on this one, but I will try to follow this discussion (as I almost bought one of these amps as a project).

    Tom

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    • #3
      "Are you talking about R3 and R6? 100ohms, 5w?"

      Yes.

      "Don't you have 1 resistor for both V1 and V2 and another for V3 and V4?"

      Not the way I read it.

      "And what about the voltages for R155 and R159? There are notes on the right side of the schematic."

      I am seeing 0v across those resistors, but this is with the tubes out.
      It's weird, because it WAS working fine.....

      Comment


      • #4
        Originally posted by Randall View Post
        "Are you talking about R3 and R6? 100ohms, 5w?"

        Yes.

        "Don't you have 1 resistor for both V1 and V2 and another for V3 and V4?"

        Not the way I read it.

        "And what about the voltages for R155 and R159? There are notes on the right side of the schematic."

        I am seeing 0v across those resistors, but this is with the tubes out.
        Enzo knows which circuit this is , juan also I think. Wanna say it's similar to a musicman output.

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

        Comment


        • #5
          It resembles the Musicman, but the PV schematic tells all.

          This amp uses cathode drive. I suspect the turning blue side with its zero cathode voltage, might be indicative of a shorted cathode transistor.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            This is a cathode driven amp. Signal is fed to the cathodes instead of grids. Bias should be 20-40 mV across the cathode resistors and no more than 10 mv difference between the two (per schematic). Your bias is low on both tubes, if I read your measurements right. It's not uncommon for any combination of Q6,7,8,9 to go bad, especially if a tube shorts. I'd start by checking those and associated resistors.

            Edit: I see Enzo beat me!
            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

            Comment


            • #7
              And both Enzo and Dude beat me

              Can only add that grids are held positive so tubes can reach Class AB2 .

              * Where does the grid current come from? ... from the +15V supply through 100 ohm resistors, so there´s up to 150mA available if needed.
              That said, I guess grid current at max drive will be in the order of, say, 10mA ; for an exact value should check datasheets, not many include values for positive grid though, but Mr Musicman first and Peavey second must have measured that in house, so ...

              * cathode voltage varies wildly from, say, +65V or so when passing 5mA or less (at idle) to ground/0V when passing maximum current which I estimate around 250mA or even higher.So if your cathodes are at or near 0V at idle I **strongly** suspect shorted driver transistors and/or very dead (no emission) tubes because a power tube with +15V grid to cathode and >500V on plates and screens must not even turn red but **explode**.
              Juan Manuel Fahey

              Comment


              • #8
                Cathode voltages without tubes in are meaningless here.
                Biggest question is 'what's wrong with blue glow' ?
                I'd guess the blue side may be running a bit cooler with higher plate voltage, thus blue glow.

                Loud hum, that is a problem!

                edit: if I was not clear, 'blue glow' is considered normal with higher plate voltages and is not considered a fault.
                Last edited by g1; 07-25-2019, 12:27 AM.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


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                • #9
                  Oh, I missed the part "with tubes out". Agree- cathode voltages are meaningless without tubes. Plate voltages will likely go lower with tubes installed, also.
                  Last edited by The Dude; 07-25-2019, 12:32 AM.
                  "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

                  Comment


                  • #10
                    Originally posted by g1 View Post
                    Cathode voltages without tubes in are meaningless here.
                    Biggest question is 'what's wrong with blue glow' ?
                    I'd guess the blue side may be running a bit cooler with higher plate voltage, thus blue glow.

                    Loud hum, that is a problem!

                    edit: if I was not clear, 'blue glow' is considered normal with higher plate voltages and is not considered a fault.
                    Unless it's gassy, then it might need some alka selzter. Where the blue glow is determines if gassy or just plasma I believe.

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

                    Comment


                    • #11
                      So are we saying that it's OK to take measurements with tubes in, even though one side is blue with a loud hum?
                      It's weird, because it WAS working fine.....

                      Comment


                      • #12
                        Yes, if there is no red-plating then put the tubes in and measure cathode voltages. Should be in the 65 to 90V range I'd think.
                        Then check the voltage across those resistors and see how the idle current is.
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #13
                          Also missed the no tubes cue, specially because 10 seconds earlier blue glow had been mentioned
                          Plus ... who would post cathode voltages without tubes there?
                          Juan Manuel Fahey

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                          • #14
                            "Plus ... who would post cathode voltages without tubes there?"

                            Me Juan, that would be me. I didn't think it would matter with tubes out, but guessed maybe someone would ask, so I included for expediency.
                            Last edited by Randall; 07-25-2019, 05:07 PM.
                            It's weird, because it WAS working fine.....

                            Comment


                            • #15
                              OK, with tubes in for more than 10 seconds, the side that was blue is actually red plating in a big way. Before, I didn't leave it powered uplong enough for this to occur.
                              It's weird, because it WAS working fine.....

                              Comment

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