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  • Dr Z redplating

    I have a Dr Z Remedy head in that I mentioned in another thread for a different reason. Came in with two dead 6V6s, one cracked, and one just not conducting or producing anything on my tester. The remaining two are red plating. This is cathode biased with two cathode resistors, one for each pair of tubes. Thinking the cathode caps were shorted, since they are right up touching the cemeent cathode resistors, but that means both of them must be bad, because the tubes are red plating in all sockets. But, both 150R 10 watt resistors measure 151R in circuit, so it sure doesn't look like the caps are shorted, but I lifted one anyway. I put a tube in one of the two sockets that connect to that cathode resistor, and the tube still red plated. So did a substitue tube from my stash.

    So I ask, what could be happening? Pretty simple circuit, so if the resistor measures right, and the cap is out, what else could make a cathode biased tube pull around 100mA? No schematic available that I could find.
    It's weird, because it WAS working fine.....

  • #2
    150R seems very low for a 6V6 shared cathode resistor. My amp has 300R. Plate current is 70mA per side, 35mA and just over 12W per 6V6.

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    • #3
      I should have pointed out that I got 100mA measuring with a Hoffman bias probe, so that is cathode current. But still.
      It's weird, because it WAS working fine.....

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      • #4
        Originally posted by Randall View Post
        I put a tube in one of the two sockets that connect to that cathode resistor, and the tube still red plated. So did a substitue tube from my stash.
        Are you saying you put a tube in one of the two sockets connected to a shared cathode resistor and the other socket was empty?
        A shared cathode resistor needs two conducting tubes. If one tube is missing or not conducting the other tube will red plate.

        Edit: I measured 77mA cathode current for each pair of 6V6s on my amp.

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        • #5
          Yes that is what I did, and just realized this. With two tubes inserted sharing the same resistor with the cap lifted, I still get 70 -75mA on a single tube, and this is with a substitute pair. At that level, a Russian EH red plated, but a used JJ in the other socket did not, not that I was willing to wait very long and see. Still something amiss here.
          It's weird, because it WAS working fine.....

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          • #6
            An LTspice sim of my amp with the cathode resistor changed to 150R gives a cathode current of 65mA which is a total dissipation of 23W per tube!
            I wouldn't want to run it like that.

            Edit:
            I've just had a thought. It's not mixed bias is it?
            What is the control grid voltage?
            Can you see if the grid resistors connect to ground?

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            • #7
              The 1.2K grid resistors connect in pairs at each coupling cap. Those cap connections are both connected to ground via a 91K resistor for each side.

              There is no grid voltage.
              It's weird, because it WAS working fine.....

              Comment


              • #8
                Originally posted by Randall View Post
                There is no grid voltage.
                If this is true, then this is your problem. The grid must have a voltage reference, even if it's 0V. Measure the grid to ground and grid to cathode voltages and be sure you are getting "no voltage".
                If the grid is open, that will cause the tube to run-away and redplate.

                edit: if the grid is lifted and floating look for an open resistor - the 91k and 1k2.
                if those seem okay, then test the DC voltages on either side of the resistor. Most likely you'll find a bad solder joint or broken wire somewhere between those resistors and the pin of the tube socket. (or at the pin of the tube socket)
                good luck
                If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                • #9
                  "The grid must have a voltage reference, even if it's 0V."

                  I do not understand this. In my mind no voltage = 0V, just as no apples = 0 apples.

                  "Measure the grid to ground and grid to cathode voltages and be sure you are getting "no voltage"."

                  I see 91.4K ohms from each grid to ground. That confirms the 1.2K grid resistors thru their respective 90K to ground.

                  Grid to ground = 0v. Grid to cathode = approx -21.5v on each tube, and still rising slightly when I pulled the plug.

                  "Most likely you'll find a bad solder joint or broken wire somewhere between those resistors and the pin of the tube socket. (or at the pin of the tube socket)"

                  Those connections all look solid. Remeber this is two pairs of 6V6s, each pair running on it's own cathode resistor, each tube running way too hot. Doubt it could be a problem at one socket.
                  It's weird, because it WAS working fine.....

                  Comment


                  • #10
                    Originally posted by Randall View Post
                    "The grid must have a voltage reference, even if it's 0V."

                    I do not understand this. In my mind no voltage = 0V, just as no apples = 0 apples.
                    What you're saying is not wrong in that sense, Randall. But, here's another way of looking at it, and what I mean by that (with an example of what I was concerned may have have happened). If you were to connect the grid of one of those 6L6s to 0V ground (the way it was designed in this case), then the circuit is works because the voltage difference across the bias resistor creates the negative grid/cathode difference needed to set the idle for the amp. Obviously, I'm not telling you something you don't already know. The grid voltage is grounded at "zero" volts. However, if you left the grid pin on the 6L6 disconnected from anything. The grid would have "no voltage" in reference to the circuit and become a floating electrode. In this scenario, the 6L6 would runaway and blow a fuse or overheat and fail. I just wanted to make sure that this was not the case

                    "Measure the grid to ground and grid to cathode voltages and be sure you are getting "no voltage"."

                    I see 91.4K ohms from each grid to ground. That confirms the 1.2K grid resistors thru their respective 90K to ground.
                    this checks out cool. Sorry if there was any confusion... carry on
                    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                    Comment


                    • #11
                      OK, I get that. I should have been more susinct in my language. We are talking 6V6s here however, just to keep things clear. Not that it would make a difference.

                      So, what in the heck is causing these tubes to draw so much current? I am seriously scratching my head over this. I put out a request to Dr Z for assistance, we shall see where that leads.
                      It's weird, because it WAS working fine.....

                      Comment


                      • #12
                        Have you actually measured grid voltage to verify? I'm thinking maybe leaking coupling caps?
                        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                        • #13
                          Originally posted by Randall View Post
                          Grid to ground = 0v. Grid to cathode = approx -21.5v on each tube
                          And the cathode resistors measure 151R

                          Do both pairs measure the same? It's unlikely both pairs would have the same fault.
                          There may not be a fault. I think a 150R cathode resistor is too low for 2 x 6V6.
                          It's biasing the tubes a lot hotter than we usually see.
                          150R is a value I'd expect to see as a common cathode resistor for 4 x 6V6 not two.
                          What's the plate voltage so we can calculate how toasty it really is.

                          Comment


                          • #14
                            Originally posted by Randall View Post
                            ...We are talking 6V6s here however, just to keep things clear. .
                            You’re right. Should have paid closer attention not to mix those up
                            If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                            • #15
                              That amp has a half power switch. I'll bet the cathode resistors aren't wired as one per side, but rather have a series connection for two tube operation. What is the power switch position when you are testing with only two tubes plugged in? And WHICH two tubes you have plugged in may also be relevant. Check how the switch is wired. Is it possible that someone else may have worked on it and kludged the wiring? And how is it supposed to be wired for that matter.?. I can't find a schematic for the circuit.

                              I don't think the good doctor would run a pair of 6v6's at any useful plate voltage with a shared 150R cathode resistor.
                              "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

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