Announcement

Collapse
No announcement yet.

Mesa Boogie High Phase Inverter Plate Voltage and Blocking Distortion

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Mesa Boogie High Phase Inverter Plate Voltage and Blocking Distortion

    I picked up a Mesa Boogie 50 Caliber Plus for cheap some months ago. The amp needed some repair and TLC. I replaced some burned resistors (Screen Resistors and Power Supply resistor to LDRs). Also replaced some of the carbon comp resistors that had drifted out too far. Also replaced the E Caps for the bias supply (one was bulged) and retubed with Mesa Power tubes and a mesa branded phase inverter. Cleaned all the pots, jacks, and tube sockets. I checked power tube bias (~-47 volt on grids) and phase inverter bias voltage(~1 volt on cathode-close enough) and everything looks good. Amp was working well, sounded really good and plenty loud. About a month later the amp is really blatty and farty (like crappy blocking distortion) when the master volume is turned up a little beyond bedroom volume level. Also the amp is not anywhere as loud as it should be especially on the clean channel. I plugged a guitar in the return jack and turn the master up and the blocking distortion is there. Plugged the guitar in the front input jack and run the mesa loop send out to the return jack of another amp and it sounds fine. So problem is in the phase inverter/power tube section. Tried various new spare tubes with no luck so far. I went back in and took a look at the voltages around the power tubes and phase inverter and the only thing that looks out of whack is the plate voltages of the phase inverter. The plates are at 417 volts and the Mesa schematics floating around list ~240-260 volts. Regardless the 417 volts seems awful high. I am only dropping a few volts across the phase inverter plate resistors. The plate resistors are the original carbon comps but have only drifted up 2-3K ohms. The bias voltages are also where they should be. Kind of stumped at the moment and was wondering if anyone could offer a suggestion as to where to go next. I would really appreciate any suggestions.
    Attached Files
    Last edited by Fred2202; 11-06-2015, 02:31 AM.

  • #2
    Are the heaters lit in the PI tube?
    How about the point F that sets the PI bias? Does it appear at pins 3 & 8 of the socket?
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    Comment


    • #3
      The phase inverter heater is lit. There is 6.6 volts AC between pins 4 and 9 and 5 and 9. The tube filament is getting power. I measure 1 volt DC on pins 3 and 8. The phase inverter cathode is tied to the bias supply. That whole Boogie bias circuit is kind of strange as the negative voltage kind of floats with no reference to ground. I think the power tube and phase inverter bias interact somewhat. I spliced a pot in the circuit in place of one of the resistors and can only move the phase inverter bias up or down a few volts. I tightened up the power tube and phase inverter socket pin holders tonight also and it didn't help. That 417 volt plate voltage on the phase inverter still has me puzzled. Seems way too high.

      Comment


      • #4
        Originally posted by Fred2202 View Post
        I measure 1 volt DC on pins 3 and 8.
        There should be nearly -2 volts on the cathodes. Are you getting a +1 volt there?

        The high plate voltage just shows that the tube is not conducting.

        Comment


        • #5
          How about at point F in the power supply (derived from bias circuit)? Do you have the -1.8V there?

          Originally posted by Fred2202 View Post
          That whole Boogie bias circuit is kind of strange as the negative voltage kind of floats with no reference to ground.
          R/Y wire of bias winding is connected to ground.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          Comment


          • #6
            I have 1 volt at the point F. The phase inverter tail seems to pull that voltage down to where it wants to be since it floats with no reference to ground. I put a pot in place of one of the bias resistors and twisted it all the way through it's range. The bias voltage only changed about +/- 1 volt. From what I can gather that -1.8 volts on the schematic is wrong and it should be a 1-2 volts positive. I found an earlier thread in this forum discussing the crazy floating bias on that amp. Enzo said that being off a few volts on the phase inverter tail will not make much difference because that triode is still swinging hundreds of volts. I am also wondering if I have a coupling cap leaking DC voltage around the phase inverter. I am going to do a little probing tonight to if I have any DC voltage on the phase inverter grids.

            Comment


            • #7
              I am pretty sure that bias is floating. The voltage is taken off the transformer winding and then goes through a backwards pointing diode that gives a negative voltage. There is no return path to ground that I can see. If I turn the amp off and check point F for resistance to ground ,I get >5 million ohms. I think the only thing pulling the bias voltage down at point F is the conduction through the tube. If I pull the phase inverter tube out and turn the amp on, point F sits at about -47 volts. Same as the grids of the power tubes. I am not 100% sure but I think that is what is going on with the bias circuit. I could rip that whole mess out and put in a Marshall style bias with a pot and ground reference but am afraid it might change the sound character of the amp.

              Comment


              • #8
                Point F comes off a 15K that comes from the -bias supply. What is the voltage at the side of the 15K that connects to the 47K/68K junction? Is the 15K and it's connections good?
                This is not a design flaw, the amp has a fault. No need to think about Marshall style bias.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                Comment


                • #9
                  Originally posted by Fred2202 View Post
                  From what I can gather that -1.8 volts on the schematic is wrong and it should be a 1-2 volts positive.
                  I think you are correct here, sorry for the confusion. Pin 7 of the PI goes to a 1500R which goes to ground. So the cathode would need to be a positive voltage. (http://music-electronics-forum.com/t26924/#post232255)
                  Check that the 1500R at pin 7 and it's connections are good.
                  Have you tried another tube in that PI spot?
                  It is not conducting which is why you are getting such high voltages on the plates.
                  Measure voltage at both PI grids with your black probe at the cathodes (instead of black probe to ground).
                  Last edited by g1; 11-07-2015, 02:08 AM.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #10
                    I am seeing 1500 ohms off of Pin 7 to ground that looks OK. At the junction of the 47K/68K I am seeing -53 volts DC and about -34 volts at pins 3 and 8 on the phase inverter. This is with the amp on and there is no phase inverter tube in the socket. As soon as I put a 12AX7 in the phase inverter socket and turn the amp on the voltage at pins 3 and 8 goes to about a volt positive. With a tube in the socket and the amp in standby it is still dropping the -34 volts down to that one volt on the cathode even without any voltage on the phase inverter plates. It shouldn't be doing that as the tube has no voltage at the plate to draw anything through the cathode. Plate voltage on the phase inverter is 417 when the standby is off and is only dropping a few volts across the plate resistors. G1 you are right that the tube shouldn't be conducting but I can still get some decent volume out of the amp like this but some really bad blocking distortion. It is not as loud as it used to be though. I tried numerous new tubes and they all do the same thing. I measured the voltage between the cathode and the grid pins (2 and 7) and it is -5.1 volts DC. I am beginning to think the tube socket is somehow bad. It looks like a job to get that board flipped around to get a new socket in there.

                    Comment


                    • #11
                      If you have -5.1V at the grid relative to the cathode, and cathode is at 1V, that means the grid is at -4.1V with respect to ground. How can that be across the 1500 ohm resistor? Maybe resolder or replace the 1500R ?
                      That seems to be why the tube is not conducting, grid should be about -1V with respect to cathode.
                      What is pin 2 with respect to cathode?
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


                      Comment


                      • #12
                        I have the amp on the bench right now and doubled checked. Both pins 2 and 7 read -5.1 volts DC relative to the cathode. Resistance from pin 7 to ground is 1468 ohms. This is with a tube in the socket. With a tube in the socket pins 4, 5, and 9 show 50 ohms to ground. Pins 1, 6, 3 and 8 are >2 meg and pin 2 is about 30 ohms with the master volume on zero and about 122K when the master volume is on 10. The resistances seem to check out. I am going to try and patch a pot in place of the 15K resistor in the bias string and see if I can adjust the bias voltage on the cathode a bit.

                        Comment


                        • #13
                          Didn't want to just leave this thread just hang there as I hate reading posts that don't come to a conclusion. Still having problems getting the bias corrected on the phase inverter tail. I really cannot adjust it by changing to lower value resistors or subbing in a pot to try and adjust. Even jumped across that 15K resistor right before point F and it still wants to sit at 4.5 volts positive now. When I do that the -46 volts on the power tube grids drops down way too low to about -29 volts. The cathode tail is now sitting at +4.5 volts and will not move no matter what I do. Swapped about 5 different tubes in there and still 4.5 volts positive on every one. There is about -147 volts right after the diode off the transformer lead so it seems like there is plenty of negative voltage there before it hits the resistors. Seems like something is draining away the bias voltage at the socket. Even when I pull the phase inverter tube the bias voltage on pins 3 and 8 of the socket will drop to between zero and 1 volt but will not go negative which doesn't make sense as there shouldn't be any load there to pull it down. Something is draining away the bias voltage there at the socket and the only thing I can come up with a bad tube socket. I ordered a few pcb mount nine pin sockets. If that doesn't work it's off to the local amp tech for a fix. Either way I will report the outcome when this mystery is solved. Thank you for the comments g1.

                          Comment


                          • #14
                            Have you tried talking to anyone at boogie service? Phone would probably be the best bet.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment

                            Working...
                            X