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Bugera T50 - Intermitent (mostly no) Sound at the Speaker Out, but signal at the FX Send

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  • #46
    I did get a chance to try and measure AC signal last night. I set the function generator to 150Hz and measured the following using the Craftsman DMM:
    Note the first reading is on the lead of the resistor closest to the EL34s and the second reading the other lead.

    "Good" amp
    R71: 29.56/9.01
    R61: 29.59/13.60
    R68: 18.30/3.10
    R67: 18.39/2.91
    PI pin 1: 13.49
    PI pin 6: 9.14

    "Bad" amp
    R71: */11.23
    R61: */12.18
    R68: 18.59/2.96
    R67: 18.47/2.81
    PI pin 1: 12.25
    PI pin 6: 11.30

    The "*" indicates I could not get a reading. However I'm not sure that means there is not a signal there. I had a tough time getting a reading on pretty much all of the measurements. The meter would jump all round including "OL" pretty much every time I took a reading. If I would lift one of the leads and try again sometime it would give a steady reading. Sometimes it took multiple tries before I would get a steady reading. It seems like the DMM was "confused" easily. I did try MANY times to get the 2 readings indicated by the "*" but eventually gave up. It doesn't make sense to me to have a reading on one side of the resistor and not something on the other side.

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    • #47
      I checked the idle current again on the bad amp, making sure I didn't anything stupid and still read 0. I measured the cathodes to ground as glebert suggested and read 5.1 ohms on the bad amp and 5.2 ohms on the good amp. I check the bad amp with and without the EL34s in place and got the same reading. I'm not sure what to make of that.

      I don't have a cap with a voltage rating above 400 so I can't use that as a DC blocker to read AC with my DMMs or to use to put a signal tracer together.
      Last edited by stoneattic; 07-14-2020, 02:16 AM.

      Comment


      • #48
        Originally posted by stoneattic View Post
        I did get a chance to try and measure AC signal last night. I set the function generator to 150Hz and measured the following using the Craftsman DMM:
        Note the first reading is on the lead of the resistor closest to the EL34s and the second reading the other lead.

        "Good" amp
        R71: 29.56/9.01
        R61: 29.59/13.60
        R68: 18.30/3.10
        R67: 18.39/2.91
        PI pin 1: 13.49
        PI pin 6: 9.14

        "Bad" amp
        R71: */11.23
        R61: */12.18
        R68: 18.59/2.96
        R67: 18.47/2.81
        PI pin 1: 12.25
        PI pin 6: 11.30

        The "*" indicates I could not get a reading. However I'm not sure that means there is not a signal there. I had a tough time getting a reading on pretty much all of the measurements. The meter would jump all round including "OL" pretty much every time I took a reading. If I would lift one of the leads and try again sometime it would give a steady reading. Sometimes it took multiple tries before I would get a steady reading. It seems like the DMM was "confused" easily. I did try MANY times to get the 2 readings indicated by the "*" but eventually gave up. It doesn't make sense to me to have a reading on one side of the resistor and not something on the other side.
        If R71/R61 are the PI plate resistors, they should have signal only at one end (plate side) as the other ends should be tied to B+ (DC).
        Please try to measure signals directly at the power tube socket pins 5. You can use a 400V series cap as long as circuit point voltages are below 400 VDC.

        Also measure power tube screens DCV (pins 4).
        Last edited by Helmholtz; 07-11-2020, 03:35 PM.
        - Own Opinions Only -

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        • #49
          Originally posted by Helmholtz View Post

          If R71/R61 are the PI plate resistors, they should have signal only at one end (plate side) as the other ends should be tied to B+ (DC).
          Please try to measure signals directly at the power tube socket pins 5. You can use a 400V series cap as long as circuit point voltages are below 400 VDC.

          Also measure power tube screens DCV (pins 4).
          I don't know what the voltage should be at pin 5 on the power tubes since I don't have schematic, but my previous measurements were below 400 VDC so I soldered a .1F/400V cap on the tip of a probe and measured the following on the bad amp:

          EL34 #1
          Pin 5
          @150Hz sine = 1.095 VAC
          Pin 4 = 374 VDC
          @300Hz sine = 1.45-1.51 VAC (bouncing around in this range)
          Pin 4 = 375 VDC

          EL34 #2
          Pin 5
          @150Hz sine = 1.203 VAC
          Pin 4 = 374 VDC
          @300Hz sine = 1.58-1.66 VAC (bouncing around in this range)
          Pin 4 = 375 VDC

          It looks like the DMM doesn't like frequencies much above 150Hz as it appears to struggle even at 300Hz. I have no idea what sort of VAC values I should be reading there.

          Comment


          • #50
            Bias is the cathode/grid voltage, current through the tube is what the bias adjusts. So current is what we measure so we can set the bias to achieve the desired flow.
            Education is what you're left with after you have forgotten what you have learned.

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            • #51
              Originally posted by stoneattic View Post
              It looks like the DMM doesn't like frequencies much above 150Hz as it appears to struggle even at 300Hz. I have no idea what sort of VAC values I should be reading there.
              Which DMM did you use? I recommended the cap probe for use with the Harbour Freight meter, because this seemed to have a useful frequency response and high enough input resistance.
              If that doesn't work either it's time to get a decent DMM.

              If everythig is ok (including meter), the AC signals at the power tube grids (pin 5) should be only slightly less than the PI plate (pins1,6) signals.

              Screen DC looks ok.

              From all we've seen I think that your problem (is it still intermittent?) is restricted to the power tube grid circuit. The symptoms being missing/low grid signal at both tubes and large negative bias voltage are unusual (the fact that you don't see idle currents may be due to your meters not being able to display low mVs).
              It seems that something is muting the grid drive and it's very likely that it's caused by a defective Infinium circuit as being said earlier.
              Last edited by Helmholtz; 07-13-2020, 03:37 PM.
              - Own Opinions Only -

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              • #52
                Originally posted by Enzo View Post
                Bias is the cathode/grid voltage, current through the tube is what the bias adjusts. So current is what we measure so we can set the bias to achieve the desired flow.
                I guess I need to remember that it is FLOW that matters. So what does it mean when I'm measuring -51VDC bias but measuring no current with the bias probe? This is really stumping me.

                Comment


                • #53
                  Originally posted by Helmholtz View Post

                  Which DMM did you use? I recommended the cap probe for use with the Harbour Freight meter, because this seemed to have a useful frequency response and high enough input resistance.

                  If everythig is ok (including meter), the AC signals at the power tube grids (pin 5) should be only slightly less than the PI plate (pins1,6) signals.

                  Screen DC looks ok.
                  I used the Craftsman DMM. It claims to be good at 400Hz. I will try with the HF tonight.

                  Comment


                  • #54
                    So what does it mean when I'm measuring -51VDC bias but measuring no current with the bias probe? This is really stumping me.
                    It means the tube has some reasonable bias voltage, but is not conducting current. Why would a power tube not conduct current? First suspect is open screen grid resistor. Is there B+ on pin 4? Next maybe the heater is not running on that tube, look and see. Also if the B+ never reaches the plate, so is there B+ on pin 3? Or the cathode circuit is open, measure resistance to ground from pin 8, it should either be zero ohms or maybe 1 ohm when using a sense resistor.

                    -51v is higher than I would expect for an EL34, but not near enough to put it in cutoff.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment


                    • #55
                      Originally posted by Enzo View Post

                      It means the tube has some reasonable bias voltage, but is not conducting current. Why would a power tube not conduct current? First suspect is open screen grid resistor. Is there B+ on pin 4? Next maybe the heater is not running on that tube, look and see. Also if the B+ never reaches the plate, so is there B+ on pin 3? Or the cathode circuit is open, measure resistance to ground from pin 8, it should either be zero ohms or maybe 1 ohm when using a sense resistor.

                      -51v is higher than I would expect for an EL34, but not near enough to put it in cutoff.
                      On pin 4 I measure: "Bad amp" 381 VDC, "Good amp" 365 VDC
                      Is ~4% difference normal/acceptable? Is the bad amp being higher an indication of something? But either way there is B+ there.

                      I can see the heaters glowing on both EL34s. Is is possible that they are not heating enough?

                      On pin 3 I measure: "Bad amp" 382 VDC, "Good" 368 VDC
                      The same relative difference between the good and bad amp. This would seem to indicate that the heaters are working, correct?

                      I measured 5.1 ohms on the bad amp and 5.2 ohms on the good amp between pin 8 and ground. It's higher than the 0 to 1 you stated. Is that high enough to be a problem? Could the Infinium circuit throw off some measurements vs a normal fixed of cathode bias setup?
                      I found a schematic (only 2 out of 3 pages though) online that appears to be someone's attempt at tracing/reverse engineering an Infinium model. I'm not sure what model, but it's definitely not the T50 because it has 4 EL34s. Maybe the 1960 or 333XL? It shows 4.7 ohm resistors between the cathodes and ground. If the T50 has the same 4.7 that could be what I'm reading.

                      In all of the above measurements I got~same between both EL34s so I only listed the measurement once.

                      Comment


                      • #56
                        Originally posted by Enzo View Post

                        It means the tube has some reasonable bias voltage, but is not conducting current. Why would a power tube not conduct current? First suspect is open screen grid resistor. Is there B+ on pin 4? Next maybe the heater is not running on that tube, look and see. Also if the B+ never reaches the plate, so is there B+ on pin 3? Or the cathode circuit is open, measure resistance to ground from pin 8, it should either be zero ohms or maybe 1 ohm when using a sense resistor.

                        -51v is higher than I would expect for an EL34, but not near enough to put it in cutoff.
                        As he has reasonable plate and screen voltages and obviously 5R cathode to ground resistance, I strongly doubt that there is zero idle current. Rather I suspect that the meter doesn't correctly display mV readings.
                        Unreliable/unsuitable meters (and bad meter batteries) can waste a lot of time.
                        Last edited by Helmholtz; 07-13-2020, 09:30 PM.
                        - Own Opinions Only -

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                        • #57
                          Originally posted by stoneattic View Post

                          On pin 4 I measure: "Bad amp" 381 VDC, "Good amp" 365 VDC
                          Is ~4% difference normal/acceptable? Is the bad amp being higher an indication of something? But either way there is B+ there.

                          I can see the heaters glowing on both EL34s. Is is possible that they are not heating enough?

                          On pin 3 I measure: "Bad amp" 382 VDC, "Good" 368 VDC
                          The same relative difference between the good and bad amp. This would seem to indicate that the heaters are working, correct?

                          I measured 5.1 ohms on the bad amp and 5.2 ohms on the good amp between pin 8 and ground. It's higher than the 0 to 1 you stated. Is that high enough to be a problem? Could the Infinium circuit throw off some measurements vs a normal fixed of cathode bias setup?
                          I found a schematic (only 2 out of 3 pages though) online that appears to be someone's attempt at tracing/reverse engineering an Infinium model. I'm not sure what model, but it's definitely not the T50 because it has 4 EL34s. Maybe the 1960 or 333XL? It shows 4.7 ohm resistors between the cathodes and ground. If the T50 has the same 4.7 that could be what I'm reading.

                          In all of the above measurements I got~same between both EL34s so I only listed the measurement once.
                          What are cathode (pin 8) DC voltages?

                          This would seem to indicate that the heaters are working, correct?
                          Measure heater voltage (AC) between pins 2 and 7.
                          - Own Opinions Only -

                          Comment


                          • #58
                            Originally posted by Helmholtz View Post

                            As he has reasonable plate and screen voltages and obviously 5R cathode to ground resistance, I strongly doubt that there is zero idle current. Rather I suspect that the meter doesn't correctly display mV readings.
                            Unreliable/unsuitable meters (and bad meter batteries) can waste a lot of time.
                            I used the same probes, meter and EL34s on both amps:

                            Good Amp
                            Ik (bias current): 33.2/33.6
                            Vp (plate voltage): 355.6/355.6

                            Bad Amp
                            Ik (bias current): 0/0
                            Vp (plate voltage): 365.4/364.0

                            Since it is not impossible that I screwed something up taking the readings, I took them again the following night with the same results. This is the probe I have:
                            https://images.reverb.com/image/uplo...qk9osetdwh.jpg

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                            • #59
                              Originally posted by stoneattic View Post

                              I used the same probes, meter and EL34s on both amps:

                              Good Amp
                              Ik (bias current): 33.2/33.6
                              Vp (plate voltage): 355.6/355.6

                              Bad Amp
                              Ik (bias current): 0/0
                              Vp (plate voltage): 365.4/364.0

                              Since it is not impossible that I screwed something up taking the readings, I took them again the following night with the same results. This is the probe I have:
                              https://images.reverb.com/image/uplo...qk9osetdwh.jpg
                              And you probably connected the probe to one of your cheap meters.
                              At a bias of -50V a typical EL34 should have a cathode current below 5mA.
                              I wonder if your meter can measure voltages below 5mV.

                              Anyway, idle currents that low don't allow proper class AB amp operation. But that's not the reason for the low grid signals.
                              Last edited by Helmholtz; 07-13-2020, 10:56 PM.
                              - Own Opinions Only -

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                              • #60
                                Is the amp still giving some sound intermittently? Wondering if you can "catch" a current reading when it is actually making sound. Not really the current value, just to see if there is measureable current during that time.

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