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  • Sizzling & Crackling in HR Deville Problem

    2001, model with 4-10's. Problem occurs with or without input jack. Neither volumes affect noise level. On standby, there's no noise. I replaced the tubes, inspected & cleaned the sockets, big resistors measured in spec, all solders appearing to be good around the pins. Any other specific suspects before I dig into it tomorrow?


    One other odd thing I noticed was the bias voltage, schematic calls for 60mv but it measured 119mv, twice spec. Is that because it has 4 speakers or just that the last tech didn't adjust it properly?

  • #2
    The number of speakers makes no difference whatsoever when adjusting bias, as long as overall impedance is correct.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      Even with incorrect load, that still ought not affect bias.
      Education is what you're left with after you have forgotten what you have learned.

      Comment


      • #4
        Originally posted by The Dude View Post
        The number of speakers makes no difference whatsoever when adjusting bias, as long as overall impedance is correct.
        Good to know, would double the spec'd bias mean that the 6L6's are running lower current or 'cooler'....and would that contribute to the crackle & sizzle?

        Comment


        • #5
          No. Higher current means higher current. They are running hotter. Current times voltage is power. So if you had say 450v on the plates, and 60ma, then 450 x 0.06 = 27 watts for two tubes. 120ma is twice the current, so 54 watts for two tubes. I see it says 485, so 485 x .119 = 57 watts. for two. Will it adjust to spec?

          Did the old tubes measure the same current as the new, more or less?

          Plug your guitar into the POWER AMP IN jack, turn it down. Does the noise remain or go away? Likewise, run a cord from the PREAMP OUT jack to some other amp for a listen, do you hear the noise in the other amp now? Turn up the reverb. When the noise occurs, is it reverberated? Or does it sound the same regardless of reverb setting? Those tests will narrow it all down to the preamp, the power amp, or the middle of the amp.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Originally posted by Enzo View Post
            No. Higher current means higher current. They are running hotter. Current times voltage is power. So if you had say 450v on the plates, and 60ma, then 450 x 0.06 = 27 watts for two tubes. 120ma is twice the current, so 54 watts for two tubes. I see it says 485, so 485 x .119 = 57 watts. for two. Will it adjust to spec?

            Did the old tubes measure the same current as the new, more or less?

            Plug your guitar into the POWER AMP IN jack, turn it down. Does the noise remain or go away? Likewise, run a cord from the PREAMP OUT jack to some other amp for a listen, do you hear the noise in the other amp now? Turn up the reverb. When the noise occurs, is it reverberated? Or does it sound the same regardless of reverb setting? Those tests will narrow it all down to the preamp, the power amp, or the middle of the amp.
            Ok, I'll try that first thing in the morning and report back here but I'm confused and curious. I didn't say the current msmt was 119ma, I said the bias voltage msmt was 119mv. I haven't taken a current msmt reading yet.

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            • #7
              Look at your schematic. The bias test point is a one ohm resistor from power tube cathodes to ground. The beauty of one ohm is that in Ohm's Law, it means voltage and current are the same.. In other words 119ma through one ohm will drop 119mv across the resistor. SO when you measure 119mv, it tells us that 119ma is running through the resistor. And the current through the resistor is all coming through teh tubes.


              And my internal nit picker is emerging. Really, the bias voltage is the negative volts you put on the grid of the power tube. What you are measuring then is not bias voltage, but in fact it is tube current. The bias voltage on the grid controls the tube current. It is current through the tube times voltage across the tube that yields dissipation in watts.

              I admit I am guilty of saying it wrong. I try to refer to bias voltage and tube current, but sometimes I slip and call the test point "bias."

              While I am at it, power off, measure resistance to ground from that test point in the corner. Is it one ohm? Remember with such low resistance, always touch your ohm meter probes together and note the resistance. Half an ohm more or less is fairly common. Just keep in mind that extra half an ohm will be included in your resistance reading, so if the probes show 0.5 ohms and your resistor measures 1.5 ohms, that means the resistor is right on one ohm.

              I don't recall ever having one of those resistors off value, but if a power tube failed, it may have damaged it at some point. SO a quick reading of resistance will at least tell us the test point is going to give accurate results.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Originally posted by Enzo View Post
                Look at your schematic. The bias test point is a one ohm resistor from power tube cathodes to ground. The beauty of one ohm is that in Ohm's Law, it means voltage and current are the same.. In other words 119ma through one ohm will drop 119mv across the resistor. SO when you measure 119mv, it tells us that 119ma is running through the resistor. And the current through the resistor is all coming through teh tubes.


                And my internal nit picker is emerging. Really, the bias voltage is the negative volts you put on the grid of the power tube. What you are measuring then is not bias voltage, but in fact it is tube current. The bias voltage on the grid controls the tube current. It is current through the tube times voltage across the tube that yields dissipation in watts.

                I admit I am guilty of saying it wrong. I try to refer to bias voltage and tube current, but sometimes I slip and call the test point "bias."

                While I am at it, power off, measure resistance to ground from that test point in the corner. Is it one ohm? Remember with such low resistance, always touch your ohm meter probes together and note the resistance. Half an ohm more or less is fairly common. Just keep in mind that extra half an ohm will be included in your resistance reading, so if the probes show 0.5 ohms and your resistor measures 1.5 ohms, that means the resistor is right on one ohm.

                I don't recall ever having one of those resistors off value, but if a power tube failed, it may have damaged it at some point. SO a quick reading of resistance will at least tell us the test point is going to give accurate results.
                I'm glad you nitpicked, I've never quite understood those bias TP's that well, now I do. I know the B+ is below 485 but those tubes are still running hot and pretty close to max dissapation! I'll do the tests tomorrow & make the adjustments.

                Comment


                • #9
                  Originally posted by Perkinsman View Post
                  I'm glad you nitpicked, I've never quite understood those bias TP's that well, now I do. I know the B+ is below 485 but those tubes are still running hot and pretty close to max dissapation! I'll do the tests tomorrow & make the adjustments.
                  For what it's worth (I'm a relative newbie) crackling and poping for me has been do to dirty or losely tensioned input jacks, dirty potentiometers, tube sockets (a wiggle test), bad tubes, or last but not least a bad plate resistor. If you've checked and cleaned all the jacks, solder joints, tubes, pots, and sockets and I believe you said you did, it may a plate resistor or faulty ground.

                  Chopstick everything again, if you didn't already. I would do this twice as it's easy.

                  Bad plate resistors are the hardest to diagnose IMHO, but you can start with the V1 plate resistor and poke that and see if you can get it to snap, crackle, or pop. I replaced two plate resistors on different amps over the last couple of years that looked almost perfect visually, but internally they were shot. They did however measure a bit off spec, but if I recall not too bad. Those two amps I replaced the plate resistors on are now dead quite, but before they sounded like bacon on a grill. Also, make sure your pots are clean, quick and easy if you didn't already do it.
                  " Things change, not always for the better. " - Leo_Gnardo

                  Comment


                  • #10
                    Originally posted by Enzo View Post
                    No. Higher current means higher current. They are running hotter. Current times voltage is power. So if you had say 450v on the plates, and 60ma, then 450 x 0.06 = 27 watts for two tubes. 120ma is twice the current, so 54 watts for two tubes. I see it says 485, so 485 x .119 = 57 watts. for two. Will it adjust to spec?

                    Did the old tubes measure the same current as the new, more or less?



                    Plug your guitar into the POWER AMP IN jack, turn it down. Does the noise remain or go away? Likewise, run a cord from the
                    for a listen, do you hear the noise in the other amp now? Turn up the reverb. When the noise occurs, is it reverberated? Or does it sound the same regardless of reverb setting? Those tests will narrow it all down to the preamp, the power amp, or the middle of the amp.
                    POWER AMP IN jack, turn it down-Noise still present
                    PREAMP OUT jack to some other amp-Noise still present in Deville, not other amp speaker
                    Turning up reverb-No difference in sound
                    Measured exactly 1ohm on TP30
                    All preamp tube plate resistors & the two 5W resistors measured to spec.
                    Lowered bias current from 119 to 68, couldn't get lower. Deox pot, got it down to 64 but no further.
                    Deox all pots & switches. No change.

                    This crackle is still intermittent, quiet for a few minutes & then crackle , crackle.

                    I did notice a small amount of leakage stuff on the + side of C35. It is a 2001 so the filter caps are about 19 years old but I'd hate to change them all out & find out it wasn't the caps. Should I just replace the one cap?
                    Last edited by Perkinsman; 11-16-2019, 09:06 PM.

                    Comment


                    • #11
                      When you said you replaced the tubes, did that include the 6L6 power tubes?
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


                      Comment


                      • #12
                        Originally posted by g1 View Post
                        When you said you replaced the tubes, did that include the 6L6 power tubes?
                        Yes

                        Comment


                        • #13
                          Originally posted by g1 View Post
                          When you said you replaced the tubes, did that include the 6L6 power tubes?
                          Originally posted by Perkinsman View Post
                          Yes
                          Ok, so I shut down the amp & changed out the 6L6’s again & took the bias msmt again. This time it measured 35mV?!?! It had originally been 119, but I adjusted down to 68 but for some odd reason, it lowered itself almost half of my adjustment. How can I determime whether the bias adjustment pot is bad?

                          Comment


                          • #14
                            You measure the bias voltage at the tube grid. If the pot turns that voltage up and down, the pot works.

                            Do I have this right? You put new power tubes in it, and found 119mv on the TP. Then you went back and put different tubes in it and the reading was 35mv? What changed was the tubes, I'd suggest either the first new set was extra hot - it happens - or ONE of them was faulty. Remember the reading is for both tubes together. So if both are at 30ma, then 60mv results. If one is 30ma, and the other 90ma, then 120mv results.

                            The bias control only adjusts the voltage on the grids, it is up to the tube to draw the current.

                            Test your power tubes one at a time. Put just one into a socket and measure the test point. Set it for half on the TP, so 30mv. If it won't go right to 30mv, just decide on something in the ball park. Now DO NOT adjust the control, but pull that tube and now stick the other tube in the same socket. We want to test each tube alone in the same socket with the same setting on the bias control. Now you will know if one of your tubes is odd, and also how much variation you have between tubes.

                            Do not assume some setting of the bias control is supposed to be some particular current. So one set of tubes will result in one thing and another set something different. You had some hot tubes in it and turned the bias way down to control it. You then put cooler tubes in it, so that setting ressults in a much colder current.
                            Education is what you're left with after you have forgotten what you have learned.

                            Comment


                            • #15
                              Originally posted by Enzo View Post
                              You measure the bias voltage at the tube grid. If the pot turns that voltage up and down, the pot works.

                              Do I have this right? You put new power tubes in it, and found 119mv on the TP. Then you went back and put different tubes in it and the reading was 35mv? What changed was the tubes, I'd suggest either the first new set was extra hot - it happens - or ONE of them was faulty. Remember the reading is for both tubes together. So if both are at 30ma, then 60mv results. If one is 30ma, and the other 90ma, then 120mv results.

                              The bias control only adjusts the voltage on the grids, it is up to the tube to draw the current.

                              Test your power tubes one at a time. Put just one into a socket and measure the test point. Set it for half on the TP, so 30mv. If it won't go right to 30mv, just decide on something in the ball park. Now DO NOT adjust the control, but pull that tube and now stick the other tube in the same socket. We want to test each tube alone in the same socket with the same setting on the bias control. Now you will know if one of your tubes is odd, and also how much variation you have between tubes.

                              Do not assume some setting of the bias control is supposed to be some particular current. So one set of tubes will result in one thing and another set something different. You had some hot tubes in it and turned the bias way down to control it. You then put cooler tubes in it, so that setting ressults in a much colder current.
                              So by design, putting in a 1 ohm resistor, allows TP30 to measure the plate current. Why then does the schematic state TP30 in mV? Is it just allowing for an easier msmt on a DMM? I did measure pin 5 of the 6L6's and both measure -55vdc. Is this correct?

                              There is however some other issue going on. I've just noticed an intermittent problem with this amp that is affecting the measurements to vary. It's not the tubes because the TP30 measurements I've mentioned (119v, 68v, 35v) were with the same set of 6L6's. I tried different tubes but that wasn't the problem so I put the original set back in. The problem is when I turn the amp on, it sometimes redplates the tubes and then the measurements go wacky. I turn the amp off for a few minutes and then back on and the TP measurement is fine and no redplating.

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