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MAGNATONE M10 - 9-11 KHz Squeak when pushed into distortion

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  • #31
    Vibrato intensity is not vibrato speed - different pots.

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    • #32
      sorry about the confusion. yes it changes when increasing the intemsity (if i increase it goes beyond my meter's scale on shows OL)

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      • #33
        Perhaps it could be 'normal'. The thread started with high frequency noise. Do you now hear anything abnormal with the amp, or by looking at the speakers (eg. cone movement) ?

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        • #34
          My thoughts exactly.
          Went from high freq to low freq in one thread.
          What are we attempting to repair?

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          • #35
            Originally posted by TelRay View Post
            i understand this 12DW7 is the Phase Inverter
            My question did not refer to schematic but to the actual tube position in the amp.

            Precisely, I wondered if you pulled the tremolo tube instead of the PI.

            I agree that we deviated from the topic (still need to really hear a "squeak").

            Nevertheless it seems that the LF modulation at the power tube plates doesn't allow to assess idle currents.

            - Own Opinions Only -

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            • #36
              thank you all for your input, ideas and for expressing your confussion when "linearity" seems to be lost. allow me to summarize.

              - the post started with a high frequency squeak issue (could be electronic or speaker related)
              - the audio file might not have been clear enough and a new one was overdue as the amp was not operative (i attach one now to this post)
              - when analyzing the speaker output with the sope there was an oscillation
              - we discovered that oscillation might have been caused by a short with a 6.3V AC connector that is right by the speaker connector (there is still a tiny oscillation though)
              - i have been asked about the bias and when attempting to measure it i found very unstable values on PIN 7 of the 7189A output tubes
              - we have investigated the origins of that instability (by pulling the PI tube and finally found a relation to vibrato intensity. i'm thinking maybe i need to find a foot switch and see if the oscillation is still there with the vibrato killed and not just with the intensity pot turned down)

              let's go back to square one if you will and asses the attached audio file. always looking forward to your guidance... and patiance

              thanks!

              Attached Files

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              • #37
                1) the vertical wiggling that worries you isn plain old hum or ripple modulating signal amplitude.

                In a way, it is some kind of tremolo, buy too fast for us to decode it as such, notice vibrato/tremolo frequencies are always way lower, an few Hz.

                2) the sideways wiggling shown in the second picture is the actual vibrato, the signal frequency shifting type, the real thing.

                3) I hear no high frequency squealing but more important: the scope screen shows none,so .....
                Tinnitus?
                Juan Manuel Fahey

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                • #38
                  gracias Don Juan Manuel,
                  your point #1, that's understood. solved after consciously getting away from the 6.3 VAC wire close to the speaker connector (still some vertical shifting present post#19... maybe i need to be more tolerant?)
                  point #2, exactly my conclusion on post #1. thanks
                  point#3, defenitely not... the squeak / squeal, etc is on top of my tinitus... hahahaha
                  there is still zero views to the mp3 attached to the post just above yours (#37) so I gues you havent listened to it yet
                  i would say the scope doesn't show it might have different reasons:
                  - the 400 Hz sine wave injected into the amp is not producing the effect
                  - the noise is not generated by the circuit but the speaker itself
                  - etc...
                  thx again,

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                  • #39
                    If asked already, my apologies. Have you tried the amp with a known good speaker?
                    "Yeah, well, you know, that's just, like, your opinion, man."

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                    • #40
                      Originally posted by The Dude View Post
                      If asked already, my apologies. Have you tried the amp with a known good speaker?
                      a perfectly valid question, thank you. the straight answer is "no". a while ago, i have run a sweep (maybe 20 Hz to 20 KHz) leaving the speaker in the enclosure (to check for enclosure vibration too) and driving it with an external amp.
                      i might need to do a more "updated mechanical research"
                      Last edited by TelRay; 02-25-2022, 12:40 PM.

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                      • #41
                        Originally posted by TelRay View Post

                        i'm thinking maybe i need to find a foot switch and see if the oscillation is still there with the vibrato killed and not just with the intensity pot turned down)
                        All I can hear in your new soundfile is some not very pleasant distortion.
                        Distortion always adds higher frequencies.

                        You can stop the "vibrato" oscillator with a simple wire to ground instead of the footswitch.
                        Last edited by Helmholtz; 02-25-2022, 03:25 PM.
                        - Own Opinions Only -

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                        • #42
                          Originally posted by Helmholtz View Post
                          All I can hear in your new soundfile is some not very pleasant distortion.
                          Distortion always adds higher frequencies.
                          You can stop the "vibrato" oscillator with a simple wire to ground instead of the footswitch.
                          that's the guitar straight into the amp with the amp's volume at 50%

                          for me the distortion is great... minus the 9-11 KHz frequencies i want to get rid of

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                          • #43
                            Originally posted by TelRay View Post
                            there is still zero views to the mp3 attached to the post just above yours (#37) so I gues you havent listened to it yet
                            I listened yesterday already, and others have mentioned too. For some reason or other, 'views' are not being registered.

                            Same here as Helmholtz, I hear some ratty distortion, but nothing I could differentiate as 'squeak'. Is it coming in at a certain point on the volume control? Maybe you could post a 'with, & without' version of a simple guitar note or chord?
                            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                            • #44
                              maybe my bad english describing the effect as a squeak. but definitely something that should not be there in the 9-11KHz range.
                              i can try feed a pure sine tone into the amp and see at what low frequency the 9-11 Hz over tone is generated and upload that file. it might be easier than deciding what a "good" guitar distortion should sound like
                              i also have no experience with diagnosing a faulty speaker, so maybe this could also be a good exercise
                              Last edited by TelRay; 02-26-2022, 03:29 AM.

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                              • #45
                                hello all and, again, thank you for your guidance and patience
                                here attached a pure sine wave sweep with the volume at the amp quite low (25%)
                                you should be able to hear it now. the "high frequency disturbance" starts to be noticeable on top of the pure tone at 540 Hz and cannot be heard anymore beyond 730 Hz. i sweep back and forth around that range a couple of times at the end of the file.
                                Looking forward to hearing your thoughts.
                                Thank you,
                                Attached Files

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