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

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

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

    Hello all,

    I hope everybody is doing alright

    it's been a while since I've worked on any of my amps but I thought it was about time to get more serious about that MAGNATONE M10 I bought a couple of years ago and with which you guys helped me big time (as you always do) at the time of choosing the right POWER TRANSFORMER and REVERB TANK

    i've been hearing lately some squeaks around 9-11 KHz when pushing the amp into a soft distortion (no pedals connected to the amp)

    SOUND SAMPLE attached [Magnatone M10 Squeak.mp3]: has 3 parts. Flat EQ, +6 dB at the annoying frequencies, -10 dB at those frequencies (what i feel it should sound like)

    I looked for mechanical sources to this squeak and could not find anything. Sweeped the SPKR and it seems to be fine (i might need to try taking the speaker out of the enclosure or even try a different speaker).

    Next step was to hook a scope and see what's happening.

    I fed the amp with a 100 mV RMS 400 Hz SINE
    Hooked a 16 Ohm dummy load to the speaker output
    Hooked the scope to the speaker output



    (scope settings 0.5 V/div and 0.5 ms/div)
    this is channel 1 (which includes the VIBRATO and REVERB, disengaged at this point) with the volume at 50%

    there's this vertical shifting (or oscillation) which (i tested) is not affected by the VIBRATO speed (my initial thought was that the vibrato signal might be leaking even with the vibrato volume all the way down, but then thought this should be more of a HORIZONTAL shifting as this is a true vibrato and not a TREMOLO).

    this vertical shifting occurs at whatever volume.

    in that case if i engage the vibrato the shifting does look different (there is an X shifting added on top of the Y axis one). VIDEO 2



    My understanding up to this point isthat the Y shifting is not generated by the VIBRATO circuit (i might be wrong)

    looking for something on the scope that shows what we can hear on the sample file I pushed the amp's volume up to 75% until I saw the first "shoulder", with a second one showing up near full volume

    Scope settings 1 V/div / 0.5 ms/div

    Click image for larger version  Name:	CH-1-Shoulders.jpg Views:	0 Size:	49.6 KB ID:	952976

    CHANNEL 2 (no vibrato or reverb capabilities on this channel) shows the same Y axis shifting



    My conclusions so far (apart from being clueless about what the scope is showing) are:

    - there is a vertical shifting at the output that is not an amplitude change, hence i do not hear a volume change (a tremolo effect) when i am playing the guitar. but it's deffinitely something strange.
    - channel 2 does not show the "shoulders" that channel 1 does but i can hear the same squeal coming out too.

    I'd love to hear your thoughts / advice

    thanks in advance!

    PS: added schematic
    https://www.magnatoneamps.com/schema...natone_M10.pdf
    Attached Files
    Last edited by TelRay; 02-20-2022, 04:26 PM.

  • #2
    That up and down shifting of the curve is a low frequency modulation, probably 60Hz or 120Hz.
    Squeeze the time axis by setting the time base to 2ms/div to see the envelope.

    The shoulders at large output are caused by power stage crossover distortion - quite normal.
    - Own Opinions Only -

    Comment


    • #3
      thank you for the reply!
      here a video at 2 ms/div
      is that "wavy" movement the envelope?
      (sorry about the CRT based monitor flickering)

      Comment


      • #4
        Is the amp showing nominal circuit idle voltages, and are the output tubes acceptably balanced (you would need an add-in bias tester or add in cathode sense resistors or do the DCV drop test across the OPT half-primaries and have measured the OPT half-primary resistances)?

        Can you remove some of the circuitry to try and localise where the issue could originate. Eg. just use channel 2 and remove the 7025 and 12AU7 valves from channel 1 circuitry and 12DW7 from oscillator circuitry, and 12AU7 from reverb circuitry ? If that stops any problem signal, then perhaps add valves back in - but one at a time.

        Comment


        • #5
          Originally posted by TelRay View Post
          is that "wavy" movement the envelope?
          Yes, it's an added low frequency.
          Maybe set time base to 10ms/div and try to set the trigger to get a "standing" display.

          I'm afraid, I can't identify a high pitched squeak in your sound clip, probably because of the massive reverb.
          - Own Opinions Only -

          Comment


          • #6
            This is likely one issue that could benefit from a soundcard+software bench tool to provide a cheapy but powerful spectrum analyser plot that can very easily identify unexpected or mis-heard frequencies.

            Comment


            • #7
              Originally posted by trobbins View Post
              Is the amp showing nominal circuit idle voltages, and are the output tubes acceptably balanced (you would need an add-in bias tester or add in cathode sense resistors or do the DCV drop test across the OPT half-primaries and have measured the OPT half-primary resistances)?

              Can you remove some of the circuitry to try and localise where the issue could originate. Eg. just use channel 2 and remove the 7025 and 12AU7 valves from channel 1 circuitry and 12DW7 from oscillator circuitry, and 12AU7 from reverb circuitry ? If that stops any problem signal, then perhaps add valves back in - but one at a time.
              thank you,
              actually when I am trying to check the bias by measuring the DC voltage drop from the plate (pin 7) of each one of the 7189A output tubes and point A on the schematic i get very unstable readings:

              TUBE 7189A #1: voltages range from 1.59 to 3.42 VDC
              TUBE 7189A #2: voltages range from 2.97 to 4.96 VDC

              I do not believe my bias calculations would be very accurate when those voltages are quickly going up and down by 30%. I am either doing something wrong or there's something strange here.

              Comment


              • #8
                I personally don't like the anode to B+ measurement because it is a hazardous measurement, and it requires a DC only responding meter (given that there will be hum current inducing an AC voltage as well). Not all meters adequately measure only a DC level. You also have to measure the DC resistances of each plate-to-B+ winding - voltage drops by themselves don't mean much.

                Comment


                • #9
                  yes, i measured the 2 resistances and plate voltages
                  but with those unstable voltage drops the power dissipation values could be anything

                  Comment


                  • #10
                    here is to illustrate how unstable the voltage drop is between PIN 7 of the 7189A output tube and point A



                    measuring both to ground:
                    Point A shows a steady voltage
                    PIN 7 / plate is showing a 3-4 V variation

                    Comment


                    • #11
                      Can you duplicate the 'squeak' with the reverb turned off?

                      Comment


                      • #12
                        Originally posted by TelRay View Post
                        PIN 7 / plate is showing a 3-4 V variation
                        Are pin 7 voltages stable with PI tube pulled?
                        - Own Opinions Only -

                        Comment


                        • #13
                          Originally posted by Jazz P Bass View Post
                          Can you duplicate the 'squeak' with the reverb turned off?
                          thanks for the question, I will answer in two parts as i can understand it in two different (and valid) ways:

                          1.- i can reproduce the squeak with the REVERB off and even when playing through channel 2 (which has no reverb). meaning, the squeak might not come from the reverb.
                          2.- i can re-record the squeak without REVERB (this is what Hr Helmholtz has already asked). i will reassemble the amp and make a new recording tonight.


                          Comment


                          • #14
                            Originally posted by Helmholtz View Post
                            Are pin 7 voltages stable with PI tube pulled?
                            I'll remove that 12DW7 / 7247 and report back tonight. Thank you for the suggestion.

                            Comment


                            • #15
                              Originally posted by Helmholtz View Post
                              Are pin 7 voltages stable with PI tube pulled?
                              still unstable voltages with 12DW7 PI tube out

                              Comment

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