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

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  • #16
    See if the bias voltage at point F is stable.
    - Own Opinions Only -

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    • #17
      i will measure point F… a slightly off topic photo of that 12DW7 Mullard tube

      Click image for larger version  Name:	Created-with-RNI-Films-app-Preset-Fuji-Astia-100-F-v-2.jpg Views:	0 Size:	106.6 KB ID:	953164
      Last edited by TelRay; 02-22-2022, 01:06 AM.

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      • #18
        Originally posted by Helmholtz View Post
        See if the bias voltage at point F is stable.
        stable at -13.3 V (i’ve tried both with and without the PI tube)

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        • #19
          there was something bugging me with this low frequency oscillation so i disconnected anything that was attached to the circuit and reattached again.
          now i see the oscillation is still there but minimum (i hope you guys can still see it on the video)
          my theory is that one of the dummy load connectors was either attached to or touching one of the 6.3 VAC wires that is very close to that terminal (could that have caused such an effect? in any case there is still some shifting, but maybe this is to be tolerated).



          in any case, after this, the voltage drop is between PIN 7 of the 7189A output tube and point A is still as jumpy as before (video on post #10)

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          • #20
            Originally posted by TelRay View Post
            my theory is that one of the dummy load connectors was either attached to or touching one of the 6.3 VAC wires that is very close to that terminal (could that have caused such an effect? ).
            Yes, something like that could explain the added 60Hz in the output signal - and why you didn't hear a 60Hz hum with speakers.

            Your plate voltage instability could have a number of reasons:
            - A bad (gassy) tube
            - Leaky coupling caps
            - A bad cathode resistor
            - Bad socket contacts.

            If it's the coupling caps, lifting them will show.

            - Own Opinions Only -

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            • #21
              i'm going to start with replacing the coupling caps and the cathode resistor (it looks a bit fried but measures spot on at 100R - edited -)

              Click image for larger version  Name:	replace.jpg Views:	0 Size:	78.2 KB ID:	953266

              maybe change those 1K resistors as well (as I have already replaced the 270K ones in the past)


              Click image for larger version  Name:	IMG-7069.jpg Views:	0 Size:	506.3 KB ID:	953267
              Last edited by TelRay; 02-23-2022, 12:05 AM.

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              • #22
                Originally posted by TelRay View Post
                i'm going to start with replacing the coupling caps and the cathode resistor (it looks a bit fried but measures spot on at 100K)
                I only replace components which are causing problems.

                Cathode resistor should be 100R not 100k.
                Lifting one end of a coupling cap will stop any leakage current, so will show if it's the culprit.
                - Own Opinions Only -

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                • #23
                  yes, 100R, edited above
                  I understand what you say about changing only the trouble components (I have no sentimental attachment to carbon resistors or old film caps, unless they are the blue molded ones).
                  I've changed the 2x 0.047 uF coupling caps and the 100R cathode resistor with no luck, voltage is a jumpy as before
                  happy to keep trying things (unfortunately i do not have spare tubes to try, but i can swap the 12DW7s on the VIBRATO section and the PHASE INVERTER and see if that changes anything (but we've tried without the PI before, so i do not know if there's any value in doing that)
                  thanks for the suggestions!!!

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                  • #24
                    Post #4.

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                    • #25
                      Originally posted by trobbins View Post
                      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.
                      good call sending me back there. after some experimentation, the voltage instability at PIN 7 of the 7189A tubes stops with the Vibrato 12DW7 tube pulled out



                      Click image for larger version

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                      • #26
                        See if the instability level changes when vibrato intensity is varied. If not then the issue likely relates to 'C' supply rail. If it does vary then likely related to 'B' supply rail. Apart from the supply rails, there could be some interaction through grounds that are common, or a close trace that couples the ~5-8Hz through to the output stage (more likely related to LFO ripple on the bias rail as that is more sensitive). Note that the rectified B+ appears to take a long trip through the standby switch to its filter cap - that lead has a lot of noise on it.

                        Note that it is not uncommon for LFO (5-8Hz) to permeate throughout an amp as it normally doesn't become noticeable via the speaker - and so what you can't hear can't hurt sales

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                        • #27
                          Originally posted by TelRay View Post

                          good call sending me back there. after some experimentation, t[B]he voltage instability at PIN 7 of the 7189A tubes [U]stops with the Vibrato 12DW7 tube pulled out
                          Now that is surprizing.

                          I don't see how the LF signal could get to power tubes with the PI pulled.
                          And you reported that point A and point F voltages are stable.
                          There isn't even a screen supply.

                          - Own Opinions Only -

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                          • #28
                            Originally posted by Helmholtz View Post
                            I don't see how the LF signal could get to power tubes with the PI pulled.
                            And you reported that point A and point F voltages are stable.
                            There isn't even a screen supply.
                            that is way beyond my understanding
                            I confirm, point A and F show stable voltages. Instability comes from PIN 7 of both 7189A power tubes
                            So far the way to make those voltages stable is to remove the 12DW7 tube (leaving all other tubes in)

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                            • #29
                              The reason I suggested to pull the PI tube was to cut off the power tubes from any signal earlier in the amp.
                              As your plate voltages still were varying, I suspected the power tube supplies at points A and F.
                              If voltages at points A and F are stable, I have no idea how any LF signal could get to the power tubes and cause the plate voltages to vary with the PI out.

                              Sorry to ask, but are you sure which of the 12DW7s is the PI?
                              - Own Opinions Only -

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                              • #30
                                Originally posted by Helmholtz View Post
                                Sorry to ask, but are you sure which of the 12DW7s is the PI?
                                that is a fair question and I will double check physically later on today

                                i understand this 12DW7 is the Phase Inverter

                                Click image for larger version

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                                Originally posted by trobbins View Post
                                See if the instability level changes when vibrato intensity is varied... If it does vary then likely related to 'B' supply rail(
                                right you are, the instability speed increases when vibrato speed is increased

                                (i'm following the fluctuating bar at the bottom of the display. the vibrato speed range is not huge, a mod is pending, but i can notice the difference)



                                i guess my next assignment is to check these components?

                                Click image for larger version  Name:	B-rail.jpg Views:	0 Size:	25.9 KB ID:	953381
                                Last edited by TelRay; 02-23-2022, 04:21 PM.

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