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K-MOD101 Kit DYI Amp -40% Voltage at V1 plate (12AX7 pin 6)

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  • #31
    Originally posted by TelRay View Post

    - completely removed the VOLUME POT from the circuit and measured 20 Ohm to 1,016 Ohm from the wiper to both HOT and COLD lugs (it is a 1M pot). So I think the range the pot is controlling is OK
    This confuses me. Is that 1,016 kohms? I would expect it to get a bit closer to zero ohms the other way as well. The fact that it doesn't respond to bass and treble does make it sound like crosstalk like Enzo said.

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    • #32
      Is the 22µ cathode cap good and well connected? If not, some signal will be coupled via the V1A and V1B cathodes bypassing tone and volume controls.
      In this case signal would spill over even if pin 7 is directly grounded.

      Another possibility would be a bad (high ESR) node E filter cap.
      Last edited by Helmholtz; 06-07-2020, 12:57 AM.
      - Own Opinions Only -

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      • #33
        Originally posted by glebert View Post
        This confuses me. Is that 1,016 kohms? I would expect it to get a bit closer to zero ohms the other way as well. The fact that it doesn't respond to bass and treble does make it sound like crosstalk like Enzo said.
        my bad, 1,016 KOhm as you pointed out
        thx

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        • #34
          Originally posted by TelRay View Post
          my bad, 1,016 KOhm as you pointed out
          thx
          Well, that's very high. If this is at the grounded end of the pot, it will let some signal pass. Around 60dB down from full volume.
          - Own Opinions Only -

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          • #35
            Originally posted by Helmholtz View Post
            Well, that's very high. If this is at the grounded end of the pot, it will let some signal pass. Around 60dB down from full volume.
            isn’t that the expected value for a 1MOhm pot?
            maybe the original typo (now corrected in the post) brought some confusion

            the pot’s range goes from 20 Ohm to 1.02 MOhm

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            • #36
              Originally posted by TelRay View Post
              isn’t that the expected value for a 1MOhm pot?
              maybe the original typo (now corrected in the post) brought some confusion

              the pot’s range goes from 20 Ohm to 1.02 MOhm
              Oh, my bad. Seems I misinterpreted. Funny "translation" error. Reason is that over here 1.000 means 1k and 1,000 means 1. We don't use a decimal point but rather a decimal comma.
              So I thought you had 20R minimum resistance at one end (between wiper and outer termination) and 1k minimum resistance at the other end.
              Last edited by Helmholtz; 06-07-2020, 04:20 PM.
              - Own Opinions Only -

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              • #37
                Originally posted by Helmholtz View Post
                Oh, my bad. Seems I misinterpreted. Funny "translation" error. Reason is that over here 1.000 means 1k and 1,000 means 1. We don't use a decimal point but rather a decimal comma.
                So I thought you had 20R minimum resistance at one end (between wiper and outer termination) and 1k minimum resistance at the other end.
                i feel you brother!
                coming from metric system world too i had a period of “americanizing” myself to the decimal point and thousand separator comma myself

                i’ll check those capacitors soon and report back to headquarters

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                • #38
                  Originally posted by Helmholtz View Post
                  Is the 22µ cathode cap good and well connected? If not, some signal will be coupled via the V1A and V1B cathodes bypassing tone and volume controls.
                  In this case signal would spill over even if pin 7 is directly grounded.

                  Another possibility would be a bad (high ESR) node E filter cap.
                  both caps measured (they were OK with the MMT, I do not have an ESR checker) and replaced.
                  The problem is still there (sound with volume at zero not affected by treble or bass)

                  You guys were mentioning "cross-talk"earlier is this what I have just tried to solve by testing those capacitors or is it something else?

                  thx!

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                  • #39
                    You guys were mentioning "cross-talk"earlier is this what I have just tried to solve by testing those capacitors or is it something else?
                    thanks in advance for any additional ideas

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                    • #40
                      Originally posted by TelRay View Post
                      You guys were mentioning "cross-talk"earlier is this what I have just tried to solve by testing those capacitors or is it something else?
                      thanks in advance for any additional ideas
                      To me crosstalk just means an unwanted signal spillover between different circuit points, often caused by unfavorable arrangement of wiring. A picture of the input wiring could help.
                      Also make sure that you have zero resistance between the neg. terminal of the 22µ cathode cap and ground (-> input jack ground to chassis contact).

                      Next step would be feeding a 100mV signal to the input and check for zero signal at V1 cathodes and point E (high voltage probe!).
                      - Own Opinions Only -

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                      • #41
                        ahhh... great suggestions (as usual)!!!
                        will check if the DIY manual suggests a specific wiring arrangement and follow the signal with the scope through those points.
                        thanks for the warning about the probe.!!! mine is a velleman 60MHz which says "Working voltage: 600V DC incl. peak AC" I think it should be OK
                        Last edited by TelRay; 07-11-2020, 09:40 PM.

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                        • #42
                          - checked the 22 µF cathode cap connection to ground and it is OK
                          - regarding the wiring, my friend did not follow any of the instructions recommendations (i've noticed before the heaters wire pairs were not twisted together). so i don't even know where to start with the wires (I think friendship has it's limits and I am not going to desolder every wire on this thing)

                          but... what's troubling me is that something weird happened when I was about to check the signal at V1s cathodes

                          now that I have my variable transformer back I noticed there is a bump in current draw when I turn the volume up (it goes back down to 0.6 A when I tirn the volume down again).

                          Normally (volume at 0) the amp is drawing 0.6 A from the mains but as soon as I move the Volume know it jumps to 1.2 A (I did not want to push it further as the FUSE is 2A)
                          Additionally, checking the components I've noticed the 360 Ohm 10W resistor is very hot (I guess it's directly related to double the current going through it)

                          I thing a more gradual jump is to be expected.

                          Please do let me know if you have any ideas

                          PS: load connected to the speaker out
                          Last edited by TelRay; 07-14-2020, 06:08 PM.

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