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  • Two Stroke Amp Build - Issues..

    Hello Everyone,
    I started building Dave Hunter's two stroke amp. The circuit appears to be a lot like a Tweed Champ with a 'boost' circuit that works by changing the bias of the pre-amp tube.

    Anyways, I am having an issue, which I think is isolated to the pre-amp / tone stack though I wanted to ask you for your opinion on what you thought.

    Issue Summary
    I do not get any sound from the input jack when I plug a guitar in. I also noticed the amp's hum / hiss does not increase as I turn the volume up. I do hear some general hum, which I believe is coming from the power transformer.

    If I insert an audio probe on the 68k resistor on the input jack (with my iphones headphone jack as the source turned the hole way up) I can hear a faint sound. If I connect the audio probe to the pre-amp tube at pin 2 bybassing the 68k resistor I get a slightly louder sound. If i connect the audio probe to pin 7, which is connected to Lug 2 of the volume pot, I also get a sound.

    I also noticed if I connect the audio prob to lug 3 or 2 of the volume pot, I get sound. Lug 1 (furthermost left, looking down with the lugs facing towards the open back of the chassis) is ground. The sound I do hear, is not loud, nor does it change when I turn the volume knob.
    It also appears that the tone knobs does not work when inserting signal with the test prob.
    The filter cap voltages seem good. The rectifier voltages seem good.

    I was wondering if you anyone has any thoughts on what might be the cause of this issue.

    Layout
    http://www.tubeampnetwork.com/wp-con...isedLayout.jpg

    Schematic
    http://www.tubeampnetwork.com/wp-con..._Schematic.jpg

    --

  • #2
    First, double and triple check your work. Also, make sure the input jack is not bent or otherwise deformed. Have you tried a new preamp tube? Different cable? Different guitar, and all possible pickup combinations? Cover the basics first...

    Can you put up a pic of the input jack wiring? Ans the rest of the, too... why not? Could be as simple as a mislabeled resistor, or wrong value. Can you inject the guitar (or CD player, music, whatever) . directly into the grid of the preamp, using jumpers? It may be a bit noisy - static, radio, etc, but it should allow you to pump out volume if that's the issue. BE SAFE!!! That 68k is for suppressing radio interference, so shouldn't affect much volume-wise if bypassed.

    And BE SAFE! Not sure of how experienced you are, but we could all still use the reminder. But I'm not saying you don't know what you're doing here...

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

    Comment


    • #3
      Hi! Welcome to the place. You'll find plenty of helpful, insightful advice from experienced hobbyists, techs, and designers here. I wish you a safe and enjoyable ride through your amp-building journey.

      I'm going to ask you to post the voltages from each PSU point (A, B, C) and each tube pin. Use chassis ground (or ground bus, etc.) as a reference. Include the heater pins just for completeness' sake.
      As a troubleshooter, I'd start with the power tube: testing voltages and injecting signal right at the grid (indicated as pin 5) of the 6V6. You should get a clear (not necessarily loud, but clear) signal out of the amp with the power tube driven from your ipod.

      edit: simulpost. What Justin says - be safe!! And any method you use to troubleshoot is valid as long as you understand what you are trying to do. My routine is simply starting you at the "other" end of the signal flow.
      If it still won't get loud enough, it's probably broken. - Steve Conner
      If the thing works, stop fixing it. - Enzo
      We need more chaos in music, in art... I'm here to make it. - Justin Thomas
      MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

      Comment


      • #4
        Originally posted by Justin Thomas View Post
        First, double and triple check your work. Also, make sure the input jack is not bent or otherwise deformed. Have you tried a new preamp tube? Different cable? Different guitar, and all possible pickup combinations? Cover the basics first...

        Can you put up a pic of the input jack wiring? Ans the rest of the, too... why not? Could be as simple as a mislabeled resistor, or wrong value. Can you inject the guitar (or CD player, music, whatever) . directly into the grid of the preamp, using jumpers? It may be a bit noisy - static, radio, etc, but it should allow you to pump out volume if that's the issue. BE SAFE!!! That 68k is for suppressing radio interference, so shouldn't affect much volume-wise if bypassed.

        And BE SAFE! Not sure of how experienced you are, but we could all still use the reminder. But I'm not saying you don't know what you're doing here...

        Justin
        I'll check my work again...throughout troubleshooting, I feel like I am making a sloppy mess of the amp, removing components disconnecting items as suggested by others.

        I am able to inject audio at Pin 2 and Pin 7 (the grid) of the 12AX7 and I get audio. I get volume, however it is not loud. Perhaps equivalent to a television on 1 or 2.

        I'll take some pictures soon.

        Comment


        • #5
          Originally posted by eschertron View Post
          Hi! Welcome to the place. You'll find plenty of helpful, insightful advice from experienced hobbyists, techs, and designers here. I wish you a safe and enjoyable ride through your amp-building journey.

          I'm going to ask you to post the voltages from each PSU point (A, B, C) and each tube pin. Use chassis ground (or ground bus, etc.) as a reference. Include the heater pins just for completeness' sake.
          As a troubleshooter, I'd start with the power tube: testing voltages and injecting signal right at the grid (indicated as pin 5) of the 6V6. You should get a clear (not necessarily loud, but clear) signal out of the amp with the power tube driven from your ipod.

          edit: simulpost. What Justin says - be safe!! And any method you use to troubleshoot is valid as long as you understand what you are trying to do. My routine is simply starting you at the "other" end of the signal flow.
          Here are the voltages.

          Valve 3 - Rectifier

          Pin 1 - 0/1 vdc it would move back and forth a bit.
          Pin 2 - 383
          Pin 3 - 0/1 vdc it would move back and forth a bit.
          Pin 4 - 5 vdc
          Pin 5 - 0/1 vdc it would move back and forth a bit.
          Pin 6 - 5 vdc
          Pin 7 - 0/1 vdc it would move back and forth a bit.
          pin 8 - 384 vdc

          Valve 2 - 6l6GC

          Pin 1 - 0 vdc
          Pin 2 - 23 vdc
          Pin 3 - 378 vdc
          Pin 4 - 340 vdc
          Pin 5 - 1 vdc
          Pin 6 - 1 vdc
          Pin 7 - 23 vdc
          Pin 8 - 26 vdc

          Valve 1 - 12AX7

          Pin 1 - 200 vdc
          Pin 2 - 0 vdc
          Pin 3 - 1 vdc
          Pin 4 - 23 vdc
          Pin 5 - 23 vdc
          Pin 6 - 197 vdc
          Pin 7 - 0 vdc
          Pin 8 - 1vdc
          Pin 9 - 23 vdc

          Filter Caps
          From power transformer to pre-amp
          Cap 1 - 382 vdc
          Cap 2 - 240 vdc
          Cap 3 - 198 vdc

          Observation

          On Valve 1 Pins 4,5, 9 and Valve 2 Pins 2 and 7 I should be seing 6.3 volts DC and not 23 volts DC.

          Comment


          • #6
            Originally posted by JonMiller View Post
            Observation
            On Valve 1 Pins 4,5, 9 and Valve 2 Pins 2 and 7 I should be seing 6.3 volts DC and not 23 volts DC.
            did you modify the amp to take the heater CT to pin 8 of the 6V6 instead of ground? that might explain the 23vdc. what do have for AC on heater pins?
            If it still won't get loud enough, it's probably broken. - Steve Conner
            If the thing works, stop fixing it. - Enzo
            We need more chaos in music, in art... I'm here to make it. - Justin Thomas
            MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

            Comment


            • #7
              Originally posted by Justin Thomas View Post
              First, double and triple check your work. Also, make sure the input jack is not bent or otherwise deformed. Have you tried a new preamp tube? Different cable? Different guitar, and all possible pickup combinations? Cover the basics first...

              Can you put up a pic of the input jack wiring? Ans the rest of the, too... why not? Could be as simple as a mislabeled resistor, or wrong value. Can you inject the guitar (or CD player, music, whatever) . directly into the grid of the preamp, using jumpers? It may be a bit noisy - static, radio, etc, but it should allow you to pump out volume if that's the issue. BE SAFE!!! That 68k is for suppressing radio interference, so shouldn't affect much volume-wise if bypassed.

              And BE SAFE! Not sure of how experienced you are, but we could all still use the reminder. But I'm not saying you don't know what you're doing here...

              Justin
              Here is a link to the pictures. https://drive.google.com/folderview?...EU&usp=sharing

              Comment


              • #8
                pin 4 of the 6L6 can't have 340vdc on it if the screen node is 240vdc. I'm assuming point B on the PSU is actually 340vdc?

                edit: OTOH, if the screen node resistor is 10k as per the schematic, and it's dropping only ~40vdc, then after subtracting 2ma for the preamp tube the screen is only taking about 2ma current. I'd expect more, even at idle.

                Was this designed for a 6L6 or a 6V6?
                Last edited by eschertron; 02-15-2016, 06:00 PM.
                If it still won't get loud enough, it's probably broken. - Steve Conner
                If the thing works, stop fixing it. - Enzo
                We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                Comment


                • #9
                  Originally posted by eschertron View Post
                  pin 4 of the 6L6 can't have 340vdc on it if the screen node is 240vdc. I'm assuming point B on the PSU is actually 340vdc?
                  This is where my lack of education comes into play. I'm not sure what the B is.

                  The red wires should be around 275 VCA

                  http://www.tubeampnetwork.com/wp-con..._Schematic.jpg

                  Comment


                  • #10
                    Originally posted by eschertron View Post
                    did you modify the amp to take the heater CT to pin 8 of the 6V6 instead of ground? that might explain the 23vdc. what do have for AC on heater pins?
                    I see 6.4 VAC on the Power Tube / Preamp tube heaters 5VAC on the rectifier

                    Comment


                    • #11
                      Was this designed for a 6L6 or a 6V6?[/QUOTE]

                      Is says the circuit can take 6v6, 6L6, El34, KT66 depending on what tap from the Output transformer. I am using the 6l6 / EL 34 taps

                      Comment


                      • #12
                        Originally posted by JonMiller View Post
                        This is where my lack of education comes into play. I'm not sure what the B is.

                        The red wires should be around 275 VCA

                        http://www.tubeampnetwork.com/wp-con..._Schematic.jpg
                        On the schematic, there's a B labeling the connection between the power supply and the screen of the power tube. The voltage on that PS cap C2 and the voltage on pin 4 of the 6L6 are the same. 340vdc?

                        Originally posted by JonMiller View Post
                        Is says the circuit can take 6v6, 6L6, El34, KT66 depending on what tap from the Output transformer. I am using the 6l6 / EL 34 taps
                        I understand. There may be room to optimize the power tube's cathode resistor .. after the amp is running properly.

                        [summary] the DC voltages look OK except I'm confused by the 240/340 numbers. the AC voltages look OK too. Good so far. Supplying the grid of the 6L6 tube with a line-level audio signal should produce audible results. Maybe I'll do that to my Champ when I get home to see just how loud. Let me know what you find. [/summary]
                        Last edited by eschertron; 02-16-2016, 12:35 AM.
                        If it still won't get loud enough, it's probably broken. - Steve Conner
                        If the thing works, stop fixing it. - Enzo
                        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                        Comment


                        • #13
                          Originally posted by eschertron View Post
                          On the schematic, there's a B labeling the connection between the power supply and the screen of the power tube. The voltage on that PS cap C2 and the voltage on pin 4 of the 6L6 are the same. 340vdc?



                          I understand. There may be room to optimize the power tube's cathode resistor .. after the amp is running properly.

                          [summary] the DC voltages look OK except I'm confused by the 240/340 numbers. the AC voltages look OK too. Good so far. Supplying the grid of the 6L6 tube with a line-level audio signal should produce audible results. Maybe I'll do that to my Champ when I get home to see just how loud. Let me know what you find. [/summary]
                          I noticed there wasn't a center tap on the heater voltage. So I created an artificial center tap.
                          The artificial center tap took care of the DC in heaters.

                          I checked the resistance on the input and it appears good. Audio prob on the input jack is barely audible, it was louder before though still very quiet. Audio probe on the volume pot is not audible.

                          Resistance on Pin 7 of the preamp tube to audio probe measures correctly.

                          If I put the audio probe on pin 6 of the preamp and inject audio at that point I gets sparks inside the output tube.

                          Any thoughts?
                          Thanks

                          Comment


                          • #14
                            Originally posted by JonMiller View Post
                            If I put the audio probe on pin 6 of the preamp and inject audio at that point I gets sparks inside the output tube.
                            pin 6 of the preamp tube has high voltage to ground, I'd advise against testing the isolation rating of your ipod that way. Downstream of the coupling capacitor C6 would be a safer choice. if C6, R9, and R11 are soldered in correctly, placing the audio probe at the C6/R11 junction should have the same results as placing the audio probe directly on pin 5 of the power tube; hopefully some sound.

                            Just a thought... are you testing with P1 and P2 fully CW? be sure to do that.

                            Originally posted by JonMiller View Post
                            Resistance on Pin 7 of the preamp tube to audio probe measures correctly.
                            What are you testing there? Did you try to inject audio at pin 7? should be LOUD at that point.
                            If it still won't get loud enough, it's probably broken. - Steve Conner
                            If the thing works, stop fixing it. - Enzo
                            We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                            MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                            Comment


                            • #15
                              Originally posted by eschertron View Post
                              pin 6 of the preamp tube has high voltage to ground, I'd advise against testing the isolation rating of your ipod that way. Downstream of the coupling capacitor C6 would be a safer choice. if C6, R9, and R11 are soldered in correctly, placing the audio probe at the C6/R11 junction should have the same results as placing the audio probe directly on pin 5 of the power tube; hopefully some sound.

                              Just a thought... are you testing with P1 and P2 fully CW? be sure to do that.



                              What are you testing there? Did you try to inject audio at pin 7? should be LOUD at that point.
                              In the updated schematic / diagram it is now a single pre-amp and power tube. so 3 total, valve 1, preamp, valve 2, power, valve 3 rectifier.

                              I'm packing to hit the road for a week, I'll get back to this project sometime next week. But when I injected audio at pin 7, i got nothing, which is strange that I can measure the resistance of the volume pot from that location.

                              Comment

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