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First question on my build

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  • #61
    I don't expect you've done any dirt to your OT. I think something about your grounding must be hinky. There's no reason a reduction in gain via a ground connection should cause oscillation unless there is a problem with the ground. Just to be sure, this is the arrangement John intended. Please check the ground you are using with your meter to be sure it is a ground.
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    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #62
      Chuck

      That's what I did and it was a no go. As soon as it was turned on it squealed bad. Adding the 220K was all that was done and it squealed then when the 220k was removed the distortion started and continued, I had to walk away for a while. I will recheck the grounding tomorrow and maybe try it again but first I have to make sure the amp is working again that the distortion is gone. Right now there are only 2 ground connections, 1 - star ground between the filter cap board and PT and one by the low switch for the 1M resistor. On the layout image I put up, that is pretty much what it looks like when looking into the chassis. All of the grounds shown that are not connected to anything go down to the star ground but as stated with the exception of the 1M at the low switch. I know what you mean about reduction in ground with the resister should not cause an issue. There has to be something in or around the PI circuitry I am not seeing or since it's a single channel based on the AA864 maybe there needs to be something changed in the FB circuit from the way it was done. Just taking guesses.
      It's all about the bass. Lock in the groove and stay out of everyone else's way.

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      • #63
        Just got offered a free oscilloscope. Maybe that can help with this issue? Of course I still have to get it and figure out how to use it. OH boy more learning!
        It's all about the bass. Lock in the groove and stay out of everyone else's way.

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        • #64
          Originally posted by J Luth View Post
          With the 100R connected to the other side of the speaker jack which would be the tip it is not loud and does not appear to be distorted but I need to check further. I ran out of time for a bit. Also while it was distorting I change the selector between 4, 8 and 16 ohms, it only appeared to distort it at 8 and 16.
          Connected like that it's feeding 100% of the output back to the PI. The extra voltage of the 8 and 16 ohm taps could be overloading the PI.

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          • #65
            Dave H

            I'm back at it again today.

            The 100R was connected to the ground terminal of the speaker jack not the tip, the 820R was connected to the tip of the jack. I may have typed that wrong not sure I am trying so many things I may be getting confused sorry for the confusion. It's an age thing It was suggested that the lead for the 820R be connected directly to the 4 ohm tap of the OT. I tried that earlier but once I had the problem I put everything back to the way it was originally and will start again today doing one thing at a time. One question about the 820R location. Would it actually make a difference between connecting it to the 4 ohm tap or the jack? When connected to the jack tip connection it is also connected to the output side of the 3 position switch which would connect back to the OT to whatever tap was selected by the switch. Wouldn't that basically be the same as connected to the 4 ohm tap?
            It's all about the bass. Lock in the groove and stay out of everyone else's way.

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            • #66
              Chuck H, eschertron, Dave H,

              Where the amp stands today.

              100R installed, tried a 220K the gain was a bit much switched to the 27K. The 27K and 100R are tied together and connected to ground, the 820R is still connected to the speaker jack tip terminal which is connected directly to the output side of the 3 position switch. If the 820R should be moved directly to the 4 ohm tap I will try it, I'll wait to hear. Wouldn't having the 820R connected there be basically the same as the 4 ohm tap? It would still be connected to the OT through the switch but it would change which lead it is connected to when the selector switch is changed, 4, 8 16. The red/yellow wire from the PT is connected directly to the ground of the first filter cap. There is no loud hum, oscillation or distortion. The minimal hum I have is no more than what it had before anything was changed. Not sure what the final fix was, if it is fixed, maybe trying all of your suggestions inadvertently fixed something else in the process. Maybe it is finally working properly I will need to try it more and see.
              Hopefully it is. Only time will tell.
              Thank You for all of your help and putting up with a rookie trying to figure it out and learn.
              It's all about the bass. Lock in the groove and stay out of everyone else's way.

              Comment


              • #67
                The 4 ohm tap would be the stock AA864 circuit. Connecting direct to the jack means the feedback level changes with switch selection. If you happen to be using the 4 ohm tap to the jack, that would be like the stock circuit. If you happen to be using the 8 ohm tap that would be more NFB than the AA864. The 16 ohm tap would be a lot more than the stock circuit. For the amp to operate consistently with all impedance selector choices the 820R needs to be connected to one tap all the time. If you like the jack connection as it is now, and you are using the 8 ohm tap, then you can connect the 820r to the 8 ohm tap and the amp will behave as does now with all impedance selector settings. This would be more NFB than the stock AA864, but that doesn't matter as much as what you like since it's your amp.
                "Take two placebos, works twice as well." Enzo

                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                "If you're not interested in opinions and the experience of others, why even start a thread?
                You can't just expect consent." Helmholtz

                Comment


                • #68
                  OK, I mostly use the 8 ohm tap but occasionally use the 4 ohm also but never the 16 ohm I will give it a shot connected to the 4 ohm tap then the 8 ohm and see what happens and pick one.
                  From what I gathered here, it didn't have any NFB before I started this. I'll see if there is any difference between the 8 and 4 ohm taps.

                  Going out tomorrow to possibly get an oscilloscope, have to see what it is, and probably a few more things from a friend. Didn't know he had the equipment. Hey, if it's free it's for me. If it works out, let the learning begin, I haven't used one since I was a little kid with my dad.
                  It's all about the bass. Lock in the groove and stay out of everyone else's way.

                  Comment


                  • #69
                    On the 4 ohm tap sounds fine, I'll try it for a while and see how it is at the next gig.

                    Thanks again for all the help. Chuck H, eschertron and Dave H
                    It's all about the bass. Lock in the groove and stay out of everyone else's way.

                    Comment


                    • #70
                      Oscilloscopes are fun when you actually have a use for them It makes you feel like you're in on a secret no one else knows about the amp
                      "Take two placebos, works twice as well." Enzo

                      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                      "If you're not interested in opinions and the experience of others, why even start a thread?
                      You can't just expect consent." Helmholtz

                      Comment


                      • #71
                        One thing a scope is, is a volt meter for your eyes. You get a graphic image of the voltage. it can flesh out the monodimensional view of your meter. Your meter would never detect one side of a waveform clipping a couple volts, your scope shows it instantly. Lots to see and understand.

                        TO those guys who say "I don't need no scope, I get along just fine with a meter." I say, "Yep, and you don't need nothin' more than a 12-bar progression in a 1-4-5."
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment


                        • #72
                          Revisiting this one.

                          Turns out the amp still hums with the 100r installed although it is noticeably less, it's still there and loud enough to warrant fixing if for no other reason than just because it not correct without it even though the amp is dead quite without it installed.

                          What I did since the last post was redid the grounding, moved a bunch of wires around, added a ground buss to the back of the pots, rewired the filaments with different colored wires so that they could be easily connected the same on all tubes and checked the underside of the eyelet board for stray solder blobs.

                          Also tried something I read on another thread.
                          Disconnected 820r resistor from the eyelet that connects to the wire that goes to the speaker jack. Connected a signal generator set a 1k tone loud enough to hear with the amp running, pressed the 820r back down onto the eyelet and the tone got noticeable lower.

                          If anyone has any other idea's or suggestions I'd appreciate it. If a gut shot would help I can probably get one in a day or so.
                          It's all about the bass. Lock in the groove and stay out of everyone else's way.

                          Comment


                          • #73
                            Originally posted by J Luth View Post
                            If a gut shot would help I can probably get one in a day or so.
                            Please. And please keep in mind that we need to know what goes where. So if there's wires under the board or any failure to overlap shots or show front or rear chassis connections we won't be able to tell what's going on.
                            "Take two placebos, works twice as well." Enzo

                            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                            "If you're not interested in opinions and the experience of others, why even start a thread?
                            You can't just expect consent." Helmholtz

                            Comment


                            • #74
                              Maybe I am just old, but when you say the 100r is omitted, I have yet to see if you replaced it with a wire. The 100 ohm or a wire need to be there to complete the cathode circuit to ground.

                              If you are just removing the 100R and leaving a space, then your tail circuit goes to ground through the 820R and speaker winding. And that would explain some of the problems.
                              Education is what you're left with after you have forgotten what you have learned.

                              Comment


                              • #75
                                Originally posted by Enzo View Post
                                Maybe I am just old, but when you say the 100r is omitted, I have yet to see if you replaced it with a wire. The 100 ohm or a wire need to be there to complete the cathode circuit to ground.

                                If you are just removing the 100R and leaving a space, then your tail circuit goes to ground through the 820R and speaker winding. And that would explain some of the problems.
                                I think I covered that on page two It's been a trip and I'm not looking forward to attempting to get my head back into this one.
                                "Take two placebos, works twice as well." Enzo

                                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                                "If you're not interested in opinions and the experience of others, why even start a thread?
                                You can't just expect consent." Helmholtz

                                Comment

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