Announcement

Collapse
No announcement yet.

New 18 watt lite build, faint buzz/distortion on low notes- HELP!

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #16
    Chuck - I just changed the preamp cathode resistor to 1.2K. No improvement.

    Comment


    • #17
      Huh.?. (need head scratch emoticon)

      Any DC measured at the input grids at idle?

      Are you using a shielded cable to the input grids?

      Do you have any long grid leads in the build?

      EDIT: Under rated resistors (current OR voltage) can crackle when you put the screws to them. If you are using a 1/2W plate resistor you should change it to a 1W. If you don't have a 1W 120k resistor you could parallel two 220k resistors.

      That crackle is little voltage spikes. They may be happening on the lowest notes because that's when the current is greatest. You don't hear it until the note is decaying because it's not going up and down in intensity in the signal chain analogous to the actual signal. So it's only apparent when the root note is lower in volume. This could indicate that it's NOT at the input I suppose. At least not in the signal chain. I'm starting to suspect the shared HV node for the PI and preamp tubes. But I'm just guessing. Just because other amps of the same schematic didn't do this isn't necessarily a reason to suspect failure instead of design. Very small differences in part spec (like PS filter ESR) can do funny things. IMHE, changing parts (but not component value) in a known design sometimes can require a rework.

      Do you have parts you can use to add an HV node?
      Last edited by Chuck H; 12-24-2016, 05:16 PM.
      "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


      • #18
        The noise it makes is not a crackle. It's more of a faint distortion sound - not spiky, but soft around the edges. The noise is present even at very low volumes. If the amp were in a club or noisy room, you'd never hear the noise. I've got to walk away from this thing for today before it drives me nuts. I'll get back on after I install the 2 - 220K in place of the 120K. Yes, input to grids are shielded cables, with the shield grounded only at the jack.

        Comment


        • #19
          Do you have a way of injecting a signal right at the volume control?
          This way you can rule out either the pre or power amp for sure.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          Comment


          • #20
            Ok. I'm sorry to have put you through such paces then. I've been working from memory on what seems to be a semi regular problem with some amps. It sounds like a light, phasey crackle. Sometimes like a buzz, but cuts in and out as the low E decays. Even on clean tones. Since this is a sort of common issue I hope you can see where I might have thought that's what you were dealing with. Sooooo...

            To take your original post more literally at it's description

            When you play the low E string, as the note is dying out there is a slight buzz/distortion sound, kind of like when your speaker has a rub and is on it's way out. (but we understand that it isn't a speaker issue)

            Parasitic oscillation due to lead dress and/or component location and/or grounding scheme.

            Photo's are what we would need now. Text descriptions of lead routing, length, component location, ground schemes and such are a PITA. Because we are remote from the actual amp, solutions to these sorts of problems from the forum are usually on threads with pics.
            "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


            • #21
              When my 18W did it it turned out to be something more prosaic, dirty tube pins/sockets on the EL84s. Try wiggling the EL84s a bit in their sockets. Does that make it crackle?

              Comment


              • #22
                Click image for larger version

Name:	DSC_0532.JPG
Views:	1
Size:	252.5 KB
ID:	844368Click image for larger version

Name:	DSC_0531.JPG
Views:	1
Size:	216.4 KB
ID:	844369Click image for larger version

Name:	DSC_0530.JPG
Views:	1
Size:	247.8 KB
ID:	844370Click image for larger version

Name:	DSC_0529.JPG
Views:	1
Size:	242.0 KB
ID:	844371Click image for larger version

Name:	DSC_0528.JPG
Views:	1
Size:	256.6 KB
ID:	844372Click image for larger version

Name:	DSC_0527.JPG
Views:	1
Size:	257.7 KB
ID:	844373

                Before everyone starts screaming at me, I know the output grids are long because of my use of a PPIMV. But if that is the cause of my problem, then I've been real lucky with the 4 others I've built just like this that had no issues. OK, no more excuses. Critique away. By the way, the amp is very quite using the brass plate for the preamp grounds. Everything else grounds at the PT. (PS - ignore photo #5&6)
                Last edited by rockybottom16; 12-24-2016, 08:23 PM.

                Comment


                • #23
                  The noise is not really a crackle. And I've had the tubes in and out a couple times while trying different tubes. New Beldon sockets. Pretty snug. But thanks for your input!

                  Comment


                  • #24
                    That's better than I had hoped. The layout and lead dress seems well considered, but for one thing...

                    "I" would have laid out the controls Vol, Tone, Vol (master). Only because placing the tone control on the end places it over the OT, increasing the probability of output/input interaction, and creates more lead proximity between earlier and later signal stages. This is the only place I can see that I could suspect parasitic interaction to be coming from.

                    I can't actually SEE much of the grounding scheme, so I'll just need to say that you don't want any daisy chained grounds. All grounds for individual circuits should be run on their own lead to the ground point.

                    If you're willing... You could try bypassing the master volume and just run the shortest possible leads from the PI to the power tube grids. You would need to disconnect any leads that run to the master volume pot otherwise it's proximity can't be ruled out. This won't do anything about the pot and leads for the tone control being in closer proximity to the OT, but changing the control order is out because you already have a face plate. If doing this eliminates the problem you can try using some shielding perhaps. So we can try to locate a source for the problem and then work on a solution other than changing the layout.
                    "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


                    • #25
                      Thanks Chuck, and Happy Holidays to you. I guess we're on the same page, as today I planned on taking the MV out of the circuit to see if that was the culprit. The chassis is a standard 5e3 piece that was already screen printed. I guess it never dawned on me that putting the PPIMV between the volume and tone controls was a bad idea. Duh! Well , nobody has ever accused me of being brilliant... I'll let you know how it turns out.

                      Comment


                      • #26
                        Did you try the suggestion at post #19? Or you could take the signal off the volume wiper and run it into another amp.
                        At least then you can rule out the power amp (or preamp).
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #27
                          Well, I took the MV out of the circuit and made the shortest possible run to the output grids. The noise is still there. Then I disconnected the 500pf treble cap. No change. I do look forward to how much smarter I'll be when this finally gets sorted out... :-0

                          Comment


                          • #28
                            g1 - This sounds like a good idea (I don't have any kind of generator). Do I just hook a shielded cable to the wiper and ground the shield on the back of the pot, then plug it into another amp?

                            Comment


                            • #29
                              Since it does it noticeably with the guitar I think using the guitar as the signal is the best idea.
                              "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


                              • #30
                                I guess I'm confused as to how I'm to do this. If I come off the vol. wiper into another amp, won't the signal overwhelm the input of the other amp? I assume I need to disconnect the wire going to the PI., correct?

                                Comment

                                Working...
                                X