Announcement

Collapse
No announcement yet.

Need Help: Gibson GA-5T Skylark Crestline: low volume, modulated distortion & more

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

  • Need Help: Gibson GA-5T Skylark Crestline: low volume, modulated distortion & more

    Hi, everyone. I’m looking for some help debugging this amp and you guys were so helpful last time, I thought I'd bug you again. I’m using this project to learn more about tube amps. FWIW, I’m not a total newbie; I promise I can safely take readings and have a basic understanding. I just really want to learn everything about these things.

    When we first got this amp, it had a hum, extremely low volume, bad decay and a nasty modulated distortion. The tremolo was also non-functioning. After changing to a three-prong grounded plug, removing the death cap and replacing all of the electrolytic capacitors, as well as reflowing a few old solder joints that looked iffy, the hum was gone and the tremolo started working again. Yay! I also replaced a couple of the old resistors that were failing/failed and, because I didn’t have a 100k on hand for one of those, I changed the input resistors to 68k so I could use one of the 100k's there that tested good, figuring it was more important to have a working 100k in the circuit than at the input jack.

    At any rate, the volume is still low. At 3, the strings are louder than the amp and 10 is maybe half what this amp should put out. Also, the higher notes cut out at first, then come in and lower notes start strong, but then decay. The distortion starts between 5 and 6 and hangs throughout all the way to 10. (Now, I just turned the amp on to get those exact numbers and the blasted tremolo isn't working again. Darn it.)

    So, I've taken voltage readings off all the tube pins (installed), in the hope that one (or more!) of you all will be able to help me figure this out.

    I have a couple of questions right off the bat, too. First, should the entire 6v on the filaments be on one side like that? Second, I wasn't completely sure whether I should be testing for VAC or VDC on some of these and in figuring it out, I realized that I seem to have DC voltage on some of the grids, which I’m pretty sure is a bad thing, since my understanding is that the grid should be the signal coming in from the plate of the previous stage, where a capacitor should block DC. Am I understanding/seeing that right? I’m thinking that might be causing the modulated distortion, maybe?

    Thanks so much for any help you can give me. I’m really excited to be learning all about tube amps! Also, please also let me know if you’d like me to take any other readings. I did take readings at all the schematic test points I could figure out, but I didn’t think to write them all down at that point and since I’m learning to “see” the layout from the schematic (because correct wiring diagrams for these are so hard to find anyway). What I can tell you is that I remember seeming to have too high of voltages at the test points along the power tubes and too low along the preamps, but the test point right after the 6X4 was almost dead on. If anyone knows a good resource for learning this, I’d love to go reading up on that. (btw, I’m too old for video learning lol)

    Thanks again!
    Gina

    Tube complement: 2x 6EU7, 2x 6AQ5, 1x 6X4

    V1: 6EU7
    Pin #1 Htr 0vac
    Pin #2 Htr 6.9vac
    Pin #3 Not Used X
    Pin #4 Cathode 2 .559vdc
    Pin #5 Grid .092vac (0vdc)
    Pin #6 Plate 2 46.1vdc
    Pin #7 Plate 1 45.5vdc
    Pin #8 Grid 1 0.0
    Pin #9 Cathode 1 0.0

    V2: 6EU7
    Pin #1 Htr .01v
    Pin #2 Htr 6.9vdc
    Pin #3 X 99.1vdc **Note, Tube datasheet says not used, but this one is
    Pin #4 Cathode 2 16.91vdc
    Pin #5 Grid 2 .092 AC (10.57vdc)
    Pin #6 Plate 2 62.7vdc
    Pin #7 Plate 1 53.5vdc
    Pin #8 Grid 1 0.097vac (0vdc)
    Pin #9 Cathode 1 0.0

    V3: 6AQ5
    Pin #1 Grid 1 .085vac (.041vdc)
    Pin #2 Cathode 5.92vdc
    Pin #3 Heater .004vac
    Pin #4 Heater 6.9vac
    Pin #5 Plate 301.8vdc (1.9vac)
    Pin #6 Screen Grid 99.2vdc
    Pin #7 Grid 1 X

    V4: 6AQ5
    Pin #1 Grid 1 .004vac (.044vdc)
    Pin #2 Cathode 5.93vdc
    Pin #3 Heater 6.9vac
    Pin #4 Heater .10vac
    Pin #5 Plate 301vdc (1.9vac)
    Pin #6 Screen Grid 99.3vdc
    Pin #7 Grid 1 X

    V5: 6X4
    Pin #1 Plate 252.4vac
    Pin #2 Not Used X
    Pin #3 Heater 6.94vac
    Pin #4 Heater 0.17vac
    Pin #5 Not Used X
    Pin #6 Plate 252.3vdc
    Pin #7 Cathode 304vdc

  • #2
    For starters, Gibson amps are known to have many variations. So are you confident that the schematic matches your actual amp?
    And can you please post the schematic you are using.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    Comment


    • #3
      Is this the correct schematic? Many Gibson models do not match exactly...
      http://images.gibson.com/Lifestyle/S...tics/GA-5T.jpg
      When the going gets weird... The weird turn pro!

      Comment


      • #4
        Yes, that's the one. Sorry I forgot to attach it!
        Gina Veesaert

        "Opportunity is missed by most people because it is dressed in overalls and looks like work." ~Thomas Edison

        Comment


        • #5
          Originally posted by LastLS1 View Post
          I have a couple of questions right off the bat, too. First, should the entire 6v on the filaments be on one side like that? Second, I wasn't completely sure whether I should be testing for VAC or VDC on some of these and in figuring it out, I realized that I seem to have DC voltage on some of the grids, which I’m pretty sure is a bad thing, since my understanding is that the grid should be the signal coming in from the plate of the previous stage, where a capacitor should block DC. Am I understanding/seeing that right? I’m thinking that might be causing the modulated distortion, maybe?

          Schemo shows one side of the filament to chassis ground, a common move in old amps to save some construction time and cost, no worries then having 6V "all on one side".

          The little bits of DC on 2 of the preamp tube grids and vibrato triode grid, no worry.

          V2 one triode is used as phase splitter, its cathode calls for 45V on the schematic, you only have about 17V, BUT at least the grid is negative with respect to cathode at 10.57 VDC. Plate voltage very deficient compared to the schematic, makes me wonder if the 100K resistor supplying it has drifted way up in value, as they do in old amps. Heck this amp is simple enough, I'd check ALL the resistors & replace the widely drifted ones. Once you get the plate voltage on this one up to snuff, the cathode & grid should follow & then let's see if that "modulated distortion" doesn't clear up.

          Vibrato usually goes bad due to the trio of caps in the oscillator circuit getting funky with old age. That's the .01 and two .02's, put in some nice new film caps and I bet you'll have all the wobble you could want. A

          I take it LastLS1 and AmpGal are one and the same? (In Mel Brooks 2000 year old man voice): HEY There's ladies heah! - - - - - About time ! ! ! ! Pleased to make your acquaintance Gina, yes indeed.

          Looks like you've done well so far on this Gibson, all the right stuff.
          This isn't the future I signed up for.

          Comment


          • #6
            As well as the low voltage Leo mentioned, it seems all the others are low. Except the power tube plates. So check that first 1K resistor in the supply, right after the rectifier.
            And the higher voltage at the rectifier & power tube plates may be due to the power tubes not drawing proper current, there should be 16V across their cathode resistor (270 ohm), so check it too.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #7
              Originally posted by Leo_Gnardo View Post
              ...I take it LastLS1 and AmpGal are one and the same?...
              I would hope not. It's extremely bad form to use two identities on the same forum.

              Comment


              • #8
                No, I'm me and Last is my husband. I was on his computer for the first post and forgot to log in as me before posting.

                I'd heard somewhere that those disc caps rarely go bad. As a result, I only checked their solder joints and not them. I see my folly, of course, as they do hit each of my problem areas.

                I'll let you know it goes! Thanks!
                Gina Veesaert

                "Opportunity is missed by most people because it is dressed in overalls and looks like work." ~Thomas Edison

                Comment


                • #9
                  Pics, in case anyone wants. (It's Robert's Imgur account, before I get jumped on. Haha)

                  Imgur
                  Imgur
                  Imgur
                  Imgur
                  Imgur
                  Imgur
                  Imgur
                  Gina Veesaert

                  "Opportunity is missed by most people because it is dressed in overalls and looks like work." ~Thomas Edison

                  Comment


                  • #10
                    Originally posted by Tom Phillips View Post
                    I would hope not. It's extremely bad form to use two identities on the same forum.
                    Well, it's lookin' like, but no bad intent, no sock puppet follies. Left me a bit cornfused for a moment. Pick one & stick with it. I vote for AmpGal, like the avatar too.
                    This isn't the future I signed up for.

                    Comment


                    • #11
                      Originally posted by AmpGal View Post
                      No, I'm me and Last is my husband. I was on his computer for the first post and forgot to log in as me before posting.
                      Just to reiterate. Sorry for the confusion there.
                      Gina Veesaert

                      "Opportunity is missed by most people because it is dressed in overalls and looks like work." ~Thomas Edison

                      Comment


                      • #12
                        Looks to me like a 100K coming off the rectifier where there should be the 1K ?
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #13
                          Oh, my Lord. I can't believe I didn't see that. It should be brown black red there. I really need to stop working on amps late at night. The original was 1k testing at 1.3m and I only brought it down to 100k instead of all the way to 1k. Makes sense.

                          I'll fix that today and see where we're at. Don't I feel silly. Thanks!
                          Gina Veesaert

                          "Opportunity is missed by most people because it is dressed in overalls and looks like work." ~Thomas Edison

                          Comment


                          • #14
                            You guys are awesome! Thanks so much. I'm now getting much better readings after fixing my "dumb broad" resistor mistake.

                            Changes across the board:

                            -V5 rectifier's plate pins are now at 226.5v (was 252.3v) and its cathode is now 240v instead of 304v.
                            -V1 preamp's plates went from 46.1v and 45.5v to 119v and 133v.
                            -V2 preamp's plates went from 62.7v and 53.5v to 141.1v and 78v
                            -V3 power tube plate is at 237v, down from 301v (The screen grid went from 99.2v to 230v - don't know if that's good or bad: I need to learn more about this piece.)
                            -V4 power's plate also decreased from 301v to 233v. It's screen grid also increased to 230v.

                            Tremolo is back and beautiful and the modulated decay is outta here! It's still pretty quiet, but I still need to lift and check those disc caps. We're certainly on our way! It's amazing to me that a single power of ten increase in resistance at just one spot can change so many things in the circuit. I can't wait to understand this all enough to design my own stuff.
                            Gina Veesaert

                            "Opportunity is missed by most people because it is dressed in overalls and looks like work." ~Thomas Edison

                            Comment


                            • #15
                              Originally posted by AmpGal View Post
                              It's amazing to me that a single power of ten increase in resistance at just one spot can change so many things in the circuit.
                              A competitor, couple towns away, makes this kind of mistake all the time. Often he's put 4K7 screen grid resistors where there should be 470R. Does he work in the dark or color blind, can't tell red from brown. You would think he could hear the amp sounds naff, heck he plays guitar in a professional band.

                              Good on ya for finding & fixing your error. Screen grid voltages look fine, they're typically just a couple volts off the plate. And you don't have to change that trio of caps in the vibrato circuit, what a relief.
                              This isn't the future I signed up for.

                              Comment

                              Working...
                              X