Sound City mark 3 - SLO100 conversion

Discussion in 'Other Amps' started by StingRay85, Nov 27, 2021.

  1. StingRay85

    StingRay85 Well-Known Member

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  2. neikeel

    neikeel Well-Known Member

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  3. StingRay85

    StingRay85 Well-Known Member

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    But I already have a mid-70s 4 hole Hiwatt converted L120. And its a one trick pony, loud, clean, tight and bright. But it does this trick very well! But only need one of those... Looking more towards a SLO-100 now
     
    Last edited: Nov 27, 2021
  4. South Park

    South Park Well-Known Member

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    So what mods are planing on doing . If are looking for gain push things with a pedal try that first . If you plan on doing a slo preamp you need to bleed off a lot of guitar signal
     
  5. StingRay85

    StingRay85 Well-Known Member

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    I just use the simplified schematic at Rob's site. As it is a four holer, with 1 free triode, I adapt it to a two channel split cathode, and use the low input on one of the two channels to position a preamp volume control. This way I can use the same faceplace, the original 2nd volume control become MV. The channels can be used separately, and also still jumpered. The back the reverb in and out becomes the depth control (NFB), and still undecided on the second hole. Probably a switch but not sure yet where it could be useful.

    For the 2nd channel I go for 1.5K + 250 uF, no treble peaker on the 470K attenuator or preamp volume, and a 68nF coupling cap. Lets see how the design will cope with these frequencies
     
  6. StingRay85

    StingRay85 Well-Known Member

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    20211128_205802_copy_1032x774.jpg

    Gutting the preamp completed. Caps were already done by previous owner. Screen grids and diodes renewed. Preamp sockets and tagboard cleaned up. Already made the layout of the preamp on the 34 positions of the board. So most of the thinking is already done, now its an evening or two of wiring and soldering
     
  7. Rudy v

    Rudy v Well-Known Member

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    Why not a snorkler cantrell amp
     
  8. Tatzmann

    Tatzmann Well-Known Member

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    Because its sounds like compressed horseshit,
    thats why. And that fckn Drop D Tuning wont
    help very much in the man department either:p
     
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  9. StingRay85

    StingRay85 Well-Known Member

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    Snorkler is mostly based on 2204 circuit IIRC? I already have made one of those. The Soldano just takes that design one step further with an additional gain stage, and the other triode I could use to make it a two-channel amp. I'm very careful with the leads dress, the position of the heaters, the grounds and where the B+ is coming from. On paper it looks very nice, but I need a few more evenings to solder everything together. Got all the components figured out. Will use NOS 2.2 µF 250V tropical fish caps for the cathode bypass caps, mustard caps for all the rest. And then a mix of metal film 1W Vishay, Piher and Iskra.
     
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  10. StingRay85

    StingRay85 Well-Known Member

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  11. StingRay85

    StingRay85 Well-Known Member

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    V1 and V2 wired

    20211202_232638_copy_1032x774.jpg
    Solid core, wired flying everywhere but well separated from each other. B+ and final GND connections will only be done after the pots and board are completely done.
     
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  12. StingRay85

    StingRay85 Well-Known Member

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    20211204_162137_copy_774x1032.jpg
    Grounds still need to be secured and completed. Then finally the B+ connections of the preamp need to be sorted out and the amp is ready for cleaning up and first tests on the light bulb
     
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  13. StingRay85

    StingRay85 Well-Known Member

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    All leads are colored solid core ripped out of an early 60s reel to reel recorder. Wired for performance not for looks. First priority is keep all wires as far away from the heaters as possible. Then keep the B+ wires away from other signal carrying wires. Ground stays on one side, B+ on the other side. And then maximize spacing after wire length is minimized. Since its solid core its quite easy to make some kind of 3D optimized design. Most noticable is of course the yellow NFB wire
     
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  14. neikeel

    neikeel Well-Known Member

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    It is good to see progress. Some aspects I would do differently (like the yellow wire crossing heaters obliquely and the preamp socket wires I’d have the plate wires a bit longer and push them straight down and run grids across and then down. As for the NFB wire flying in mid air…….:facepalm:
    But I guess the proof will be in the pudding!!
     
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  15. StingRay85

    StingRay85 Well-Known Member

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    20211204_211315_copy_2064x1548.jpg
    I see your point. I just went for smooth bends instead of 90 degree turns. I know on first sight it looks messy and a hack job, but you should see it in person, its not that bad. Why would I run the NFB all across the amp while I can fly it shorter 2 inches above the turret boards. It does look untidy but I'm sure its a good solution.

    Final task now, check the entire schematic once again, and measure continuity on the connections
     
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  16. StingRay85

    StingRay85 Well-Known Member

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    20211204_231349_copy_1032x774_1.jpg
    The beast is awakened. Measured behind the PI, both channels work. No power tubes installed yet, B+ voltages were considerable... when cranking the preamp, at some point the amp starts making an internal noise, this needs to be checked. It's surely a component that gets under stress, need to find out which one. When compensating output with the master volume, the sound is not there, so it must come from before the tone stack. Probably start with the preamp tubes

    Besides that, I call it a succes already
     
  17. StingRay85

    StingRay85 Well-Known Member

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    A little troubleshooting was still needed. The noise probably came from the ecc81 that was still in the cathode follower slot, previously the PI on the Mark 3. Also a bad contact at the 68 nF coupling cap. Think it also had a bad contact on the impedance selector. Now it has a quad of Xf2 double ring getters in there, but under load I again see some strange things happening on the scope. At least the preamp is sorted out already
     
  18. neikeel

    neikeel Well-Known Member

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    Is it oscillating?
     
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  19. StingRay85

    StingRay85 Well-Known Member

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    Not oscillating, it seems to trip when it reaches a certain signal level. I now measure with the oscilloscope at the dummy load. Could be the tubes, because I didn't see that when I tested it with 2 old Telefunken tubes. I could go close to blocking distortion with them, biased at their minimum (12-15 mA). Or possible traced back to the bias supply. With all volumes off (but 100mV at input), when I rotate the right bias pot above a certain point, there's some strong AC signal flowing to the output (60mA+ and visible on the scope). So probably up next is cleaning out all power tube sockets, check the grounds of the power section, and probably try a less expensive pair of tubes first.
     
  20. tschrama

    tschrama Well-Known Member

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    Cool project and well done. Will keep on following
     
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