From the Ashes of a Great but Unloved Amp...

Discussion in 'Shock Brother's DIY Amps' started by shortcircuit, Nov 20, 2019.

  1. shortcircuit

    shortcircuit Tele-Meister

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    The GA-100 is no more, but Dr. Frankenstein has been busy...

    overview.jpg

    Just need to hook up the transformers and the NFB/presence circuit. All octal tubes...

    cf.jpg

    ...except for this little guy. There will be a voltage divider going to the grid of the second triode. I'll clip in a 1M pot and figure it out once everything's hooked up.
     

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  2. mabinogeon

    mabinogeon TDPRI Member Ad Free Member

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    I have three quick questions, if you don't mind:

    1 - Where do you get that pre-drilled turret board? None of my usual vendors have it.

    2 - Where do you get that clear lead insulating tubing? I use scraps of stripped wire, but am running low.

    3 - What are those spirally looking things I see on a couple of the resistors and caps? Solderable lead extenders? Where do you get those?

    Thanks much for your time
     
  3. shortcircuit

    shortcircuit Tele-Meister

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    amplifiedparts.com

    https://www.ebay.com/itm/5M-PTFE-F4-Teflon-Tubing-Rigid-Pipe-ID-0-3mm-1-3mm-High-Temperature/331536271458

    I got the 0.7mm, 0.9mm, and 1.1mm ID tubings rated for 600V. I have all of my bases covered with the three sizes.

    https://www.tdpri.com/threads/5f2a-ish-with-tremolo.983519/#post-9406820
     
  4. shortcircuit

    shortcircuit Tele-Meister

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    All done.

    done.jpg

    I set the voltage divider in front of the second triode at 470K/470K, boring. It doesn't seem need a bright cap across the top half. I'm trying to decide to whether to use a 6SL7 or a 6SN7 for the phase inverter. I had designed it so either tube can be used.
     
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  5. mabinogeon

    mabinogeon TDPRI Member Ad Free Member

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  6. jtcnj

    jtcnj Tele-Holic

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    Nice work; this is like I'm in Little League watching the pros play.

    On that insulated tubing, the (S) is the 600v, and the price listed is for a 5m length?

    If you were to only buy 1 size, which would you / most versatile? I am thinking the one suited for the orange caps in your pics?
     
  7. mabinogeon

    mabinogeon TDPRI Member Ad Free Member

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    I just ordered one of each size that he recommended above. Was $12 total for 5 meters of each size.
     
  8. shortcircuit

    shortcircuit Tele-Meister

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    The medium orange drops required the 0.9mm tubing, and the big ones, the 1.1mm. I also use a lot of solid copper wires for various duties. The 0.7mm fit 24ga, 0.9mm fit 20ga, and 1.1mm fit 18ga.
     
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  9. CCK1

    CCK1 Tele-Meister

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    Very Nice!
     
  10. shortcircuit

    shortcircuit Tele-Meister

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    Definitely 6SN7 for the phase inverter. It sounded much more smooth, plus, I don't need the extra gain of the 6SL7 because of the high gain preamp. The 6SN7 can handle a lot more plate voltage too.

    I redid the power supply. Initially, I followed the GA-100 layout, running the plates after the choke, but that was more of a Hi-Fi thing. The voltage coming out of the rectifier was lower than I had expected. I wanted a little more juice to the plates and screens, so I hooked the plates to the node before the choke, and the screens to the node after the choke. I got rid of the 2.2K resistor to raise the voltage to the preamp tubes to the level I had calculated.

    Octaltron HG-6L6 Guts.jpg

    Octaltron HG-6L6.jpg

    Final schematic with voltages.

    Ressurection Joe.png

    The plate dissipation of the 6L6's were 20.5W each, and the clean output power was 35W at 440Hz.

    Oh, yeah, the power transformer is a Hammond 273BX and the output transformer is a NRE Kustom 4.5K PP surplus job I picked up from Ebay a while back.
     
    Last edited: Nov 23, 2019
  11. bcorig

    bcorig Tele-Afflicted Silver Supporter

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    Dude, my brain hurts.
     
  12. David Barnett

    David Barnett Doctor of Teleocity

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    Why do you lace your ground bus like that?
     
  13. shortcircuit

    shortcircuit Tele-Meister

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    Using multi-strand ground bus is like a multi-core processor in a computer. It allows the power supply to ground faster, thereby reducing hum...

    I saw a build a while back with the ground bus laced like that, and I thought it looked cool. :oops:
     
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  14. sds1

    sds1 Tele-Afflicted

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    Hmm interesting, I hadn't heard this before. Can you elaborate, or do you have a link to a technical discussion on the matter?

    My assumption would be that stranded bus wire would perform the same as solid when the total mass of the copper is equal.
     
  15. RottenTheCat

    RottenTheCat Tele-Holic

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    I'm interested in the switching of the cathode bypass on the first preamp stage. You're choosing to ground the capacitors with the switch, leaving the other in place with the 470k resistor - which without doing any math, should effectively eliminate it. So ... since its more complex than just switching, is that for pop reduction when switching?
     
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  16. shortcircuit

    shortcircuit Tele-Meister

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    I was being silly.:D
     
    Last edited: Nov 23, 2019
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  17. shortcircuit

    shortcircuit Tele-Meister

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    Yep, the 470K resistors are there to prevent popping when flipping the switch.
     
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  18. sds1

    sds1 Tele-Afflicted

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    Dude it's all fun and games but next thing you know everyone is using stranded bus wire. Once it shows up in some build thread it's too late, you'll need to run with it! :lol:
     
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  19. King Fan

    King Fan Friend of Leo's

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    :lolol: Yeah, the current fluxes faster if the electrons have a choice of two wires and all intersections are like roundabouts instead of stop signs. :D

    Seriously, I saw this first on Doug Hoffman's builds...

    Doug.png

    It's pretty, true, but also "tidy" to build: consistent, fast, and easy. Doug demonstrates here:



    Notice you can use fine, pliable wire to build a sturdy bus (Doug uses 24ga), and you get easy but solid wraps around the turret -- these are not component leads you might need to replace. Plus they end up at the bottom of the turret, leaving room for leads above and on top.
     
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  20. sonicsmitty

    sonicsmitty Tele-Holic

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    Very nice looking work. You've got photos and schematic. You forgot the audio clips. :D
     
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