An Utter Novice's Simple 2A3 Build

Discussion in 'Tube Gear' started by jmathers, Feb 2, 2018.

  1. Wntrmute2

    Wntrmute2 Junior Member

    Did you remember the fuse?
     
  2. jmathers

    jmathers Junior Member

    Doh! Thanks.
     
  3. paulbottlehead

    paulbottlehead Active Member

    Ground the "negative" end of each output transformer secondary. It's OK to ground these to either the chassis or the electrical/signal ground, whichever is convenient.
     
  4. jmathers

    jmathers Junior Member

    Interesting. So just run a wire from each negative speaker terminal to ground? Can I use the ground bus as the ground point?

    Why would you need to ground these? I'm not seeing it on any of the other 2A3 builds. And my speaker terminals are actually attached to a metal plate in the rear of the amp. Although this is not connected to the main chassis in any way.
     
  5. Wntrmute2

    Wntrmute2 Junior Member

    Are not some OT grounded through the mounting holes/screws? Not saying this is the case here. Just wondering as my Yashimotos mention something like that in the hard to translate instruction sheet.

    How's the test Jeff? If your lacking courage, have Ally do it. She won't care is something starts leaking smoke!
     
  6. jmathers

    jmathers Junior Member

    Well, I was about ready I thought but @paulbottlehead brings this up so I'm waiting on him to chime back in.
     
  7. paulbottlehead

    paulbottlehead Active Member

    Yes.
    Sure.
    If the OT primary shorts to the speaker winding (rare, but it happens), having the secondary grounded will blow the mains fuse. Without that there, you may end up with ~300V at your speaker terminals.

    I would probably ground that metal plate on the back too. If something comes loose and raises that plate to your B+ potential, that could hurt when you go to try new speaker cables.

    smokey.jpg
     
  8. jmathers

    jmathers Junior Member

    Fair enough. Adding those now. Thanks Paul.
     
  9. marantzfan

    marantzfan Administrator Staff Member

    I think it is time to...

     
    Wntrmute2 likes this.
  10. jmathers

    jmathers Junior Member

    Well, fired it up and nothing exploded. All the tubes glow. I checked some of the voltages and they appeared okay. However, when I hooked up my phone to play music, nothing came through the speakers. I wasn't even getting any hum or thump from the speakers when powering on. Interestingly, I did get music from what appears to come from the output transformers - soft, tinny, sound like an old transistor radio. Nothing comes through the speakers I have hooked up however. Any ideas?
     
  11. Wntrmute2

    Wntrmute2 Junior Member

    Sounds familiar. The experts will be along shortly I'm sure. Lack of smoke or sparks is a step in the right direction. Oh, you weren't feeding it NPR were you?
     
  12. jmathers

    jmathers Junior Member

    Ha. No NPR. Perhaps it doesn't like Laura Marling? Yes, no smoke. Maybe that's what I'm missing?
     
  13. paulbottlehead

    paulbottlehead Active Member

    Check your DC voltages again. Generally it's good enough to just look at cathode bias voltages.

    Since you have a phone hooked up, you can play a 60Hz tone into your amp. Measure the AC voltage at the RCA jack (ground the black probe for all of these measurements), then measure the AC voltage at the grid of the 2A3, then measure the AC voltage at the output transformer.

    The voltage at the 2A3 grid should be higher than at the RCA jack. The AC voltage at the speaker terminals should be less than the voltage at the 2A3 grid.

    You might want to pop up another photo of your build with the small tweaks.
     
  14. paulbottlehead

    paulbottlehead Active Member

    This is rare, but certainly happens. You'll usually see something like this:
    [​IMG]
     
  15. Wntrmute2

    Wntrmute2 Junior Member

    Are you saying the soft, tinny sound is not being heard through the speakers?
    I assume you grounded the RCA's to the chassis not just to the aluminum insert or grounded the insert itself
     
  16. jmathers

    jmathers Junior Member

    Yes, exactly. No speaker output.

    I did ground the aluminum insert that serves as a rear mounting panel to chassis ground as per Paul's recommendation.

    I'll take some measurements tonight and see where things stand.

    One area of concern is the OT primary connections at the last big can cap. I've run the grey leads (shown in yellow) to the final +ve on that cap and then another set of wires (shown in green) from there to the 27K resistor that goes to the plate of the first half of the 6SL7. I'm not sure this is correct. That is, I'm not sure if the OT wires should run to the can cap or just be connected to the wires leading to the 27K resistor.

    Here's a pic:

    completedzoom.jpg
     
    Last edited: May 17, 2018
  17. Wntrmute2

    Wntrmute2 Junior Member

     
  18. jmathers

    jmathers Junior Member

    Here's how @marantzfan hooked his up. The red wires are the OT B+ wires.

    marantzfan2A3OTwiring.jpg
     
  19. paulbottlehead

    paulbottlehead Active Member

    Are those speaker posts insulated? (They have to be if you are going to mount them to a piece of metal)


    This is incorrect.

    If you look at the schematic, one of the primary wires from the OT must connect to pin 2 (the plate) of the output tube. This is the only connection for this wire. The other end of the primary connects to the power supply. The 27K dropping resistor will connect to this node as well.
     
  20. jmathers

    jmathers Junior Member

    Well, this may be the issue right here. There's an insulator on the outside but I don't remember one on the inside of the mount. I'll have to check when I get home.

    Also as @Wntrmute2 alluded to, as far as I know my rca jacks may also be un-insulated at this plate.
     

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