An Utter Novice's Simple 2A3 Build

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

  1. That's going to look real nice and tidy on the rack. Professional looking. Kudos to you.
     
  2. Paul,
    I used some milspec coaxial from rca in rear to volume pot in front...are there any disadvantages?
    Would 2 wire with shield be a better choice?
    Thanks, Robert
     
  3. paulbottlehead

    paulbottlehead Active Member

    You can always put the coax in first, then swap it out for STP later if necessary.
     
  4. jmathers

    jmathers Junior Member

    After a brief vacation, I got the components mounted this weekend. And, let me head off some criticisms from the outset lest dowto1000's head explodes.

    I'm not happy with the leftmost 2A3 location either, but at this point it is what it is. Hate to have it so close to that choke but you live and learn.

    And I will take the advice of of many here and run shielded wire from the inputs to the signal tubes. And I think I have the filament transformers in an okay position now.

    Certainly, however, if there's anything glaringly wrong here feel free to point it out. I've got plenty of learning ahead.

    topside1.jpg

    backside1.jpg

    underside1.jpg
     
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  5. Wntrmute2

    Wntrmute2 Junior Member

    Simply beautiful! BTW, dowto1000 is no longer a part of HiFi Haven.
     
  6. Prime Minister

    Prime Minister Site Owner Staff Member

    That is truly lovely.

    Nicely done.
     
    jmathers likes this.
  7. So clean - pretty. I reckon my next build will be wood frame with top plate.
     
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  8. Olson_jr

    Olson_jr Active Member

    Looks great!

    Jeff, did you ever mention who the SMAC wood guy is?
     
  9. jmathers

    jmathers Junior Member

    SMAC wood guy wants to remain anonymous. He's got plenty of personal projects and did this as a favor. I'll tell you next time we meet but he's avoiding any further commissioned projects.
     
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  10. paulbottlehead

    paulbottlehead Active Member

    Man, this looks miles better than probably the first 5-10 amps that I made! How much heat are you sending through those chassis mount resistors?
     
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  11. jmathers

    jmathers Junior Member

    Thanks!

    Can I calculate in some fashion how much heat is going through those resisters? Or are you asking their value? They are 880 ohm 25W resistors with a 100uf 100W bypass cap.
     
  12. Wntrmute2

    Wntrmute2 Junior Member

    Peggie-Back question @PBH. What can/should be done to minimize heat formation and maximize heat transfer? Would spacing them off the chassis help or hinder heat dissipation?
     
    Last edited: Apr 16, 2018 at 11:16 PM
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  13. paulbottlehead

    paulbottlehead Active Member

    They are 880R with 54V across them. This is 3W of dissipation. The 25W Ohmite resistor should be treated as a free air part unless your chassis is enormous and very thick, or if you have them mounted to a heatsink. This puts the dissipation at 12.5W @25C per the datasheet. Tube amps get hot, so I would assume that you will be running at more like 50C, which takes you down a bit more to 40% of the initial rated power. This is a 10W dissipation rating, which would mean you'd be near 150C dissipating 10W. I try to keep things much cooler, so I wouldn't run more than 3-1/3W through it. You're in the safe zone, but whenever a chassis mount/heatsink mount part is used, this kind of calculating has to be done.

    I should add that this rule of thumb is frequently not well observed. What I've noticed is that a 10W resistor running at over 5W will die within 5-10 years. A 10W resistor running at 3W will likely outlast the rest of the components around it. A 10W resistor running close to its rating will also burn the crap out of anything around it. You'll end up with a charred PC board, or in the case of a chassis mount part, you can actually cook the finish right off your panel.
     
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  14. paulbottlehead

    paulbottlehead Active Member

    If you space the resistor off the chassis, the resistor will get a little hotter. If you mount the resistor to the chassis, then it will warm up the chassis. With this kind of part, I will put little cooling slots on either side of the resistor. This will help add some airflow and I like to tell myself that it prevents the majority of the chassis plate from heating up, but I haven't actually taken any measurements.
     
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  15. Wntrmute2

    Wntrmute2 Junior Member

    Another piggie-back question? If you used a 50 watt resistor 880 ohm, would that improve the situation?
     
    Last edited: Apr 16, 2018 at 3:40 PM
  16. paulbottlehead

    paulbottlehead Active Member

    The situation isn't really dire, but a 50W chassis mount resistor will handle the heat better. The 880 Ohm value is kind of a PITA to find though.
     
  17. Wntrmute2

    Wntrmute2 Junior Member

    ^ I found that out.
     
  18. Wntrmute2

    Wntrmute2 Junior Member

    Thanks Tieung, I picked up a couple for insurance.
     
  19. jmathers

    jmathers Junior Member

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