5e3 center tap wiring

Discussion in 'Shock Brother's DIY Amps' started by Steve77, Jan 25, 2020.

  1. Steve77

    Steve77 TDPRI Member

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    Looking for help wiring my heaters on a 5e3. It’s a Mojotone Kit I built a bunch of years ago. Was going through cleaning up wiring and trouble shooting a bit of hum, when I realized that the power transformer I added a few years back actually has a center tap.
    It’s a classic tone 40-18078. I have it wired up currently with an artificial center tap. The two green 3.15V wires to the pilot light, two resistors to ground there and then down to the tube filaments.
    Question: how do I wire up the heaters using the one center tap wire off the transformer?
    I seem to be only able to find diagrams using the two greens for the artificial CT.
    And: what do I do with the two greens if I use the actual center tap? Wire to ground? Cut and cap them off?
    I can just stick with the artificial CT, but reading robrobenette’s 5e3 mod page he mentioned using the actually CT if you’ve got it.
    Is there any benefit to using the actual CT over the artificial one?
     
  2. muchxs

    muchxs Doctor of Teleocity

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    Center tap goes to ground. Green wires go to the pilot light and then on to your filament string.

    Don't ground anything except the center tap and lose the 100 ohm resistors. You will burn the filament winding out of your transformer if either of the green leads are shorted to ground.

    It may meter as a short to ground because you're metering through your pilot light bulb. Remove the pilot light bulb, you're still metering through your transformer.
     
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  3. King Fan

    King Fan Friend of Leo's

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    What Much said, as always. :)

    What Rob said, as always. :D
     
  4. Steve77

    Steve77 TDPRI Member

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    Ah..
    center tap (green/yellow single wire) is already to ground. Two greens already to pilot light and on to the filaments. So.. essentially I’m just about there. What your saying is just remove the two 100ohm resistors and good to go? Guessing with the center tap to ground the resistors aren’t necessary?
     
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  5. clintj

    clintj Friend of Leo's

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    Center vs artificial is either/or. If you connect the transformer's center tap, remove the dual 100r resistors.

    I'm not sure whether my position is in the minority or not, but after getting in an amp with a dead power transformer last week I'm more inclined to use the dual 100 ohm method. The artificial center tap, if the resistors cook off and open, can limit the effects of a shorted power tube whereas the real center tap lets full plate voltage short to ground until it cooks off.

    Sent from my Pixel 2 using Tapatalk
     
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  6. muchxs

    muchxs Doctor of Teleocity

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    Yup. Get rid of the 100 ohm resistors.

    While I sound sensible most of the time every so often my dain bramage catches up to me. I have inadvertently used "alternative" filament wiring schemes with predictable if inconvenient results.

    Yeah, but...

    If the high voltage shorts to ground the mains fuse will blow.

    Mains fuse won't blow if the filaments are shorted to ground. Don't ask me how I know.

    :rolleyes: :rolleyes: :rolleyes:

    I think fuses should be fuses and resistors should be resistors.

    If I were going total "belt and suspenders" I'd add a fuse to the filament string. Why not?
     
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  7. clintj

    clintj Friend of Leo's

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    This poor amp had the plate voltage short to ground via the heater center tap long enough to burn down the heater winding, enabled by a larger than rated fuse on the mains supply. Heater fuse and an HT fuse would be a better option for sure, but this one has me wondering if maybe half a backup is better than no backup. I knew as soon as I got close to the amp there was a major problem, just by the smell.

    Just relating a repair story and my thoughts from it. You have the luxury of designing for ruggedness the right way and do a fine job of it from your reputation, I have to deal with mass produced amp design shortcuts.

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  8. Steve77

    Steve77 TDPRI Member

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    Awesome info I understand the concepts much better. I only hope I haven’t done damage to the PT. If I have, well it’s a lesson learned. I’ve corrected the heater wiring with the CT to ground and no resistors. Clint I understand what your saying. Maybe a fuse in the heater wiring isn’t a bad idea.
    Thanks all for helping me correct my lack of knowledge.
     
  9. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    Newer Fender 57 Deluxe has (2) 4 amp fuses on the board. One for the 6.3v and one for the 5v heaters. Imagine that!
     
  10. muchxs

    muchxs Doctor of Teleocity

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    That's the dreaded "fuse blew so we used a bigger fuse".
     
  11. clintj

    clintj Friend of Leo's

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    I really dislike that mindset, take a shortcut to avoid paying a few bucks more. Took what was probably a simple power tube swap and bias to major surgery and a several times larger repair bill. At least this isn't a collectible amp. Now where's that UPS package?

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  12. King Fan

    King Fan Friend of Leo's

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    As you two smart minds note, when smart minds don't see things the same it's likely they're standing in different places.

    A third smart mind, Merlin's, has still another approach: "The traditional way to reduce hum is to use a transformer with a centre tap, and connect it to ground. A refinement is to ground the centre tap through a small flame-proof resistor (anything up to 100 ohms). This will act as a fuse if there is a short between the anode and heater pins of the power valves, thereby reducing collateral damage."

    [​IMG]

    Anybody ever try this?
     
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