Need help understanding Silvertone 1481 schematic

Discussion in 'Shock Brother's DIY Amps' started by DavidV, Jan 24, 2020.

  1. DavidV

    DavidV TDPRI Member

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    So I've been trying to get into amp building for a while now with reading and watching as much as I can about tube circuits. I'd like make a clone of a silvertone 1481 and maybe try modding it as I go for hands on experience. I think I understand the basic 5f1 circuit from Uncle Doug's videos but want to branch out slightly.

    There is just one part of the schematic that has me scratching my head when I went to draw up a layout. That is the 6v secondary windings on the power transformer. On the Champ one of the 6v leads goes to the filaments and the other goes to ground as well as pin 9 on the 12ax7 and pin 2 on the 6v6. On the Silvertone those pins go back to one of the windings and there is a grounded 68 ohm resistor on both legs.

    What is the difference between these two circuits? I'm sure it's simple but because it doesn't make sense to me I feel like I'm missing something fundamental.

    https://el34world.com/charts/Schematics/files/Silvertone/Silvertone_1481.pdf
     
  2. King Fan

    King Fan Friend of Leo's

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    I'm impressed you're starting from the *schematic* -- and that you understand everything *but* this point. :)

    I'm thinking you're looking at the difference between golden-age single-strand heater wiring, where 6.3V went from a single heater wire to ground, and two-strand heater wiring where +3.15V goes to one pin and -3.15V goes to the other. (The double-strand is quieter.)

    Then I imagine you're looking at an artificial center tap (also quieter) with the 68R resistors to ground. Many modern amps use Fender's 100R (up to Merlin's 220R) but 68R would've worked.
     
    Last edited: Jan 24, 2020
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  3. King Fan

    King Fan Friend of Leo's

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    Compare my '67 VibroChamp with its original single strand (green) heater wiring...

    VC - 1.jpeg

    to my 5e3 build with two-strand (blue/white) heater wiring...

    VC - 1 (1).jpeg
     
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  4. DavidV

    DavidV TDPRI Member

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    Okay now it's making sense... Honestly I'm an electrician's apprentice so I shouldn't have as much trouble with this as I am, but it is a little different than wiring up a house. So the two leads coming off of a transformer are separate phases, if there is a center tap that is the neutral. So on an amp without a center tap the two hots power the filaments like on a 240v electric stove here in the states.

    So the circuit would 'work' without the resistors but they are there to reduce hum?
     
  5. King Fan

    King Fan Friend of Leo's

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    Yep, it would work fine. Did you check out the link to Merlin Blencowe above? He tells us how these help (italics mine).

    Hum in Heater Supplies
    Hum and buzz is caused by leakage current between the heater and cathode, and by the electromagnetic field around the heater and associated wiring...

    and...

    The traditional way to reduce hum is to use a transformer with a centre tap, and connect it to ground...

    and...

    If the transformer doesn't have a centre tap then you can create an artificial one using a pair of resistors. The resistors should have a fairly low resistance to help shunt leakage currents from the transformer primary.​

    There's a lot more in that short chapter that's worth reading. If I don't understand or recall how something in an amp works, like heaters or ground circuits, my first step is often to Google "Valve Wizard heaters" or "Valve Wizard grounds."
     
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  6. Wally

    Wally Telefied Ad Free Member

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  7. galaxiex

    galaxiex Tele-Afflicted

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    There is a mistake in that 1481 schematic.

    That 220K resistor on the plate of V1A to ground is in the wrong place,
    it should be on the grid of V1B to ground.

    Move it to the "other side" of the .01 cap.
     
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  8. DavidV

    DavidV TDPRI Member

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    Thanks for the help guys! Adding Valvewizard to my bookmarks


    I'll make a note of that for when I start building
     
  9. rknoedl

    rknoedl TDPRI Member

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