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Heater wiring variation

Discussion in 'Shock Brother's DIY Amps' started by Mongo Park, Oct 23, 2020.

  1. Mongo Park

    Mongo Park Tele-Holic

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    74AFA5FE-FC44-4CA4-A434-EA34BB288DD9.jpeg This is from a point to point amp that I did not build. It is very quiet, very low hum of any kind.
    The heater wires from the transformer to the first tube are not tight wound, but they are not close to any other components.
    I have included a drawing of how the preamp and power tubes are connected.
    This is very different from what I have been doing. Basically I have been following how Leo Fender and many on this forum do, two tightly twisted wires going from tube to tube in series.
    The way this is done in the drawing looks simpler, any downside of this. Why is this not common in amps I see on this forum. Does this style produces “elevated “ heater.
    I have come across this style in some older P to P amps/radios that I have bought to salvage parts from so I assume this is old school which seems to have been abandoned, implying something wrong with it. Or maybe it was just forgotten over time.
    Any opinions would be appreciated
    Cheers Ron.
     
  2. KPAE

    KPAE TDPRI Member

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    That would push filament current through the chassis between the two ground lugs, wouldn't it? I believe early Fenders did use that scheme for a while, but was overtaken by the "twisted and balanced" approach used today. I guess the current would primarily circulate in the chassis between the two tubes, but mixing 60 Hz current into the ground plane used for signal reference is not a great idea.
     
  3. Mongo Park

    Mongo Park Tele-Holic

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    Yes in my limited thinking as you said the heater circuit is use I guess the chassis for part of the connections. I don’t know how this all works out kinda like mixing AC and DC together maybe.
    Thanks for sharing your knowledge.
    Cheers Ron.
     
  4. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    So they used two-conductor heater wiring, but still grounded one side at each tube socket? Not how I'd do it, but you can get away with a lot of different approaches as long as you avoid big mistakes.
     
  5. jmp81sc

    jmp81sc Tele-Meister

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    The Hammond S6 organs have many 12A_7 tubes and the heater "ground" pin 9 is attached to a tube socket grounding pin. Single conductor wire attached to pins 4 and 5 of the tubes in series. They also ground many other resisters etc to the tube socket ground pins.

    This contrary to the way we do heaters now, and star grounding, but it seamed to work. I have no idea what the noise floor level was for these organs as the ones I got were not working, but I can't imagine that they would have had an unacceptable level of AC noise.

    I read somewhere about multiple grounding points creating ground "planes" instead of loops, but way above my knowledge level.

    Interesting topic.
    John
     
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  6. tubegeek

    tubegeek Friend of Leo's

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    Some Hammond power amps (AO-15 for sure, maybe others?) are completely balanced from the tone generator input through to the push-pull outputs, and so are pretty much immune to heater hum by design. In other words, they may not be the best examples to judge unbalanced heater wiring by, in comparison to a typical unbalanced guitar amp circuit, which is inherently more susceptible to picking up heater hum.

    Not sure about the S6 though, not familiar with that one.
     
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That is not series wiring. In the diagram above pins 7 are hot, all the others are grounded. Parallel current will flow from each tubes' pin 7 through the filament then to ground.

    Series wiring would have one wire attached to 6L6 pin 7, then one wire running from 6L6 pin 2 to the 6SL7 pin 7, then ground pin 8 and ground the second heater wire. All current flows through all the tubes and total voltage drop through the tubes needs to equal the heater supply voltage.
     
  8. Mongo Park

    Mongo Park Tele-Holic

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    Thanks Rob for your explanation. Which points out that this also creates a bunch of grounds. It has other components going to these grounds on the tube sockets.
    I put it up originally because the amp is quite so it must be working on some level. Also it is a 3 tube se alp so not real complicated.
    Cheers Ron.
     
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Many old Fender amps with half of the heater wires grounded are quiet as a mouse. Heater wiring technique is over emphasized.
     
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  10. tubegeek

    tubegeek Friend of Leo's

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    OK, so if I let that one go what do I obsess on? Huh?
     
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  11. Mongo Park

    Mongo Park Tele-Holic

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    Yes even this post is obsessing on it.
    Cheers Ron.
     
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