6.3 V Heater Tap Question

Discussion in 'Shock Brother's DIY Amps' started by milocj, Jun 22, 2015.

  1. milocj

    milocj Friend of Leo's Ad Free Member

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    I figured this would be the better place to post the following question than the Technical section. First off, even though I'not a pro I've fixed a couple of amps that I've bought dead, capped/serviced all of my own and some friends' amps, and built a few amps so I know the correct answer to my question is "use one method or the other", but what happens if you use the PT's center tap and the artificial ground reference resistors at the same time when hooking up the filament heaters?

    I just picked up a 1969 Princeton non-verb. I already own one and a '67 PR plus have sold a couple in the past, and I knew this one had been worked on before. When I finally got around to open it up a few days later I discovered that both the center tap on the transformer was grounded and it had a pair of resistors soldered from the power lamp to ground. It already has a properly grounded three-prong cord added, the death cap removed, and the polarity switch disconnected and is one of the quietest idling amps that I have.

    I've usually heard that if there's a center tap that it should be used (the way method I follow) and I've also seen some recommend try both methods separately and use the quietest, but I've always read that you never use both methods together. If this amp has been used this way for over a year should I expect any issues and what are the possible ramifications of the heaters having been run this way? All the plate resistors have been changed and it appears that the artificial center tap resistors were added at the same time. The PT is a Classic Tone replacement which was already in it when the previous owner bought it. I knew what I was buying in that respect and knew that the PO had had a couple of minor issues, but planned on doing some minor mods and having to do a cap job anyway, although all electrolytics except the can have already been replaced.

    Any info on having the center taps both hooked up at the same time?

    Thanks.
     
  2. clintj

    clintj Friend of Leo's

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    I've seen that advice, too. I'm going strictly from memory here, so I'll try to get what I remember right.

    I believe it can set up a ground loop as the artificial center shunts some current to ground to balance the wires, and that will try to return to its source through the real center tap.
     
  3. milocj

    milocj Friend of Leo's Ad Free Member

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    Thanks. That's sort of what I remembered as well, but I usually just see "don't do both" and with about 35-40 years since my last electronics class I can easily talk myself into other strange scenarios. :)
     
  4. tubeswell

    tubeswell Friend of Leo's

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    You either use the heater winding centre-tap, or you use 2 x 50-100R balancing resistors, (or a humdinger).
     
    Last edited: Jun 25, 2015
  5. mRtINY

    mRtINY Tele-Afflicted

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    So, it dissipates more power if you do both.... Not that it will blow your heater taps for sure.

    The issue comes when there is an imbalance, that current creates a signal between the two ground points. The current flowing there could induce a voltage on your ground reference that is different at various places in your circuit. If it's not interfering with your preamp stages, it won't be a particular issue.

    Personally, I'd remove the extra parts....


    -tINY
     
  6. clintj

    clintj Friend of Leo's

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    Yeah, a facebook friend/fellow amp geek had a Soldano clone that had a wicked hum. Turns out both center taps were grounded and it made a ground loop. Those 100 ohm resistors should only let a few dozen mA leak to ground off each leg.
     
  7. milocj

    milocj Friend of Leo's Ad Free Member

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    Just an update.

    I contacted the previous owner who had been told by his "tech" that the PT was a replacement when he took it in to have it looked at because he had some minor problems. I asked specifically what those were and it turns out he had occasionally had noise problems and would get radio stations through the amp, so it sounds like his problems may have been from a ground loop even though it has been dead quiet since I tried it.

    I had planned on removing the resistors when I had the time as the replaced bias cap in the amp is huge and I always add an adjustment pot anyway.

    Thanks again for the info.
     
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