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Thread: power transformer secondary - grounded or isolated?

  1. #1
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    power transformer secondary - grounded or isolated?

    I'm in the process of building my first tube amp, and I'm a little confused about what to do with the secondary coil of the output transformer. I've read somewhere that the secondary should *not* be grounded to the chassis, as this could result in unpredictable and dangerous ground currents; however, I've seen several published schematics - including ones for the Gibson GA-5 and the Marshall 18W - that clearly show the OT secondary grounded.

    My question is, obviously, what is the general concensus on this issue? Is it better/safer to ground or not to ground?

    Also, when I added an eight ohm jack to my modded Valve Junior, I used a Switchcraft jack, which obviously grounds the jack to the chassis. Should I replace it with an insulated Cliff jack? (I've noticed that the chassis gets rather warm, which may or may not be due to ground current...haven't noticed anything else that would indicate a "hot" chassis.)

    Thanks,
    Peter

  2. #2
    Old Timer
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    should your post be titled "Output transformer secondary"?

    It is normal to ground the common lead from the OT secondary, admittedly not ALL amps are wired like this, but more are than aren't. It is more likely that the heat you feel from the chassis is coming from the PT and/or power tube.

  3. #3
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    Oops!!! Yes, I meant to type "output", not "power". Don't know how I made that mistake. At any rate, thanks for the feedback - general practice is to ground OT secondary common lead.

    So my VJr should be OK with the Switchcraft jack?

    I had a feeling the heat was coming from the PT & power tube (Valve Junior combo doesn't have the best ventilation, especially when it's backed against a wall), but I figured it couldn't hurt to ask. I hadn't really played with it much before adding the eight ohm jack, so I didn't know if the heat would have otherwise been there.

    FWIW, here's the page where I read about *not* grounding the OT secondary:
    http://members.shaw.ca/house-of-jim/...unding_ra.html

  4. #4
    Old Timer
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    You should be OK with the switchcraft jack, in the unlikely event that you do get any new, unwanted symptoms then maybe try the Cliff jack.

    Heat is rarely something to worry about with a proven tube amp, most parts that get hot (especially PTs, power tubes, Fender reverb driver tubes, even amp chassis) are happy that way and can take way more heat then is comfortable for a human hand! Cathode biased 6L6 amps can get so hot you ,may feel the heat radiating from the chassis at a few inches away.

    Exceptions would be if transformers are dripping wax, or if you visibly have smoke and/or flames (both very, very unlikely in your case).

  5. #5
    Supporting Member loudthud's Avatar
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    The thing you should avoid is allowing speaker current to flow thru the chassis. Don't ground the secondary near the transformer and then rely on the chassis to conduct the current to the speaker jack. Run both wires from the transformer to the jack. It's fine to ground it there.

    If there is negative feedback, things can get a little tricky. O'connor recommends not grounding at the output jack but tying to ground at the phase inverter where the feedback is connected. Unless you connected grounds ideally, where there is only one connection from signal ground to the chassis, it usually doesn't become an issue to allow a chassis ground at the speaker jack.

  6. #6
    Better Tone thru Mathematics bob p's Avatar
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    early Fender amps grounded the output transformer secondary. the only ground lead was soldered directly to the transformer frame, and connected electrically to the amp chassis through the bolt. newer ones don't do that.

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