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Thread: MM Transformer center tap warning

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    MM Transformer center tap warning

    Hi- my mercury magnetics PT for 5e3 came with a warning not to use the center tap if it was not on the schematic. They don't say which CT they are talking about- but I would ground the B+CT and use the filament CT to ground rather than floating. Anything wrong with this? Is there anything different about MM 5e3 PTs?

    I've used MM trannys before on champs and princetons but this is my first 5e3
    and the warning came with an ! so I thought I should pay attention.

    I emailed MM but I'll probably get an answer here first - anyone know what this warning means? This is a stock b+ regular 5e3 PT so I would think that schematic is pretty well known.

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    Supporting Member tubeswell's Avatar
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    Well if the CT for the HT winding is lifted, the amp won't work, so that's got to be grounded.

    Similarly the CT for the heater winding should be grounded if you've got one, otherwise won't the amp be noisier?

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    Does it have a CT for the 5V winding? If so, it will be at B+, so grounding it will short the PT. Usually, you just insulate it, and tuck it away, but supposedly, if you take your B+ from it, it should be 'cleaner' than taking it from one of the 5V taps.

    Jon

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    Yes - thats my way of thinking. I'm just a little spooked by the look of this warning and I can't figure out what they mean. Whether you choose to use the filament CT or float does not require a warning - and I don't know when you would not use the B+ CT.

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    Lifetime Member Enzo's Avatar
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    If you intend to rectify the high voltage with a full wave bridge - four diodes - then you won't want to ground the center tap. If you plan to get the full wave rectification with a pair of diodes or a tube recto, THEN you ground the CT.

    SO if you are using two diodes like most Fenders, then ground it - just like the schematic says. if you plan to wire it up like man Marshalls, then one sode of the bridge is grounded, but DON'T ground the center tap. Just like the schematic says.

    Now and then someone wires it up on a bridge, sees the unused center tap, and thinks, "Gee, center taps are always supposed to be grounded, aren't they?"

    NOOOOO!.

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    no, no CT for 5v - just b+ and heaters.

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    Sometimes on 5E3 schematics (like this one:http://www.sonicdeli.com/ThunderTweakWeb/5e3layout.htm), they show 1 leg of the 6V heater winding grounded. In that case, you wouldn't want to ground the CT either.

    But like you say, in that case, I'd ignore the schematic, and connect the CT to the cathode to elevate the heaters.

    Jon

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    That makes sense - thanks. It would seem a more appropriate warning for a generic PT than for a 5E3 tone clone. It would be more clear if they just said "warning don't do it wrong! "

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    See Enzo's reply,that is what they are talking about.

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    My power supply is a quad 1N4007 diodes rectified on a center taped transformer to gound. I have wanted to try a PS with the snubbing capacitors as depicted over at the Valve Wizard site for quite some time now. ( http://www.freewebs.com/valvewizard/fullwave.html ). On my 5E3 built the PT has worked out far better that I had expected it?s not only smooth. When pushed hard you got the sag baby, it don?t need no stinking sag resister. I used the 47nf 1Kv caps. With the four diodes and grounded the center tap for the secondaries. I really was expecting to let the smoke out but the scrubers are doing the job and behaves as if it only see's two of the 1N4007's diodes. My Power Trans is from Matt over @Musical Power Supplies rated @285-0-285, cleaned up to 370Volts. His prices are very reasonable ($32), and after two hot hours of use it was still cool to the touch.

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Borg54 View Post
    I really was expecting to let the smoke out but the scrubers are doing the job and behaves as if it only see's two of the 1N4007's diodes.
    ???
    What do you mean here.. are you referring to the caps across the diodes?
    I think that whole line of thought from valvewizard is greatly over stated and what does the "scrubbers" have to do with not letting the smoke out?

    Try doing some real Net research about bypass capacitors used like this across SS diodes and you'll run into a ton of junk written about their effect.. or non effect.
    A typical scenario is a .01uF/Kv bypass cap in parallel with a high value resistor and that RC lumped in parallel with each diode.

    The diodes legs, be it one or 100 in series, in a FW rectifier are only turned on one set at a time so it behaves that way because it has to.... as far as I know, the bypass caps do nothing with respect to that.

    $32.00 each is a stellar cost for a decent PT by the way.

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    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    No Offense taken Bruce, I was only experimenting with a center tapped transformer. As a rule the ct is not grounded when using SS rectification. I did learn this lesson the hard way and smoked a few Power transformers in the past. It's just that the thread that I responded to I will admit that I was being a SA, and showing off that yes you can ground a center taped transformer with SS rectification with out burning out the PT. I have since removed the circuit, in favor of fewer components, the less that can go wrong (KISS)

    And agreed that was a GREAT Price on a PT here is the link http://musicalpowersupplies.com/4.html
    I was planning on ordering several 325-0-325 120mADC from Matt this afternoon, but he's out of stock. He's a Great guy to deal with though.

    And while I have you, Bruce how would you setup V1&V2 with a 6SC7 in a 5E3? Fender used I believe 100v?I just ran into a boxful of them. And the truth is I just don?t trust the schematics over at Webber's site.

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    If they are NOS RCA or Tung-Sol 6SC7GTs, they sound fabulous! If they are metal envelope they sound OK but both can be microphonic.
    The normal approach is get a bunch and try them in different parts of the circuit, picking the quietest ones.
    You 'll have no trouble with them running at +160vdc or more but the preamp stages in older tweed amps did run kinda low B+... bump it up a little for more preamp headroom.

    Oh... there really is no rule of thumb with respect to using SS diodes for a full wave rectifier. Nearly 90% of all amps using SS diodes in the rectifier use a full wave with center tap... in other words, you have to use a center tap PT with a full wave rectifier to get it to work.
    Yes there are some that use a FW Bridge rectifier... such as some big Orange amps.
    These of course do not need a center tap and the ground point is the grounded anodes of the FWB rectifier.
    If you ground out a center tapped PT using a full wave bridge, yes you can do some damage...
    The main thing is to study the differences between a full wave rectifier and full wave bridge rectifier and how they work... follow the flow of the diodes and you'll see how the diodes work, as one side or the other is turned on and off (with respect to VDC potential on the cathode or anode).

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    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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