First time builder PTX ?'s

Discussion in 'Shock Brother's DIY Amps' started by checkmate110, Jun 12, 2019.

  1. checkmate110

    checkmate110 TDPRI Member

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    If I'm understanding things correctly, I should be getting about 305 between the center tap, (the red with the yellow stripe) and either one of the red wires, what I'm actually getting is 770ish. I removed it from the circuit to make sure it didn't have anything to do with my wiring. I started investigating because I was getting over 500 volts at the first filter cap.

    This is the transformer I'm using.


    http://www.classictone.net/40-18021.html

    Thanks!
    771volts.jpg
     
  2. muswell_hillbilly

    muswell_hillbilly Tele-Meister

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    This might seem like a silly question, but have you confirmed your primary side voltage? Also measure the other secondary windings to see if they make sense...

    EDIT: And then red-to-red to see what it reads...
     
  3. checkmate110

    checkmate110 TDPRI Member

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    The primary side is at 119 and change.. the others are what I assume is within range 3.4 to CT and 6.8 across on the 6 volt windings and 5.6 on the 5 volt windings (no CT).

    Thanks!

    EDIT...I didn't measure it across because my meter is only rated to 750 volts.
    do you think it's OK to try?
     
  4. muswell_hillbilly

    muswell_hillbilly Tele-Meister

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    Agreed. Those seem correct. And this would also seem to rule out a flaky meter. Hmmm...
     
  5. mherrcat

    mherrcat Tele-Holic

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    The specs on the transformer at the link you provided shows 710v.

    Your other measurements are a small percentage above the specs for them as well. Calculate that percentage and apply it to the 710v and you will be in the area of your 771v measurement. I think your meter may be reading off a little.

    I have two inexpensive meters myself and they never agree.
     

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  6. checkmate110

    checkmate110 TDPRI Member

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    I was under the impression that the voltage between the CT and one of the red wires should be about half of the total as in 355 - 0 - 355. Is my thinking wrong? I'm a confused newb...:confused::eek::confused:
     
  7. muswell_hillbilly

    muswell_hillbilly Tele-Meister

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    You're thinking is correct. What do you measure red-to-red?
     
  8. Snfoilhat

    Snfoilhat Tele-Afflicted

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    If the worry is that the transformer is miswound and that red-red is going to read 1400VAC and maybe damage the meter, then an alternative is to measure the other red to red/yel. Then shut the whole thing down and measure resistances to confirm that the leads have the correct color codes. When I re-use old transformers from vintage gear I occasionally find non-standard color coding, so making a map of the windings has become a standard practice for me, even when a schematic is available.

    Also, it may be that many medium-to-high quality multimeters nowadays are doing RMS calculations internally, and much of the documentation given out by manufacturers is already transformed in some way to make it as readable and useful to hobbyists as possible, and so matches what they expect a customer's meter to say. Classictone and others are giving measurements of the parts operating under typical loads rather than the raw numbers. This leads to errors sometimes where the user (like us) do a conversion to a number that the manufacturer already did the conversion to, as a convenience :confused:

    Good luck!
     
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  9. checkmate110

    checkmate110 TDPRI Member

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    The meter goes from all zeroes on the right to a single 1 on the left hen I energize it.
    I assume that's because the voltage is out of range of the meter?
     
  10. muswell_hillbilly

    muswell_hillbilly Tele-Meister

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    Not sure about your meter model, but that sounds like a reasonable assumption. Was this a kit, or did you purchase the PT separately? Does its part number match the model you referenced in your original post?

    As @Snfoilhat suggests, could be a matter of taking the measurement completely unloaded, in which case a higher value might be expected. But double? That's seems like a bit more than I would have expected...
     
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  11. checkmate110

    checkmate110 TDPRI Member

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    It's a kit from Boothill, I got the transformers from ampparts direct linked from the classic tone website, the part numbers match.

    I tried to get resistance values as @Snfoilhat suggested and I got readings that were all over the place, never the same twice on, both (same cheap model) meters. I'm going to borrow my buddy's fluke this evening and see whats really going on. In the meantime I'll head over to Amazon to get a decent meter..

    Thanks much for all the help Gents, I surely do appreciate it.

    Sorry if this turns out to be a wild goose chase...

    :(
     
  12. mherrcat

    mherrcat Tele-Holic

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    Based on your observations and the specs of that transformer it seems like something isn't right. The 710v is what you should be getting when you connect to both sides of the HV winding. From red to red/yellow you should only be getting half of the total HV.

    Here is a diagram of a different transformer with an 800v HV winding:

    xfmr4.gif
     
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  13. checkmate110

    checkmate110 TDPRI Member

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    @mherrcat

    Thats exactly how I saw it, its possible I got 2 meters that are flakey in the same way so I'll withhold judgement til I get a known good meter...
     
  14. Snfoilhat

    Snfoilhat Tele-Afflicted

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    355-0-355 VAC or 710 VAC is what the transformer puts out with 120VAC across the primary and when the secondary is loaded and 100 mA current is being pulled. So you can expect your unloaded voltages to be higher, as it seems everyone agrees.

    But the 355 and 710 numbers may not be peak. My multimeter proclaims itself to be 'true RMS' and it usually agrees with Classictone specs, so it could be that Classictone is reporting AC voltages in RMS. It also could be that less expensive multimeters read peak by default, which would explain why my multimeter is branded with the true RMS graphic, to draw attention to the feature. RMS values are lower than peak values. https://en.wikipedia.org/wiki/Root_mean_square#/media/File:Sine_wave_voltages.svg

    So it may not be that the meter is flakey, just measuring differently.
     
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  15. mherrcat

    mherrcat Tele-Holic

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    Please post your findings; I'm very curious as to how this is resolved.
     
  16. checkmate110

    checkmate110 TDPRI Member

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    A little convoluted, but here goes. I built a light bulb limiter and incorporated a switch as a convenient way to cut power to the Amp. Of course I wired the switch in series with the hot (black) wire. So I switch off the limiter, drain the caps and start poking around and I get zapped!

    In checking my wiring (correct) I find out the NEUTRAL WIRE IS HOT! Turns out I was plugged into the one outlet in my kitchen that was wired wrong...the one outlet in the whole house for that matter.

    Not exactly sure what was going on with the transformer but I am now getting 370ish per side. Haven't had time to do any more because I was busy checking all my outlets.
    :mad::mad::mad::mad:

    Edit - Spelling
     
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