Announcement

Collapse
No announcement yet.

OT Ratio ..funny numbers

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • OT Ratio ..funny numbers

    Has anyone ever seen different ratios on an OT when testing for turns ratio. I initially applied enough on the primary to get a.5 v on sec and calculated the ratio..it wasnt what I was expecting so I went to 1 v on the secondary and got closer..as I apply more ac voltage I get closer to what the specs should be on the OT..does this mean something is wrong. ITs an old Magnatone mp3 OT which ran 2xel34 pp which should be about 3400 primary impedence ..I was getting like 6200 with calculations applying low voltage..as Ive applied more now its closer to 3800. Anybody familiar with what this could mean?

  • #2
    How did you apply an AC voltage, and what frequency?
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

    Comment


    • #3
      I think you may be testing the OT incorrectly. You should be putting a voltage into the secondary and reading results at the primary. That way you get a multiple in volts instead of millivolts and the overall test is more accurate. It's also important to check the actual voltage from your signal generator (or other AC source) while it's hooked up to the OT secondary in test because the OT's inductance load will drop some voltage. Then you'll have an accurate figure to base your calculations on.
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

      Comment


      • #4
        I applied with a variac and 2 dmms reading at the primary taps and secondary on the 8 ohm tap..I thought it was ok to test from the sec side..at least was told it is safer with lower voltages etc...makes sense it may be inaccurate. I believe im using 60z frequency as im in California?Thanks for any suggestions

        Comment


        • #5
          How high can I go with voltage applied to secondary?

          Comment


          • #6
            I use a signal generator at 1kHz and adjust it when it's hooked up to the secondary to some easy to work with number, like .5 or 1 volt. If your using a wall wart or some other 60Hz ac device then your voltage is predetermined and must be measured as it is in test to know the actual input voltage to the secondary. (I know I'm being redundant. Please excuse.) The OT is safe anywhere under twenty volts for the eight ohm tap. This would give dangerously high voltage at the primary though, so not so safe for the tester. But it's safe for the OT
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "If you're not interested in opinions and the experience of others, why even start a thread?
            You can't just expect consent." Helmholtz

            Comment


            • #7
              Im a little confused ..but that is pretty normal for me..ive been using a variac to apply a voltage to the ot(Ive now tried it on the secondary and tested the primary side ..with pretty much the same results) with 1volt on the sec 8 ohm tap I get 24 v on primary 24:1, with 3 volts I get 68v or 22.6:1, with 10 volts i get 210 or
              21:1 which is nearly spot on the 3500 spec on the schematic. Can I trust these numbers?
              Thanks for your help Chuck

              Val

              Comment


              • #8
                The load is crazy low into a 2-4 or 8 ohm secondary with your variac. You have to be careful or you can blow the variac fuse easily.
                Also, are you measuring the actual AC voltage at the secondary wires when you do this?
                For best results, I always use two voltmeters and an actual measured 10vac at the secondary wires with my variac set to what ever it takes to measure the 10v and always delivered into the highest impedance Zed of the OT (8 or 16 ohms wires) to find the ratio.
                Yeah... there will be enough voltage transformed on the primary winding to give you a whack so watch out.
                Bruce

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

                Comment


                • #9
                  OK ..Im using 2 dmms to measure and the highest tap is the 8 ohm so this looks good to me. Im just wondering if the odd ratios whn applying less voltage are symptoms of a bad OT or just innaccurate due to my lack of understanding. THanks for the help..im hoping ot use the OT to transform this Magnatone into some type of JMP . It was running 2xel34s but it was listed as a 35 watt amp. I think it may handle more.

                  Comment


                  • #10
                    24*24 = 576
                    22.7*22.7 = 515
                    21*21 = 441...which gives 3.5K to 4.6K on th 8ohm tap, split the difference and you get 4K, which is fine.

                    You might always get slight variations each time you measure, could be your meter, human error? I'd check that you are getting an even voltage each side of the primary when measured to the centre tap, if so, don't sweat it. Part of your concern seems to be that you were "expecting" 3.4K...you have what you have, what you were expecting was irrelevant.

                    Comment


                    • #11
                      Right, 4k is fine. Not sure why you get the drift in results with different voltages but notice that the primary Z seems to go down as you increase the voltage. Maybe inhearant in this design and at actual working voltage the Z is 3.4k
                      "Take two placebos, works twice as well." Enzo

                      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                      "If you're not interested in opinions and the experience of others, why even start a thread?
                      You can't just expect consent." Helmholtz

                      Comment


                      • #12
                        Thanks chuck.. could that be the way the ot is wound

                        Comment


                        • #13
                          Measurements should be made at transformers nominal power output into nominal load with a sine signal at the center of the passband. Somewhere between 400 and 2000Hz.
                          You will get different results at differnet power levels because the transformers efficiency, or power transfer ratio if you will, is less than 100% and is highest at nominal rated power.
                          Also, measuring with a variac at mains frequency has inherent errors since 50 or 60Hz is often outside the transformers pass band and mains voltage is not a clean sine wave.
                          Strictly speaking, measurement made with a variac will only tell you that the transformer works and its impedance ratio is within ball park of what the specs say it should be.
                          Aleksander Niemand
                          Zagray! amp- PG review Aug 2011
                          Without the freedom to criticize, there is no true praise. -Pierre Beaumarchais, playwright (1732-1799)

                          Comment


                          • #14
                            Thanks for that detailed description Alex..that helps me greatly to understand the margin of error within my limited ability to test...It works...its close...lets lay it out ..wire it up and see how she sounds! Thanks!

                            Comment


                            • #15
                              OK I'm not an engineer but I disagree here... 400Hz to 2Khz? "50Hz to 60Hz is often outside the OT's passband"....?
                              I think if your OT 's "pass band" low end is 50Hz, you should consider a new one asap.

                              Really well made OTs work at +-1dB to +-2dB from 20Hz to 20KHz. and that should be a pretty flat curve.
                              Maybe some very cheap guitar amp's OTs will be closer to, +- 2 or +-3 dB from something like 40Hz to 12KHz or so, (just made those numbers up for comparison) but I don't think they should vary in impedance by applied AC voltage much.....
                              Unless the AC voltage is so low that the magnetics don't really work...?
                              Bruce

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

                              Comment

                              Working...
                              X