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  • #1

    PT test questions?

    Hello everyone,

    Possibly silly question...

    I'm trying to turn a (free) Stromberg PA into a guitar amp. It wasn't intact, some power supply components missing - power cord, caps, etc. Basically trying to use trannies & chassis. Wanted to check the PT out. It looks okay I went through RG's tests, ohming out windings (all okay) & checking voltages totally unloaded (all okay). Don't have the neon short tester, though I did do the "leave it on and powered" test for 12 minutes. It never got above the outside air temp.

    Here's my question: for fun & experimentation, I checked voltages between 2ndary windings, as in, one leg of HV to one leg of 6V heater. Got about 170 volts. Also, got about 5V between the two heater windings - 5V/6V (has tube recto). So, would this be a short, or just inductance that happens naturally? AC in a coil in close proximity to another coil, etc... since windings aren't connected together in the amp either? As I said, never got even warm. Just wondering if this is normal? I could take the PT out of another functioning amp, I guess...

    Thanks to everyone with their patience!

    Justin Thomas
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -
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  • #2
    Set your meter to AC volts, and set it on the table in front of you. Lay the black probe down somewhere. Now grasp the red probe tip with your fingers firmly. How many volts do you get? Right now on my meter I get about 2/10 of a volt. Now plug just the reb probe into the hot side of a wall outlet, how many volts do you read? Remember, the black probe is connected to nothing. I get about 12v here. The numbers do not matter, they are not a real measurement of anything useful. But they serve to illustrate that your meter has no idea what it is connected to, and your leads can act as antennas when they are not terminated by a circuit. Now imagine that same meter connected to two live power transformer windings, you are sure to get a variety of voltages.

    What matters is what voltages you get on the actual windings. If they seem reasonable, and the thing does not get hot and doesn;t blow fuses, then hook it up to some circuit loads, like tube heaters, or high voltage power supplies. And make use of it.
    Education is what you're left with after you have forgotten what you have learned.

    Comment

    • #3
      Thanks Enzo, for the quick&dirty. I got .9V with my finger & 12 in the wall. I'll go ahead with it since it didn't get hot.

      Justin Thomas
      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

      Comment

      • #4
        Rats, you blew me away on the finger voltage, but we tied on the mains outlet. Maybe if I move closer to something electrical...
        Education is what you're left with after you have forgotten what you have learned.

        Comment

        • #5
          Hey Enzo,

          I guess those few pounds of "insulation" I've got are only good for the cold weather... phooey.

          Parts for this one should be in Tuesday... this would be my first "build" if I finish it.

          I'll take a few other trannies I have lying about & check them out - I'm curious about this voltage between windings thingy. Thanks again!

          Justin
          "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
          "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
          "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

          Comment

          • #6
            Originally posted by Justin Thomas View Post
            Hey Enzo,

            I guess those few pounds of "insulation" I've got are only good for the cold weather... phooey.

            Parts for this one should be in Tuesday... this would be my first "build" if I finish it.

            I'll take a few other trannies I have lying about & check them out - I'm curious about this voltage between windings thingy. Thanks again!

            Justin
            I bet it is inductance induced... (I mean that is how they work).
            When referenced to chassis ground and when there is no rectifier plugged in, I find anywhere from 40vac to 140vac or more on the 5v rectifier windings of my PTs ... same thing with the 6.3vac ones.
            I just checked one my bench a little while ago.
            I would not be surprised to find it is not real, (well, real but not relevant)... probably just your DVMM is super sensitive and reading leakage or static voltage with tiny tiny micro amps behind it.
            Bruce

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

            Comment

            • #7
              Hey Bruce,

              Thanks for the backup test - it's just doing what it does. Between your & Enzo's responses it makes sense. Hope to be able to get it done in a week or two...

              Justin
              "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
              "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
              "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

              Comment

              • #8
                I agree with Bruce, it isn't real. Your 10 Meg meter can measure that, just as it measures voltage from the air, but as soon as you try to use that voltage for something, it cannot provide any current.



                I had a customer once bafled that his tube heaters would not light. No 6v on the heaters. But he disconnected the transformer to test it and got 6v across the 6v winding. SO he stopped ther and assumed it was OK.. Turns out the winding was open, but the phantom AC voltage just coincidently turned out to be about 6v, so he assumed incorrectly the transformer was working. "Enzo, how come this..." Keep the meter on the winding while you connect the load. 6v disappears.
                Education is what you're left with after you have forgotten what you have learned.

                Comment

                • #9
                  So, correct me if I'm wrong, but my meter is simply saying, "we have this number of electrons at point A and a different number of electrons at point B, so there is a voltage - difference of potential. Your coils are doing their job of inducting. A current would flow if it could. But it doesn't mean anything without additional information." Parts haven't come in yet to wire it up, but since it didn't heat up at all, it's reasonable to say it's usable.

                  Hopefully I'll be mostly finished with this by the weekend; I'll start a new thread then, preferably to celebrate. Thanks again, Enzo & Bruce!

                  Justin
                  "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                  "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                  "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                  Comment

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