Announcement

Collapse
No announcement yet.

mystery single ended output transformer

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

  • mystery single ended output transformer

    In the same Silvertone 1451 combo mentioned earlier I have a mystery OT labeled TF-103, and below that 10520003. It looks like a champ tranny, and shares the TF-103 part, but it measures 264 ohms on one side and 0.4 ohms on the other. The amp has a non-original 8 ohm CTS speaker and the output tube is an oddball 5OC5 7 pin mini.

    Is this even an output transformer? Do I dare use it? If not, what would work?
    It's weird, because it WAS working fine.....

  • #2
    50C5 is not really oddball, that with the 35W4 was used in a million radios. We just don't see them in guitar amps much.

    There were two transformers in this amp. One was the output transformer, which would be wired to pin 7 on the 50C5 and the other end to pin 7 of the 35W4. Check your wires. It was not rare to find the OT mounted directly on the speaker frame. Then a couple wires from the chassis run to that.

    The other transformer is the 12v transformer for the 12AU6 heater. The secondary is wired to the 12AU6 heater pins. it is not a straight 120/12v transformer, as its primary is in series with the two other tube heaters plus a resistor. In fact we can calculate what it is.

    Heater current 0.15A (150ma). SO nominal 50v and 35v dropped across the tubes, and 100 ohms drops 15v. SO 100v drops across everything but that transformer primary. leaves 20v I never saw a 20/12v transformer but it is possible.

    SO you determine which transformer in the schematic is the one you see.

    I googled TF103 and find it is a replacement Champ type OT from Triode ELectronics, (and maybe from others too) I am sure that would work just fine here.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      Enzo, I think you missed my question. The filament transformer is not in question. I am trying to figure out if the transformer wired to the speaker is actually an OT or not, given the readings I measure. The speaker is not original nor is it 4 ohms, so maybe this is the original OT, or maybe it is something else. Do those measurements sound right to you?
      It's weird, because it WAS working fine.....

      Comment


      • #4
        is this your trfm?
        http://triodeelectronics.com/tfchxfwi48oh.html

        if it has 3 wires secondarys its got a screen tap for optional pseudo-triode "ultralinear" operation.
        If you don't need it, just tape up the extra wire.

        Comment


        • #5
          Well, you were measuring DC resistance, which has little to do with impedance. dstrat found the exact web site I did. It is a Champ OT, and why wouldn't that work for you? Is it the original Sears part? No, of course not. It is an aftermarket part. But that doesn't make it unsuited.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Yes, I saw that site also, but no that is not what I have. Mine is only two wires in, two wires out. What is throwing me is it says the TF-103-48 is a 8K primary, and this one reads 264 ohms. I realize ohms does not equal impedance, but doesn't 264 ohms seem quite a lot different than 8K impedance? Or does that not matter?
            It's weird, because it WAS working fine.....

            Comment


            • #7
              Take any other OT you have laying around and measure the DC resistance of the primary. You might be surprised.
              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

              Comment


              • #8
                No, 243 ohms sounds perfectly normal to me. Yes 243 is very different from 8000, but then resistance and impedance are very different as well.


                COnsider this:

                TF103 - has four wires, the 8k primary and the 4 ohm secondary

                TF103-48 - has extra wires so your 8k primary can have 4 and 8 ohm taps. the 48 on the end means it is a model with the extra tap for 4 and 8.
                Education is what you're left with after you have forgotten what you have learned.

                Comment


                • #9
                  Originally posted by The Dude View Post
                  Take any other OT you have laying around and measure the DC resistance of the primary. You might be surprised.
                  Off the top of my head, I've seen some Fender OT primaries around 100 ohms, some Marshall's around 50 ohms. That is from CT to each side as these are not single ended, but just to give an idea.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #10
                    have a 40-18030 on the 8k tap its around 285 ohms. so yours is not that different, well not enough to make any difference.

                    Comment


                    • #11
                      Really, the main thing I use a meter for on a transformer is to see if a winding is open. The actual resistance isn't very helpful.
                      Education is what you're left with after you have forgotten what you have learned.

                      Comment


                      • #12
                        Originally posted by Enzo View Post
                        Really, the main thing I use a meter for on a transformer is to see if a winding is open. The actual resistance isn't very helpful.
                        Well, there is another good use for the meter: Feed heater voltage to the secondary and measure voltage across the full primary (several hundred volts). The ratio of primary to secondary voltages gives you the turns ratio, square it and you get the impedance conversion ratio. Multiply this by the output impedance and you get primary impedance (Raa/Zaa).
                        Last edited by Helmholtz; 07-10-2018, 04:46 PM.
                        - Own Opinions Only -

                        Comment


                        • #13
                          That is certainly true. I was only referring to using my ohm meter on a transformer.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #14
                            Originally posted by Enzo View Post
                            That is certainly true. I was only referring to using my ohm meter on a transformer.
                            As most meters can also measure AC voltage, I thought I'd give the OP a simple method the directly verify primary impedance.
                            - Own Opinions Only -

                            Comment


                            • #15
                              Originally posted by Helmholtz View Post
                              Well, there is another good use for the meter: Feed heater voltage to the secondary and measure voltage across the full primary (several hundred volts). The ratio of primary to secondary voltages gives you the turns ratio, square it and you get the impedance conversion ratio. Multiply this by the output impedance and you get primary impedance (Raa/Zaa).
                              Wouldn't feeding heater voltage into a 0.4 ohm secondary amount to a dead short?
                              It's weird, because it WAS working fine.....

                              Comment

                              Working...
                              X