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Thread: mystery single ended output transformer

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    Supporting Member Randall's Avatar
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    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?

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    It's weird, because it WAS working fine.....

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    Lifetime Member Enzo's Avatar
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    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.

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    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?

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    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.

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    Lifetime Member Enzo's Avatar
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    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.

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    Supporting Member Randall's Avatar
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    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?

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    Supporting Member The Dude's Avatar
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    Take any other OT you have laying around and measure the DC resistance of the primary. You might be surprised.

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    Lifetime Member Enzo's Avatar
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    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.

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    don't forget the joker g1's Avatar
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    Quote 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.

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    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.

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    Lifetime Member Enzo's Avatar
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    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.

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    Quote 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).

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    Last edited by Helmholtz; 07-10-2018 at 05:46 PM.

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    Lifetime Member Enzo's Avatar
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    That is certainly true. I was only referring to using my ohm meter on a transformer.

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    Quote 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.

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    Quote 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?

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    Nope. Again, that .4 ohms is DCR- not impedance. The heater voltage would, of course, need to be AC.

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    Supporting Member mozz's Avatar
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    Do you have a signal generator? I use one set at 1v across the secondary and measure voltage across the primary. Bit safer than higher voltage-higher current, easier to calculate, but you can still feel it if you touch the wires (by accident).

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    measure the resistance of any 6v winding on a power transformer. You will find it very low.

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    "Nope. Again, that .4 ohms is DCR- not impedance. The heater voltage would, of course, need to be AC"

    Yes, but I with 6.3 vac and 0.4 ohms I get (E/R = I) 15.75 amps. Am I misunderstanding?

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    Supporting Member The Dude's Avatar
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    That formula applies to DC. We aren't talking about resistance and DC voltage. We are talking about impedance and AC voltage.

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    Lifetime Member Enzo's Avatar
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    Yes. You keep thinking of the transformer winding as a resistor.

    You can run that 6v into that 0.4 ohms because the current is transformed into a different voltage/current on the other side of the transformer. If you short across the other side of the transformer, THEN it turns into a 0.4 ohm resistor.


    Imagine a typical power transformer, 120v primary and 6v secondary. That 6v secondary has a super low resistance. GO check one. The primary may only have 100 ohms resistance, but the 120v doesn't care as long as it can turn that into 6v on the secondary. But short across that 6v winding, and BAM you blow the mains fuse, because now the transformer can't transform. SO the primary becomes a short circuit across the mains.


    I view a transformer as a set of gears.

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    Impedance Z is an AC quantity, given by ratio of AC voltage to AC current. There is no general relation between the impedance and DCR. The secondary of the OT just behaves like an inductor as long as there is no current flowing in the primary. The impedance of an inductor is always higher than its DCR. The 6.3V heater voltage corresponds to an output power of around 5W in the amp, i.e. the AC current in the secondary is the same as with an output of 5W into 8 Ohms.

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    i.e. the AC current in the secondary is the same as with an output of 5W into 8 Ohms.
    Maybe you intended to say "5W into 8 ohms mean 6.3 Volts AC?"

    We can be certain of Voltage ... not so much about current because we don´t know what load is applied at the other end of the transformer.

    And if as usual it´s measured unloaded (the meter does not count as a load in this case) then current is nil.

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    Quote Originally Posted by J M Fahey View Post
    Maybe you intended to say "5W into 8 ohms mean 6.3 Volts AC?"

    We can be certain of Voltage ... not so much about current because we don´t know what load is applied at the other end of the transformer.

    And if as usual it´s measured unloaded (the meter does not count as a load in this case) then current is nil.
    You're right, of course. My bad. With open primary the secondary current at 6.3V is only determined by the secondary magnetizing inductance and thus lower by a factor than with primary terminated by the tubes' Raa.
    What I wanted to get across is that it is safe to use heater voltage (as I proposed above) and that the secondary is far from being a short.

    My recommendation to beginners: Use your DMM's AC current measuring function and measure voltage and current to determine Z. (Note: The Z of a transformer winding gets reduced by the load on the other windings. Or in other words , the current in each winding is influenced by the current in the other windings.)

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    Last edited by Helmholtz; 07-11-2018 at 10:06 PM.

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