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I'm restoring a Philco Model 46-431. The primary of the output transformer is open.
The schematic lists the primary DC resistance as 500 ohm; no listing of impedence.
I have a spare transformer from an unknown radio whose primary measures 200 ohm.
What performance issues will I encounter if I substitute this lower DC resistance transformer.
The audio output tube is a 6K6.
Any idea where I might procure a reasonable substitute if the one I have is not suitable?
Thanks for your help.
Don
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I wasn't able to locate the schematic for a 46-431 to get the audio output power. Your schematic might have the spec. So let's say it's about 5 watts. You can get a 5 watt universal OPT for a very reasonable price. Make sure the primary impedance matches the 6K6 at around 7.6k ohms (6k - 8k) and the secondary matches to the speaker voice coil impedance, probably 3-4 ohms. You may get lucky and find one that has the same mounting holes. There's a way to calculate your unknown OPT impedance but you may not have the equipment.
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Thanks, Rob.
The power output is only 1.5 watts.
What is the source info for the universal OPT you mentioned.
Also, the for the impedence calculation, if I run a 1KHz signal thru the primary of my spare OPT, load the secondary with 4 ohms and measure the AC voltage across the primary, I should be able to calculate the impedence. Right?
Thanks,
Don
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The Hammond 125ASE is a 3 watt universal.
Measuring voltages will give you the turns ratio. You apply one volt to the primary and measure the secondary voltage. Off hand I don't have the technique for getting impedance. I remember reading about it sometime ago and I know there are YouTube videos that show how to do it (restoreoldradios).
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Impedance is a function of the transformer turns ratio squared. For example if you have an output transformer and apply 1 VAC to the secondary and measure 40 VAC on the primary, then the turns ratio is 1 to 40. The impedance ratio will be 40 squared or 1600.
So if you connect a 4 ohm speaker to the secondary, the impedance seen at the primary will be 4 X 1600= 6400 Ohms.
Typically, the output transformers for small tube radios were designed for speakers of 3.2 to 4 Ohms, so pretty much any used one can be measured as described above.
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This one isn't dead on impedance wise but it will get you in the ballpark. And it's not $50.
https://www.tubesandmore.com/products/tr...ngle-ended
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
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Correct me if I'm wrong, but I seem to remember that the impedance which the tube plate circuit sees is a function of the winding ratio and the speaker voice coil resistance. In other words, if the transformer ratio was 200 to 1, primary to secondary, and the voice coil was 8 ohms, the impedance reflected to the plate circuit would be approximately 8 x 200, or 1600 ohms. For the transformer Terry linked that would work out to a turns ratio of about 625 to 1.
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Mike, you are correct, except that the impedance transformation is proportional to the square of the turns ratio. So in your example of a 200 to 1 ratio, impedance ratio is 200 x 200 = 40000. Reflected impedance is 8 ohms x 40000 = 320000 ohms
https://www.radioremembered.org/outimp.htm