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Audio Output transformer info needed - Printable Version

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Audio Output transformer info needed - 45philcodon - 02-26-2022

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

RE: Audio Output transformer info needed - RodB - 02-26-2022

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.

RE: Audio Output transformer info needed - 45philcodon - 02-26-2022

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

RE: Audio Output transformer info needed - RodB - 02-26-2022

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).

RE: Audio Output transformer info needed - Mondial - 02-26-2022

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.

RE: Audio Output transformer info needed - 45philcodon - 02-26-2022

Thank you, guys.

Don

RE: Audio Output transformer info needed - Radioroslyn - 02-26-2022

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/transformer-output-8-w-single-ended

GL

RE: Audio Output transformer info needed - mikethedruid - 02-26-2022

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.

RE: Audio Output transformer info needed - Mondial - 02-26-2022

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