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
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.
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
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).
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.
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.
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