32-8120 audio xfmr specs? - Printable Version

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+--- Thread: 32-8120 audio xfmr specs? (/showthread.php?tid=10348)

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32-8120 audio xfmr specs? - Eric T - 08-24-2014

Does anyone know what the specs for the audio output transformer (p/n: 32-8120) on a Philco 42-380?

Mine is kaput and I need to find a substitute, for which I'd also appreciate recommendations.

I'm not sure if I should go for a Hammond 125 series or one of the more expensive audiophile transformers.

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RE: 32-8120 audio xfmr specs? - codefox1 - 08-25-2014

This is just a radio, no need to get too fancy. I assume you have the PP 41 output version. Since you have nothing to lose, carefully open the old one up and see if there are any corroded or broken leads attached to the windings. This happens a lot on some of these old sets. Again these low powered sets are not too fussy about an exact match, go too far off and you may lose power, but other than that, you can certainly try whatever you can scrouge up. Well that's my opinion.

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RE: 32-8120 audio xfmr specs? - Ron Ramirez - 08-25-2014

From the 1946 Philco Quick Selector Parts Directory:

Primary 17,000 ohms; secondary 5 ohms; 6 watts; turns ratio 58:1.

As Codefox said, there's no need to get fancy. In my opinion, you would just be wasting your money to get an "audiophile" transformer since this set does not deliver high fidelity sound anyway.

A Hammond 125B (5 watts) is probably big enough since the 42-380 is only rated at 3 watt output.

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RE: 32-8120 audio xfmr specs? - Eric T - 08-26-2014

Thank you both for the information. Not only have you saved me about $60, you have also helped me figure out where to tap the secondary for the right impedance match on the primary.

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RE: 32-8120 audio xfmr specs? - Eric T - 08-27-2014

One more thing:

3 WATTS!!!???!!!???

*Reaches for earplugs*

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RE: 32-8120 audio xfmr specs? - Eric T - 09-08-2014

Just in case someone else needs help figuring out the best place to tap a Hammond 125 series universal replacement transformer when the secondary impedance isn't showing on the chart (available here: http://www.hammondmfg.com/125.htm), I have decided to post this fairly simple bit of math:

The chart does not show where a 5 ohm secondary impedance should be tapped to get 17,000 ohms primary impedance. Hammond also doesn't show the turns ratio for each tap, so you need to figure that out in order to decide the closest match for the original.

On the chart, you see that tapping the secondary at 1 & 3 with a 6 ohm voice coil impedance will get you 21,600 ohms impedance at the primary. The primary to secondary impedance ratio is the turns ratio squared.

pri/sec = turns²

So you divide 21,600 (primary) by 6 (secondary) to get 3,600.
21,600 ÷ 6 = 3,600

Then you find the square root of 3600 to get the turns ratio, which is 60.
√3,600 = 60

Now I'm looking for a 58:1 turns ratio and a primary impedance of 17,000 ohms, so I did the math on the two closest tap points on the chart and found that I'll get 60:1 at taps 1 & 3 and about 53:1 at 2 & 4.

At 1&3:
60² = 3,600
3600 x 5 = 18,000

And at 2&4:
53² = 2,809
2,809 x 5 = 14,045

So I go with tapping the secondary at 1&3 to get a primary impedance of 18,000 ohms to go with the 5 ohms secondary impedance.

Yes, I know that was easy to estimate just by looking at the chart for my situation, but not everybody is going to have as cut-and-dried choices. Punching a few numbers on the calculator is easier than hooking up the transformer and playing the trial-and-error game.

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