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Replacement model 20 output transformer - Printable Version

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Replacement model 20 output transformer - Tubesforme - 03-29-2015

Hi everyone
I have a Philco model 20, which uses PP 71A's and I need to replace the output transformer. I would like to use a Hammond 125C, which is rated at 8 watts. What I need to know is what should I be using for the primary impedance for PP 71A's.  Voice coil impedance is: VC=1.5 X DC resistance, which, in my case, means 1.5 X 1.4 so the voice coil impedance is 2.1 ohms. What taps should I use for the correct primary impedance with the PP 71A's? I have forgotten as all I used to have to do was look up the original part number and cross it over the the replacement number and put it in! By the way, my speaker isn't the original Philco, it is a Jensen, of the same size that has been reconed and has a new field coil installed too.

Thanks,
Michael

RE: Replacement model 20 output transformer - morzh - 03-29-2015

If your speaker is original, you want the outputs 3 and 4 as they provide close to 1:100 ratio you have in 20 output xfmr.

Otherwise we need to know if it is not original.

http://www.hammondmfg.com/125.htm

RE: Replacement model 20 output transformer - codefox1 - 03-30-2015

Truth is almost any output transformer from the era will be OK, worst match would mean lower maximum volume. If you get a "universal" one adjust for best tone/volume.

RE: Replacement model 20 output transformer - Tubesforme - 03-31-2015

(03-30-2015, 11:48 AM)codefox1 Wrote:  Truth is almost any output transformer from  the era will be OK, worst match would mean lower maximum volume. If you get a "universal" one adjust for best tone/volume.

Thank you to everyone. I am using an NOS transformer that had the paper work with it showing how to wire it for PP 71A's as well as all other output tubes of the era. So my problem is solved.