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City: Loxley, AL 36551
I have a Philco #76 on the bench, the output transformer primary is open. The radio uses two 45's in push-pull and the VC winding resistance is very low - less than one ohm from memory. The part number is 2848T.
Should I just go with a universal replacement transformer?
Wondering if anyone knows the proper impedance values for a replacement?
Thanks!
-Sam T.
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Joined: May 2010
City: Albuquerque, NM
Sam,
I think a Hammond universal type audio putput transformer will work for your 76. The only question is the best power rating needed. Maybe someone else with more knowledge of this model will know what the P-P 45's are capable of wattage wise. I suspect it may be only 5-6 watts audio power because of the single 27 driver tube. But I may be wrong as 45's in push-pull with the fixed bias AB2 operation is capable of a lot of power...up to 15-18 watts with sufficient drive power.
The plate to plate ideal load impedance is theoritically around 3200 ohms for this bias operation...quite low. By using the chart supplied with the transformer you can wire the secondary to get a pretty good match between the low secondary of 1.5 ohms or less to about 3200 primary impedance or so ohms. So do you need a 125 B,C,D or E transformer?
:
Best of luck. John
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The original specifications call for an audio output transformer with a primary impedance of 6,700 ohms to match a 0.7 ohm voice coil.
--
Ron Ramirez
Ferdinand IN
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My daddy told me a long time ago that I didn't need to know all the answers, but a smart man knows who to ask. Thanks again!
-Sam T.
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City: Albuquerque, NM
HI,
Out of curiosity I looked at PTOP and Gary shows he has this transformer in stock for $7.
His stock # 4541-239. He also has another that would probably work...his part # 1290-716. Go to this link to search for it.
http://www.oldradioparts.com/2a23dfl.txt
PTOP's web site is oldradioparts.com
Email Gary at
gbsptop@aol.com
Good luck, John
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Hi John, unfortunately Gary has already sold that one. When I learned that earlier today, I posted here - and now I've written Gary back with the impedance values to see if he has something similar. Thanks for looking!
-Sam T.
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I'm surprised that the audio output transformer is shot but the audio interstage one isn't. It seems that the audio interstage is almost always gone on sets with push pull #45 tubes especially the primary side, at least there is a cheap substitute you can build now with solid state parts, if you can stand having some "sand" in your old radio. It isn't really necessaty for P-P 45 tubes most of the time but some sets used interstage transformers with odd impedance ratios which you can not find stock and would be expensive to have rewound or replicated as a one off.
Regards
Arran
