01-17-2011, 04:45 PM
I have read several postings regarding replacing the output transformer on a model 70. I wish to find a replacement transformer that will fit inside the original case if possible. I'm sure this has been done before. One thing confuses me. The tube manual for a 47 output tube says the load impedance is 7000 ohms. However in the Tech Section of Ron's site http://philcoradio.com/tech/speaker.htm the primary impedance for a model 70 is listed as 5700 ohms. PERHAPS I AM MISINTERPRETING THE 5700 OHM NUMBER IN THE TECH SECTION?
I measured the voice coil at 0.7 ohms DC and 0.7273 ohms impedance at 400hz. At 7000 ohms primary impedance, the ratio would then be about 98:1 (I think I read in the Phorums that one restorer mentioned 100:1). That is a tough ratio to match! If the primary impedance was indeed 5700 ohms, it would be easier to find a replacement transformer.
Assuming 7000 ohms, I have a couple of transformers in stock that may work:
Stancor A-3856, Universal Single or Push-Pull, 4 watts, Primary DC max 35ma. The 47 tube is 2.7 watts out and 31ma plate current, so that should work. The ratios available are 7000 ohms to 1.78 ohms, 0.36 ohms, or 0.42 ohms. None of these are close to 0.7 ohms, so I assume I would use the higher impedance (1.78 ohms)? This transformer will definitely fit inside the existing case. It is a small transformer so the frequency response is probably limited (but so is the model 70 speaker, right?)
Merit A-2902, 10 watts. With this one, I can match 7000 ohms to 1.04 ohms. It is much larger than the A-3856, but I THINK it will fit inside the existing case (depends on the thickness of the existing transformer shell).
In the Phorums, there were several discussions of using Hammond transformers. Which one is recommended (must fit inside the existing case).
Thanks!
I measured the voice coil at 0.7 ohms DC and 0.7273 ohms impedance at 400hz. At 7000 ohms primary impedance, the ratio would then be about 98:1 (I think I read in the Phorums that one restorer mentioned 100:1). That is a tough ratio to match! If the primary impedance was indeed 5700 ohms, it would be easier to find a replacement transformer.
Assuming 7000 ohms, I have a couple of transformers in stock that may work:
Stancor A-3856, Universal Single or Push-Pull, 4 watts, Primary DC max 35ma. The 47 tube is 2.7 watts out and 31ma plate current, so that should work. The ratios available are 7000 ohms to 1.78 ohms, 0.36 ohms, or 0.42 ohms. None of these are close to 0.7 ohms, so I assume I would use the higher impedance (1.78 ohms)? This transformer will definitely fit inside the existing case. It is a small transformer so the frequency response is probably limited (but so is the model 70 speaker, right?)
Merit A-2902, 10 watts. With this one, I can match 7000 ohms to 1.04 ohms. It is much larger than the A-3856, but I THINK it will fit inside the existing case (depends on the thickness of the existing transformer shell).
In the Phorums, there were several discussions of using Hammond transformers. Which one is recommended (must fit inside the existing case).
Thanks!
