04-14-2007, 04:56 PM
The output transformer in my 37-2670 is shot. No continuity between any primary leads (if just HALF of it worked I could measure the turns ratio I need). The set uses push-pull 6F6's triode connected. Since they are driven by a low impedance driver transformer and push-pull 6J5 drivers, I must assume they are running class AB2. With 350 volts on the plate, the tube manual says 13 watts max output and 6K ohms plate to plate load.
I have a Stancor VAT-6 universal push-pull output transformer rated at 15 watts and 70ma. It fits on the Philco speaker using the exiting mounting holes, and is physically larger than the original.
Here is how I propose to find the correct secondary taps to use (I do not know the voice coil impedance, but the DC measures 1.6 ohms). I connected the entire primary of the transformer (plate to plate) in series with a 6.8K resistor and my HP 200A audio oscillator set at 25 volts output at 500 hz.
I carefully measured the resistors value. Then I measured the AC voltage across the resistor and the transformer with my HP 973 DVM, which is pretty good on AC measurements up to 20KHz. I then ASSUME that the reflected impedance of the voice coil can be calculated using the ratio of the voltages across the resistor and transformer (at least a good estimate): Z/(speaker volts) = R/(resistor volts).
I then connected the voice coil leads to the varous taps and measured the voltages and then calculated the estimated impedance. The closest I can come is either 6820 ohms or 5230 ohms. So I will try both once the radio is working and pick which ever sounds the best.
I did NOT have the field coil active during these tests.
Does anybody think this will work? Am I totally out in left field? Is there a better way? I don't trust my ear to pick the taps.
Dave
I have a Stancor VAT-6 universal push-pull output transformer rated at 15 watts and 70ma. It fits on the Philco speaker using the exiting mounting holes, and is physically larger than the original.
Here is how I propose to find the correct secondary taps to use (I do not know the voice coil impedance, but the DC measures 1.6 ohms). I connected the entire primary of the transformer (plate to plate) in series with a 6.8K resistor and my HP 200A audio oscillator set at 25 volts output at 500 hz.
I carefully measured the resistors value. Then I measured the AC voltage across the resistor and the transformer with my HP 973 DVM, which is pretty good on AC measurements up to 20KHz. I then ASSUME that the reflected impedance of the voice coil can be calculated using the ratio of the voltages across the resistor and transformer (at least a good estimate): Z/(speaker volts) = R/(resistor volts).
I then connected the voice coil leads to the varous taps and measured the voltages and then calculated the estimated impedance. The closest I can come is either 6820 ohms or 5230 ohms. So I will try both once the radio is working and pick which ever sounds the best.
I did NOT have the field coil active during these tests.
Does anybody think this will work? Am I totally out in left field? Is there a better way? I don't trust my ear to pick the taps.
Dave