> they have different size
Size is proportional to power, lowest frequency versus distortion, and type of iron.
5 Watts per pound is good for 50/60Hz power transformers, and for medium-performance audio.
A cheap 1-watt table radio with 150Hz response may use an OT less than a tenth of a pound. The OT for a 5-Watt Champ gitar amp is slightly less than a pound. The OT on my hi-fi 9-Watt amp was near 4 pounds.
Large transformers will be designed with just-enough iron to meet their specs. Very low-power transformers are sometimes generous, because making a very small transformer with very thin wire costs more in winding time than the saving of metal is worth.
Size has almost nothing to do with impedance. The Champ OT has 7,000 ohms one side, 4 ohms the other side. We could generally wind down below 1 ohm, and up past 10K ohms, without any change except change of wire gauge and length.
(Getting great response much above 10K requires tricks. For small power, super-iron allows ample inductance on a smaller core, with fewer turns, and with a smaller area to cause capacitance. Or a larger core allows greater insulation between layers, reducing capacitance. But in general, you prefer to stay away from high impedance audio windings.)
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> push pull line amp with 5687 output tube, i try to use a 500ma E core power transformer as output transformer(P=0-110v-220v, S=24-ct-24), it's work good! no low freq loss. The high freq is good too, but...do i missed something here?
The 5687, well loaded, can put out 1 or 2 Watts. Your OT presents about 12Kct to the plates, and 200V 40mA would be an excellent condition for the tube. That's about 6K-3K load to each plate, plate resistance is near 2K here, it should work good.
When worked all the way to 220V rms on primary, the distortion of a small power transformer at 50/60Hz is terrible. As a power transformer, "distortion" means a distorted current waveform in the primary, a strain on the utility company generator. But a few-Watt PT on a megaWatt utility system, this strain does not matter. When the source is a small tube, it matters.
48V into 600 ohms is 4 Watts, your tube can't easily do that. It is also +36dBm, an insane level for small studio work. Assuming +20dB audio peaks, you have 7.7V audio in a "48V" power winding. You are working at 1/6th the level it runs as a power transformer. While distortion at 48V would be high, at 1/3rd power-rated voltage it is usually tolerable. So at 1/6th rated voltage it may be audibly "clean", though perhaps not perfectly transparent for large bass. Distortion will rise at low frequency and fall at high frequency. Above 500Hz this iron may be far cleaner than your tube.
I expect a small simple 120V power transformer to be very roughly 2K impedance near 50Hz. Since this is near your plate resistance, I would expect some small droop by 50Hz, maybe 25hz. Your 24VA core is bigger, should do better. You may be solid to 20Hz, which is very good.
The old-old 2-4-1 power transformers were tightly wound, and CJ measured one as flat to past 15KHz. I have seen similar response on other PTs. However the increased demand for safety has led to greater separation between primary and secondary. A split-bobbin winding may be falling before 10KHz. I have a U-core PT with primary on one leg and secondary on the other leg, great for safety, but the response is falling by 2KHz. At low frequency, most of the flux is in the iron and cuts all coils. At high frequency, the iron is not doing much, you must make all coils "intimate" with each other. Primary over secondary is often good enough. Primary on one leg and secondary an inch away on the other leg, the highs don't get through.
So: it depends, I'm not going to analyze your iron from afar, you can measure it yourself, or just enjoy.
