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RE: Losses in Output Transformers

Taking as an example the Dyna A470, the end-to-end DC resistance of the primary is about 200 ohms. Usually the secondary losses are equal to the primary losses, so total about 400 ohms. The reflected load should be 4300 ohms without the wire losses, so net efficiency is 4300/4700 = 91.5%.

Obviously the above is for Class A; I'd expect a bit more loss in Class B, or in Class AB at high power, maybe 1.5 times as much. At 1kHz, iron loss should be low since it's far from saturated.

This won't add up to the 30% loss you reported, of course! Must be something more. A couple possibilities:

1) If the tubes are not fully broken in, the cathodes might not be fully formed, limiting available current peaks.

2) If the transformer impedance including DCR is not matched to the spec operating point, that might reduce available power.


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