Because pentode tubes are not presented by a load 2 to 3 times their internal resistance as is done on triodes (for reasons of reducing harmonic distortion), the output impedance at the secondary is fairly high.
So the speaker is driven by a highish impedance. As the impedance of the voice coil increases at high frequencies (the reactance of coils increases with frequency) the amplifier is presented with a higher load. So there is less voltage drop at the output of the amolifier.
So the output voltage of the amp increases at very high frequencies when the speaker is connected!
This makes cymbals so realistic on full range open back speakers from good vinyl records reproduction that it is scary to listen too.
It sounds as if a real big cymbal is inside the room.
Reference:
Applied Electronics - T.S. Gray - MIT
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