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Hi
The situation of having a grounded centre tap is entirely different compared to a non grounded centre tap. When it is not grounded you have a series circuit where the voltage the primary 'sees' is simply double the voltage from one amplifier output. To do otherwise contradicts basic laws of electricity as it is a series circuit, the current cannot go anywhere else.
While at 0.3 Hz the response may be 3dB down unfortunately when measured the 'resonance' shows a 2dB boost at 10 Hz. As i wrote earlier, reducing the cap predictably changes the resonant frequency, giving a 6dB peak at 15 Hz and very sharp cutoff below 15 Hz. If I said 'the basis of a Pultec equaliser' others may take notice.
Heavily loading the output with 600 Ohms (the way the transformer was designed) the resonance disappears and you get the predictable roll off at HF and LF.
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