This Wikipedia writeup seems relevant.
My suspicion is that there is nothing wrong with the speakers, and that some marginal instability in the amplifier is being brought out by the particular impedance characteristics of the Santana.
The following statements in the writeup particularly grab my attention, especially given that you have modified the original design to use a tube high voltage rectifier:
-- Al
My suspicion is that there is nothing wrong with the speakers, and that some marginal instability in the amplifier is being brought out by the particular impedance characteristics of the Santana.
The following statements in the writeup particularly grab my attention, especially given that you have modified the original design to use a tube high voltage rectifier:
Low frequency oscillations like motorboating indicate that some device or circuit with a large time constant is involved, such as ... the filter capacitors and supply transformer winding.Regards,
One common cause is feedback through the plate power supply circuit.[2][4] The power supply provides DC current to each tube's plate circuit, so the power supply wiring (power busses) can be an inadvertent feedback path between stages. The increasing impedance of the filter capacitors at low frequencies can mean that low frequency swings in the current drawn by output stages can cause voltage swings in the power supply voltage which feed back to earlier stages,[2][4] making the system a subaudio oscillator. This is caused by inadequate power supply filtering or decoupling.
-- Al