An output transformer designed specifically for use in a guitar amplifier should IMO have a -2db pass band wider by about an octave at each end than the loudspeaker which it will power. OT is part of a system and should be designed for particular power tubes, buying an OT with only Raa and power handling specified and nothing else is a lottery.
Given that guitar loudspeakers generally cover, give or take, 65-6000Hz the answer presents itself. A 20-20.000Hz OT in a guitar amp is asking for trouble, especially when combined with a spaghetti style lead dress. Impedance ratio should be measured at midband , with a purely resistive dummy load, at frequency where a loudspeaker impedance is also resistive, this in most cases of guitar speakers is at around 400Hz. Hope to have confused you some more
Given that guitar loudspeakers generally cover, give or take, 65-6000Hz the answer presents itself. A 20-20.000Hz OT in a guitar amp is asking for trouble, especially when combined with a spaghetti style lead dress. Impedance ratio should be measured at midband , with a purely resistive dummy load, at frequency where a loudspeaker impedance is also resistive, this in most cases of guitar speakers is at around 400Hz. Hope to have confused you some more
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