One of the design goals of earlier versions of the Hammarlund Super-Pro family was hi-fi audio capability to support critical monitoring and re-broadcast of programs. Unfortunately, while the SP-600 retains a variable-selectivity IF that can be opened up to 12 KHz, the audio amplifier is rolled off below 200 Hz to improve voice intelligibity when narrow bandwidths are used. This had become standard practice in communications receivers.
Here in the 21st Centrury, we'd like to use the old SP-600 to appreciate some of the excellent AM signals, both amateur and commercial, that are currently on the air. (I'm listening to Radio Timtron World-wide on WBCQ, 7415 KHz as this is being written.)
The fix is not too difficult. The coupling caps to the first and second audio stage and the cathode bypass on the second stage should be scaled by a factor of ten. Fortunately, the first AF amp has an un-bypassed cathode resistor, giving us a good place to apply negative feedback to further improve the situation.
SP-600 Audio Mod
The audio in the modified radio is essentially flat from 20 Hz to 9 KHz. There's a slight rise around 10 KHz before a distinct rolloff. Distortion is quite low, with just a hint of second harmonic setting in at about 30 Hz as the transformer runs out of iron. This set sounds really good through a 15-inch Altec 604 speaker.