If the 75A-3 is like the A-4, the L-C i.f. strip is too broad for congested band condx. Rather than simply using blocking caps and loading resistors in place of the mechanical filter, a better solution would be to find a good 455 kc i.f. transformer, make an adaptor, and wire or plug it into the circuit. That would tighten up the L-C selectivity a little and make a nice wide-band selectivity position for AM. I use an 8 kc filter in mine most of the time. 6 is a little too narrow for AM when the band is not crowded or there are no nearby adjacent signals. Often, even under congested conditions, I get better readability with the 8 than with the 6, particularly with the synchronous detector. Despite decades of space-shuttle-audio propaganda to the contrary widely circulated in ham radio literature, the audio frequencies between 3 and 4 kc do carry a lot of the intelligibility in the human voice. The outboard sync detector which picks the signal off the final i.f. transformer just before the the diode detector, disables the passband tuning feature, but I don't find that a great disadvantage. I just have to manually tune the signal to one side of the passband, and in CW/SSB mode, manually adjust the BFO in sync with the main tuning, just as you normally do with other receivers. I have NEVER seen an A-4 with perfect passband tuning; the heterodyne frequency always wobbles a little and/or shifts as you tune the control back and forth. If you have an early 75A-4 with the mechanical filter input wired in series with the B+ line to the plate of the tube, you should immediately do the recommended modification to feed the tube through a 2 mHy r.f. choke resonated with a 68 pf mica cap, feed the filter through a 1000 pf blocking cap, and ground the bottom side of the input winding of the filter. With the original circuit, a shorted plate connection or tube could ruin the mechanical filter by placing the full B+ from the power supply across the winding and burning it out. Collins used a 600v disc ceramic for the blocking cap with the upgrade, but even those can short out; I replaced it with a 3 kV cap in all of mine, even the ones with the factory upgrade, figuring that a 3 kV cap would be less likely to fail with only +230v or so on it, than would one rated for just a little more than double the working voltage.