If the A3 you have is a low serial number (below around 1000 I think but I have to check to make sure) you need the B type mech bathtub filter; if it is over that number you need C type. Collins changed the capacitance in the interstage location there from 330 pF (B type) to 130 pF (C type). I went through the same search for my A3--a proper B type filter for 6 kc or wider is practically unobtainium, or if one surfaces for sale it is usually a small fortune.
ER has had some articles in the past about filters for the A3 (which I found out
after I worked something up
)
See ER # 64 "Putting the 75A-3 on AM"
I wound up buying Murata ceramic filters from Mouser--they are around $3 or $4 each. They have one that is listed as 2.5 kc. But for some reason, Murata does not give passband width--that's an audio spec so it is really a 5 kc filter. The skirts are not very steep so for all practical purposes it is 6 or 7 kc. They have wider filters of course. I have found the 5 kc filter to be about perfect for ham AM. on the bottom of the filter there are three pins for ground and two for in and out. Filter is symmetrical so which are in and out doesn't matter. It needs a blocking cap, something around 500 pF 600 v, and load resistors. I have uploaded a photo. I don't remember the R values but you can see the colors on two in the foreground for a circuit that is just an interstage coupler (making the rx too wide so I eventually put a ceramic filter in that one too). Ignore please the ridiculous use of four disk ceramic caps--that was all I had on hand to cobble together to get a few hundred pF for B+ blocking
The hardest thing about the filter was getting a perf board and putting pins on it to seat into the filter socket so they mate up right and are the right gauge for the holes. I found stiff wire that matched, cut and drilled the holes on the perf board and used epoxy to hold the pins in the board. The cost of all the parts and materials was probably $20 though, maybe less, way better than a $200 mech filter
and for AM, I like the ceramic filter better actually because strong signals come in past the skirts somewhat for a higher fidelity sound but weak signals that are harder to copy are only on the top of the filter response. If you have the stock 3.1 kc mech filter hang on to it because it is very good during battle condx on 75 meters.
With the right filter, prod. dectector mod and pulling the audio out from the AF gain pot wiper to an external p.p. tube audio amp you wind up with a pretty good AM rx.
Very important: the slots in the A3 are not wired exactly as shown on the schematic. With the mech filter it doesn't matter but for other filters it does. looking in when facing the front panel, the signal path is between the outer two sockets front to back; common is the inner two pairs front to back with the grounds at the two wider pin sockets centered on each slot.
I will try to find a URL for the Murata filters I picked from.
here it is:
http://www.mouser.com/Passive-Components/Signal-Conditioning/_/N-8bzuiZscv7?P=1z0wkx5Z1z0z5ndZ1z0z7l5Z1z0z5h6Z1z1414h&Keyword=murata+filter&FS=True
Unfortunately Mouser doesn't put the 6 dB width right up front on their parts listings
so you have to click on each one and navigate to a spec. page to find that. You have to click on the data sheet link. The spec for passband will say somthing like: 6dB Bandwidth fn±4.5kHz min. the +- is the giveaway that in this case it is a 9 kc IF filter. It might be better to look at the Murata filter list, get the part no. and go back to Mouser:
http://search.murata.co.jp/Ceramy/PnsearchViewAction.do
