I just finished changing out the chips in my deluxe big muff pi. Mine is the 78 version with RC4558NB chips. I had some 741 chips laying around that I was going to use just because I had them. I checked the data sheets for the 741 I had, there several brands on the market. It is good I checked them because the pin outs on my 741 chips were slightly different from the 4558’s and they would not have worked in my big muff……I ended up buying some JRC4558D’s from ebay, 4 for less than $3, these are the chips everyone raves about for the TS808 Mod. I changed out 4 chips, I changed the compressor chips also. To make a long story shorter, they work and sound great. I think they sound better than the stock units. So check the data sheets before changing any chip. The reason I changed them in the first place is because the original ones had failed.