Just some of my thoughts on the EF-5. I purchased mine 3-4 years ago. I'm guessing it's the first revision since my pcb doesn't have a cut out in the front where the volume control is. My pcb appears to have more parts on it than the one shown here. I tried to install an Alps Blue pot and even a smaller more standard size pot such as Radio Shack sells, but neither will fit due to the pcb coming all the way to the front of the cabinet. I have ordered a small Noble carbon pot to try but haven't received it yet. If you want to check your amp to see which pcb you have you can remove the four screws in the front panel and pull the front panel forward to see if your pcb has a cutout such as the amp shown here. The pot which came in my amp is a small Alps which I think is a plastic conductive model and doesn't have the clicks.
As far as changing the OP275 opamp, be aware that some other opamps may not be ideal in this amp (or any other amp). The 'audiophile' opamps all need some form of compensation to work in a specific circuit and just replacing an opamp with another opamp that isn't optimized can cause problems. I tried other opamps in my amp but they all ran hot temperature wise which means they may not be totally suitable for this circuit. If you try another opamp or dual opamps, be sure to check the top of the opamp with your finger to see if it is getting hot. Some heat is probably ok but if it is too hot to keep the tip of your finger on then I personlly wouln't leave it in. I have seen an opamp oscillate in a circuit (not this amp) and get red hot in a matter of seconds and it will burn up if the amp is left on. In other words, just replacing one opamp with an 'audiophile approved' opamp is not always a good thing. Since my amp is an older version it may not 'like' other opamps, so the newer version may be fine, but I would check just the same.
As far as tubes go, try a 12BH7 or an E80CC/6085. I personally like these better than the 12AU7. Just like opamps just swapping to a different tube number may mean the amp isn't truly optimized for that tube, but it seems to work fine in this amp and won't cause any problems in use. I agree this is a great amp and not as appreciated as it should have been. Head-Fi is often focused on the latest and the greatest at the expense of older amps that are as good or better than the current models. If you can buy an EF-5 used for $250-$300 I don't think you will regret it. I will try to take some pictures of my older version amp when I finish working on it so you can see the difference between mine and the newer version shown here.
Good write up and observation. The cut out in the pcb of the later version seems to enable users to fit a bigger pot or originally the manufacturer wanted to use an alps pot or similar type as the space is just fit for it.
As to the opamp some of them have lower operating voltage thus will heat up or burnt if used in EF-5. Therefore check datasheet on operating voltage before swap. I agree that some opamps are less stable and oscillate by themselves if capacitance are not big enough. Check the data sheet before swapping. However the one I used is a very good opamp which is very stable and you may wish to try it out.
Edited by inter voice - 8/21/14 at 7:31am