Quite the coincidence that I'm reading this post today since I haven't been on this site in quite a while (I apologize). Anyhow, let me tell you of my very recent experience. I decided to 'dust off' my old Nak 300s after having them sit for over 10 years. Naturally the Eveready 206 batteries were dead. I ordered the Exell 206A /NEDA 1611 replacement batteries from "Brooklyn Battery Works". Their site listed them as the proper replacement for the older Eveready 206. Being that they were both NEDA 1611, I thought they would be fine. They just arrived and are a hair longer and wider (perhaps 1-2 mm) than the Eveready batteries. Of course that small difference in length shouldn't make a difference since one end of the battery basically goes on a spring. However that slight difference in width made all the difference in the world! They just would not fit!!! So I contacted "Brooklyn Battery Works" and told them about it. One of their web pages states they are compatible with the Nak 300s, however the Amazon link I bought them through apparently says no (but I didn't see that page). They will send me an RMA to return the batteries and will be issued a full refund. I was also told they will change their website. I was told that over half an hour ago. They said it should take a few minutes, and I just noticed it still has yet to be changed.
I was still upset with the company Exell. Somehow I was under the impression they were to use the same specs as the original Evereadys. When I called their number, I found it is a combination of Exell and "Batteries In A Flash". Anyhow, I spoke with a guy (Raoul?) in their tech department and he said when Exell first started making the batteries, they had a bunch of calls from disgruntled Nak 300 owners. He claims they got their specs within a range they found from different manufactures (Eveready being just one of them), and the Exell batteries are manufactured to fall with that range. A side note here...is that true? Were their other manufactures of the 206 other than Eveready? Or is he just saying that to defend his company? Who knows?
Anyhow, he also advised me against shaving the outside of the battery to make it fit, since the Naks are metal and not plastic.
In summary, my suggestion is
do not try the Exell 206A batteries. Even though they have the same NEDA number (1611),
they will not fit in the Nak 300 mics.
Needless to say, this has been a very frustrating experience.
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As a follow up to my message above, I have since ordered the 4LR44 6V alkaline batteries (apparently often used for dog collars). I just recorded two shows with them and they work perfectly. These batteries are half the length of the original 9V batteries, so you'll need two of them for each mic. They are a little bit smaller in diameter than a AA battery, so you'll need to figure out some way to get them to stay end to end. Others have suggested taping them (with duct tape or electrical tape). I didn't want any tape residue in my mics, so I tried something different and it works very nicely. I took some card stock (110 lb. paper), but I suppose you could use thin cardboard and cut strips that were 5 cm in width. I rolled the strips up and inserted them into the mics. Now my batteries just slide in easily, stay in place, and after the show they slide back out. The only downside of this method is the batteries are small and it's often dark after a concert, so it would be easy to lose one of them after you take them out of the mic.
I have to say that I was nervous about using 12V in each mic since the original Nakamichi specs called for batteries to be in the range of 8.4V - 9V. I didn't know what negative effect it would have on the mics or the recording. Some place I read the mics will take anything less than 14V, but I was still nervous. Well after recording two rock 'n roll shows with them, I am now a happy camper!
Furthermore, after recording a couple of show I was curious if the batteries would show any drop in power. I tested them and there doesn't seem to be any loss at all. Wahoo!!!