The Neumann U77 is an excellent mic.  I have owned one for a number of years and, in my opinion, it always sounds better than my U87 mics. The U77 has a tighter and extended bottom end and a more open top than a U87.  The U77 is a distinct design and is different from either the U67 or the U87. The capsule is the same as the U67 and is powered at the same voltage as the U67 (60V) unlike the original U87 (ca. 45V.)

To use a U77 you need a special 12 volt "T Power" supply but they are easy to find and I purchased one from Tracy Korby.

The circuit boards in the U77 are TOTALLY DIFFERENT from the U87 and to say the U77 is just a 12 volt version of the U87 is not correct.  In addition to the different capsule polarization voltage, the major difference between the U77 and the U87 is that the U77 is transformer-less.  (If you exclude the hefty choke- trafo- K.H.)

This was a major departure for Neumann when the mic was designed in the mid 1960's and was possible, at the time, only because of the special 12 volt T Power system used.

The U77 is great for vocals. If you sound good on an U87 you will probably sound better on a U77.  I also use the U77 for a (not too close!) kick drum mic with excellent results as the transformer-less part can handle the dynamics without distortion ( not so- the headroom spec of the mic is identical  to that of the U87- K.H.)and the mic has a great tight low end.

Compared to a U67, the U77 is not as dark. Being solid state, it lacks some of the "creaminess" of the U67 but has greater clarity, lower noise, more punch, and is easily one-half or less the cost of a similar vintage U67.

The U77 was designed as the flagship mic of the Neumann F.E.T 70 series mics introduced around 1967 - 1968.  At that time, 48 volt phantom power was not an established standard so Neumann was not constrained by the limitations of the low current of phantom power. (See David Satz's post, below- K.H.)

The U77 is not the only mic in the F.E.T. 70 series.  There were several others, all good, the most famous (other than the U77) would be the KM74. These are also great and I own and use 2 of them almost every day.  In my opinion, the KM74 sounds even better than the KM84.  

Both the U77 and the KM74 are real "vintage sleepers" and are a bargain because they are not well known.  The U77 and KM 74 can not be converted to phantom power without totally replacing all the circuit boards and there is no reason to degrade the sound by doing that when correct power supplies are readily available.  

Both of these mics were designed to be used with period Telefunken mic pres like the V376 which is perhaps the best bargain in all vintage mic pres (I own 8 of them and they are as good as anything I have ever heard and the perfect match for a U77 or a KM74).

http://repforums.prosoundweb.com/index.php?topic=7203.0

The U77 is a classic mic and would be much more famous if they made more of them, but shortly after the U77 (and KM74) came out, both of which use 12 volt T Power, Neumann invented phantom power and it became very popular and the older 12 volt T Power mics seemed a bit old fashioned and fell out of favor. Neumann then came out with the U87 and KM84 and their literature claimed they sounded identical to the U77 and KM74 but used the newer and more popular phantom power. While the U87 and KM84 are both great mics, they don't sound the same as the U77 and KM74. They are related, but the lack of a transformer and the fact the T Power fed more current to the U77 than phantom power fed to the U87 make them sound different. I have spent a fair amount of time comparing the U77 I own to the U87's I have and to the U67 and in general would describe them like this.

A U67 in good condition is probably the king, it has a midrange thing going on and really brings out the "chest" voice in a singer. The problem with a lot of U67's is that they are in bad condition and sound dark, but a good one will have excellent clarity while still being warm Any time you can combine warmth and clarity, you have something special. The high end on a U77 however, is better and more extended compared to the U67 or U87. The U77 sounds more like a U67 than a U87 despite the transformerless design, but really the U77 has its own sound and is not identical to either a U67 or U87. The U77 is an excellent choice for a good singer with a clear voice and will bring out more "head" voice. Compared to a U87, the U77 sounds like someone removed a filter or a cloth over the mic and suddenly you heard everything more clearly. It's also faster sounding than a U87 and the U87 seems to "smear" things a bit in comparison and I assume this is due to the transformer in the U87 (but sometims you might want that sound). The output of the U77 is also much higher, more like the tube U67. Although "clearer" than a U87, the U77 is in no way harsh or "zingy" sounding like a modern Neumann transformerless mic.

U77's were mostly sold new in Europe and were purchased more by broadcast facilities and less by recording studios. They were also used to record sound for film and TV as some of the popular Nagra tape recorders used for that had built in T Power supplies. The U77 can also be run quite nicely by an internal standard normal 9 volt battery unlike the U87 (but the battery life is less than 8 hours). U77's came with both the 3 blade large Tuchel connector and with an XLR connector. You need to be careful not to plug a phantom powered mic into a T volt power supply.

I love all 3 mics (U67/U77/U87) but a U77 is something special. I believe it is the best solid state large diaphragm mic ever made. I think the added current from the T Power is a big part of it's sound. When you hit a U87 hard it tends to crap out because the phantom power can't give it enough current to reproduce a loud transient sound as good as a U77. The lack of the transformer also reduces the "smear" you sometimes get with a U87 by comparison. Both mics use the FET however.