A standard ringer coil will respond to the nominal 48 volts DC of a standard voice line. For this reason, the capacitor is installed in series in order to prevent it from doing so. If the capacitor were not in place to block DC from flowing through the ringer's coil, it would place a short on the line.
That being said, even 48 volts
AC at 25-30 Hz will operate many ringers. The fact that the 50 volts being presented causes some of your ringers to respond probably covers 75% of the electronic and even cheap mechanical ringers out there. The problem is that more voltage (typically 77 volts or more) is what is and was expected by ringers that involve a significant amount of clapper throw during operation. I'd be willing to bet that you are actually receiving more than 50 volts, but since most AC test equipment is designed for 50-60 Hz, your reading may be a bit off.
You might want to try adjusting the bias spring between the gongs to the low position. This may help the clapper to travel more under the low voltage condition.
With the exception of a ring booster, it is sad to say that traditional C4 type ringers are going to become more and more difficult to accommodate. Viking Electronics makes a ring booster as well, but it isn't cheap. The model number for it is RG-10A and it can be viewed at their web site
www.vikingelectronics.com .
