If used the 6S6 bulbs as well, but rather than add a blob of solder to the tip, I have used a large drill bit to widen the hole in the base of the light so the bulb reaches further down into the socket. In a couple instances, even that didn't suffice to bring the bulb low enough to make contact in the socket. So now for my vintage C7 tulip bubble lights, what I'm doing is removing the longer bulbs currently found in new bubble lights (www.noveltylights.com;
less than $12 for a set of 7) and using those as replacement bulbs. By soaking the bubble lights overnight in water, and using a razor blade to carefully split the top and bottom halves, the bulb can be carefully wiggled out and then put in your old tulip bubble light. A bit labor intensive but the results are the best I've come up with.