Art, Correct, without the common possibility of arc over that the 866A mercury vapor tube may exhibit. Here is a helpful link: http://www.tubecollector.org/866a.htm Joe - KB9R Artmouton wrote: > Looking at ebay for similar tubes and found one sale that indicates > that the 3B28 is drop in replacement for 866A. > Can anyone confirm that info. > 73, > Art K5FNQ >