Before the Bell- Calpine Drops 6 Percent  NEW YORK (Reuters) - Shares of Calpine Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=CPN.N target=/stocks/quickinfo/fullquote"&gt;CPN.N&lt;/A&gt; fell 6  percent before the bell on Tuesday following reports that one  of its joint ventures received a default notice.