After letting it cool down I did a look see around to find anything obvious to cause the "pop". Everything looked fine, fired it back up without the preamp running and did some voltage checks, all normal. Turned it off again and "pop" kind of loud. What I found out to be the problem was one of my driver tubes, an old 59 RCA clear top 12au7a. It played fine but pops during shutdown.
I had some more of these tubes and replaced it, and everything is good. I'm just curious if anyone has encounter this before?
Terry from Tennessee