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Replaced KT-90s with St Petersberg Svetlana Factor...
John Morrison
The new tubes are from the legendary St. Petersberg Svetlana tube plant and were purchased from one of our new members who was lucky enough to have purchased some in the past. These were installed by Sam Papadas into the tube amps he designed and built. It required re-biasing the new tubes. 
 
We listened to the KT-90s for about an hour and it took about 30-45 minutes to install the new 6550s and get the bias set and stable. After the bias stabilized we listened for the rest of the afternoon. The differences were startling and heard by everyone. The KT-90s were warm, detailed, and musical sounding with lots of deep, rich, low end. A good description would be "sweet". The 6550s were still incredibly musical but much more focused with better detail. The 6550s created a more three dimensional presentation with additional speed and midrange clarity. The bass seemed quicker, but with slightly less impact. After the guys left, I listened for a while longer and found myself unable to skip the vocal cuts on our reference tracks like I do normally. Normally I skip them for the instrumental tracks... but not now! I found myself listening to all of the tracks and really enjoying the vocals!
 
I am certainly glad Sam talked me into the purchase!
 
Later Note: We decided, after some discussion with other designers who use these tubes that the Plate Voltage we are running in the amps may be a little low for the KT-90s to develop their full "goodness". Since we would have to replace the existing power transformer to check this out, it just ain't gonna happen!
 
John