Calvin at ARC told me the KT-120's should not be used in an VT-100's. I'm sticking with my 6550's till further notice.
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I replaced my Svetlana 6550 winged C with Tung Sol KT 120.
I got 800 hours on the tubes and everything is working perfectly.
I love my amp but always felt the low end was a bit week. Not so much anymore. When my friend heard my amp with the KT 120 he said " shit, solid state lows". It sounds more robust and powerfull.