We bought many cases of these from Groove Tubes. These are from the same batch of tubes Groove Tubes matched, graded and packaged. Each tube of the quad set we ship will measure the same mA plate current. All tubes have also been operationally tested by Angela Instruments. KT stands for "kinkless tetrode", developed by the British Marconi-Osram company in the '30s to address the non-linearity of most 6L6 family tubes. These tubes look like the "Valve Art" Shuguang KT66 still sold (for much more $$$) by many tube dealers worldwide. I've got twenty year old Valve Art tubes in my home hi-fi amps. If you've been buying "Groove Tubes" or Valve Art KT66 tubes, save your money and buy these instead. I think our testing is better and we actually listen to the tubes we sell. These tubes are sold strictly "AS IS" with no warranty whatsoever. We are almost sold out so we soon won't have any stock to pull replacements from. When you get our tubes try them out in your equipment and make sure that they're working for you. If they don't meet your expectations let us know ASAP and we will authorize a return for a refund. Tubes must be returned in working 'same as shipped' condition. Buyer is responsible for all shipping charges including returns.