Soviet copy of 6BK7/6BQ7. Sometimes falsely advertised as equivalent E88CC (to push the sales) which is not true. Curves are quite different so operating point will be sub-optimal when interchanging. Very linear and good sounding triode but has same problems as all soviet tubes – large quality and parameter variance. This particulate example always has traces of higher harmonics (below -110dB). Other factory/year – can have same clean spectrum as premium Siemens E88CC, but for a 1/100 of a price. Available for sale.