I have been using these tubes for about (2) years now in my Marshall JCM900 Amps. I usually buy a bunch of duets & rematch them in my Maxi-Matcher Digital Tube Tester - That way I can get them 1-2MA apart. These tubes last a long time when biased correctly - I ordered the medium/green duets & tightly match them into quads - they are very clean & clear with good breakup but still stay warm & dynamic - they also have tight vacuum with solid construction. They are a very underated tube - They are also a good alternative to standard Sovtek 6L6 Power Tubes - Great Value & Great Tone - Highly Recommended!!

I purchased two pair of Svetlana Duet tubes, one medium/green and one hard/blue. Note that these are sold as Duets, not matched pairs, because they most certainly were not matched. One tube of the medium duet was basically dead.

The hard duet had bias currents that differed by over 15%. I swapped tube positions and the current moved with the tubes, so it isn't the amp. I had to use the remaining medium with one of the hard ones to get a pair matched within 10%. Unmatched tubes with a 25% failure rate turn out to be very expensive.