GT-6550-R Duet Highest power pentode for use in Marshall, Hiwatt, Ampeg SVT, the GT amps and a few other higher powered amps. Also popular for HiFi amps. Newest entry, power and quality should mean long life. Excellent response, yet it"s easier to push into distortion than "A" version, a little harder sounding than "C". Country of origin: RussiaCountry of origin: Russia