Electro-Harmonix 7591A EH Duet Platinum Matched (Pair) The 7591A EH tubes are mooth, yet punchy, with a sweet harmonix finish, this year"s vintage from Electro-Harmonix. A real 7591A! Ampeg owners, rejoice! Matching power tubes for equal current draw is important for maximum performance. Electro-Harmonix"s platinum matching process consists of a 24 hour tube burn-in period followed by a bias (plate) current measurement in milliamps to ensure accurate matching. All Electro-Harmonix tubes are made in Russia.