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The 7581 is part of the category of 6L6GC, it is Russian production. Warning, the name 7581 can be confusing in trying to recall the old and found Sylvania 7581A, American manufacturing. Both tubes have in common than being modified 6L6GC: Tung Sol today is not so much as the old Sylvania 7581A remains, even today, the only tube type 6L6 to never have had a maximum power of 35W. I remember that 6L6 (also the 5881 and KT66) can diffuse and 25W 6L6GC can broadcast 30.

That said, in my ears and in my amp (Egnater Tourmaster 4100 head), the Tung Sol 6L6GC / 7581 wonders. I find his tone more defined, more transparent and clearer than those I mention below. There is a lot of headroom. As clean as saturation, the bass is precise, beautifully tense. Beware of bread, even big fat crunch blues and drooling on a handle P90: we hear.

Here's the story: the amp is new and original RUBY 6L6GC tubes were too. Having to move my gear when not cooled, the inevitable incident occurred and I had to replace my power tubes. As I was told the good of the new Tung Sol, I chose them out of curiosity. In other amp in 6L6, I also had Mesa / Electro Harmonix, JJ, Chinese badged Tube Town.

I have biasées to 34mA for a plate voltage close to 500V.