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Those National professional amps are super-rare. I've never seen one. However, Valco was also making the Aztec during the same era which also had a 5879 input stage, and I would hazard a *guess* that the 5879 input stage at least was probably the same as a the Aztec. A guess!!!! I don't know for sure. Gibson also used a 5879 input stage in one version of the GA20, very simple with a 100K plate/470K screen setup, I use that a lot and it sounds great. You have to be careful with 5879, as typical high value resistors for plate/screen which work fine with 6SJ7 do not work well with 5879. Pretty unique little pentode.
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