trinityamps.com

https://www.tubesandmore.com/tech-corne ... g-resistor Found this under Tech Articles at Antique Electronic Supply website. Amplified Parts has the same. Handy for someone like me,

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"The only problem with being retired is you never get a day off"

Thanks for the link. It's a handy tool.

It's just using Ohm's law, V=IR, rearranged to find R. That's R=V/I where V is the supply voltage minus the LED voltage drop.

I usually just eyeball it, though, and pick a resistor that gives the brightness I want.