666-R Tube Preamp Schematic

6J7 day!

I think i understand the love for that one, It's the geekiest tube!

You can sculpt its sound so much that it could be a tube I might be afraid to use!
Because you wouldn't want to limit its abilities due to your design, or if you're using it musically, if you don't succeed at making it shine as much as it could, hitting it from the right angle, it would add a pressure lol!

If you show this tube to someone, you'd want the person to have the whole experience, not only a shade of it.

It's an instant great sounding tube. I'm still a little bit agitated from having played with it, i could say a lot of things:

On a set cathode resistor value, it's not as 'whole' as a 6sn7 can be. If you use gear that has one, and the cathode resistor is around 1k, you won't have all the low end it can give! It will be a nice impression, but not the complete experience of the tube. That's why i found that it's the geekiest!

With other tubes like the ef86, you set your stuff, and bam you've got your immediate reaction back, you don't think for a second that you'd change anything lol

Whereas with the 6j7 i spent my time 'what if we try that?' around it, i even added a NFB to it, it's not necessary, but i wanted to see how it sounded compared to the ef86 in that regard. It controls nicely the highs frequencies, maybe it neuters them a lil bit too much

The cathode bypass capacitor can be switched on and off as well, both ways sound fabulous!

I put 980R by default as a fixed cathode resistor, but it was more as a center point where the anode is at 79v, i'm sorry, i've got my numeral obsessions lol
720R would be a super cool starting point if you use a fixed resistor, so you have all the low end, and it's still super musical and usable!
560R is super possible as well, it's really bass heavy!

In my opinion that tube is such a jewel that i would invite you to put a potentiometer on its cathode, so you have all the colors it can give!

I called it 666-R because i loved the cathode resistor at 666R! I had put it as a value, but i knew people could miss what the tube can do if it's their first impression, so i picked a balanced value. For my use i think i would set it from 560R to around 820R

If i had to give you an equivalent for synths, the 6J7 is the Access Virus Ti (my favorite polysynth), a 6sn7 / ef86 is a Moog

The sound files i added 720R, 960R versions so you can hear how the cathode resistor affects the low end

Sound files:

Schematic: