rebias jfet circuits for 12v supply?

[Beo]

On my phone so google powers are limited.

I'm looking at amp sim jfet circuits like bsiab2, dr boogie, plexi-drive, etc. They all seem designed for 9v with jfets biased near 4.5v (sometimes but not always with a trimpot). I like to run my pedals at 12v for more headroom, and I use 9v/5v regulators in my circuits where needed.

What happens when you use 12v into one of these biased jfet circuits, which don't have 9v regs? Do they need to be rebiased to near 6v? Does this take the jfets out of the tonal sweet spots, if such exists?
Thanks, and sorry if this is noob or rehash.

[Tony Forestiere]

I am going to say that the voltage divider that produces Vref (Vb) doesn't care what voltage comes in. If the JFETS are biased to 1/2 V+ at 9 volts, Vref should still be 1/2 V+ at 12 volts. Or I could be showing my ignorance AGAIN.

[Beo]

That simple, huh? I wondered if the effort to match jfet vgs meant that there was a sweet spot with the 4.5v value. Maybe they just need similar characteristcs, but as you say, can scale up with source voltage.

[Tony Forestiere]

I haven't built any of these circuits, but the reading I have done says most folks start at 1/2 v+ bias, and then tweak each stage "to ear".
*edit* spelling error

[PRR]

There is no absolute magic in "4.5V".

There is some preference for "half the supply", more-or-less.

Try it.

[teemuk]

I wondered if the effort to match jfet vgs meant that there was a sweet spot with the 4.5v value.

4.5V bias just theoretically allows highest voltage excursion on both negative and positive half waves. Note: Theoretically. In practice you should also consider source voltage's effect as drain voltage swing isn't exactly 0V - Vcc. It's at least the source voltage lower than that.

On the other hand, the tube circuits these FET circuits try to mimic are often biased for deliberate asymmetry (and often very notable asymmetry on that matter) so....

Anyway, no matter where you set the drain bias at though, the stupid drain trimmer design will ensure that each FET stage will have different Zout, different gain and different frequency response as these characteristics are more or less dictated by the drain resistance ....which will vary with trimmers. Unless you can design in something that actually minimizes the spread in FET tolerances you can't really build these circuits in an uniform fashion. That's at least where a little device matching comes in. With somewhat matched devices the drain resistor values end up at least close to same ballpark values.

I wouldn't worry too much past the point where the FET biases to be operational unless you are going to build more than a single unit and want that each of those units sounds the same. The matching comes in when you have several FETs and you insert them to same circuit design and want that the circuit still operates the same way. e.g. building two effect pedals of the same design. Regardless of any matching and biasing the operation will be WAY DIFFERENT in comparison to the tube circuit the FET circuit tries to mimic anyway.

...Other than FET bias I would worry about voltage ratings of other components.

[Brymus]

That simple, huh? I wondered if the effort to match jfet vgs meant that there was a sweet spot with the 4.5v value. Maybe they just need similar characteristcs, but as you say, can scale up with source voltage.

IIRC I used my dr Boogie with a wall wart rated at 12V it actually put out closer to 15V into my Boogie,but yeah it worked fine and seemed to sound better,but that may have been wishful thinking.If I still had it I would go try for you,but some kid made me an offer for it I couldn't refuse and I haven't built another yet.