Running JFet's from 9V - why a potentiometer?

[meffcio]

Hi guys! Maybe it's a dumb question, maybe it has been asked before, I don't know.. But looking at this case, everyone's still running jfets (like j201 etc. - for example in tube simulating circuits like DR. Boogey, FET-2B etc.) from 9V with a potentiometer/resistor lowering down the voltage to around 4,5V. What the hell, why is that? When the battery voltage drops down a little, the voltage on transistor goes down very hard. Wouldn't it be more usefull using 7805 AND a resistor to drop down to those 4,5V? The voltage difference is much lower then - I think that effects would run much longer. So, why nobody uses this method?
And btw. Is my post written ok? I'm a Pole and I love English, so I'd like to know how many mistakes I make, and how to correct them.
Regards,
Meffcio.

[BAARON]

The point isn't to "run" at 4.5v.  The point is that when the JFET "amplifies" a signal, you get a voltage swing across the drain resistor/potentiometer that both goes positive and negative, relative to whatever the idle voltage is... but the voltage cannot swing farther than what your power source can provide (i.e., 0-9v).  So if the idle voltage is 4.5 volts, the signal can swing about ± 4.5 volts before it reaches the limits of the power supply and starts clipping.  If the idle voltage was at 8 volts with a 9v supply, it would only swing positive about 1 volt before it started clipping one side of the waveform.

Thus, setting an idle voltage of 1/2 the voltage supply across the drain resistor gives you the most voltage swing/output from that JFET before clipping occurs, theoretically speaking.  So you see, the JFET isn't "running" off 4.5 volts.

[meffcio]

Oh, now I get it. Thanks mate.
But I still think there should be a better method.
Anyway, Admin/Mod, You can close this short topic. xd

[MohiZ]

There's a whole thread arguing about that same thing  Apparently it's just the easiest way for the designer to bias the jfets, albeit it is an inconvenience for the builder and it allows for differences between builds.

http://www.diystompboxes.com/smfforum/index.php?topic=70065.0