Dear Rowdy.
I'd love to clarify my position.
Let me use your Sports analogy, which I find interesting.
I do not see myself as a Pro Football player laughing at a High School team; that would be a totally useless (and childish) attitude and would not help them at all.
Rather consider me an old, grumpy, seasoned Trainer, yelling "c'mon guys ... faster .... faster ... catch that ball ... run ... show your training ... don't be lazy !!"
and so on.
The drain trimpot "solution" *is* being lazy, there's no other way to put it.
If you can find another word to say it, please do, I'm open to suggestions.
In fact, I *do* use Fets as gain blocks since the early 70's, have built "Twin Preamps" with them for clean sound, have used them after some Op Amp stages just to provide "tubey overdrive" ... but I select and bias them !
And I had to work a lot to be able to get some consistent results, because way back then, all we had in Argentina was the locally produced Texas 2A264 , the local labelling for Texas Instruments TIS58.
No fancy MPF102 for us.
The parameters were all over the place; Vp could be anywhere from 1.5V to over 6V.
Idss and Gm would vary as much.
I had to buy 100 of them , build some kind of socket out of a DIP8 one, (no Protoboards way back then), lay a row of labelled tuna cans and start testing and sorting.
Try this, 15 seconds per FET:
Just to see the results of the "lazy" approach and the "simple" one, please build these two versions of the classic "Marshall gain stage" side by side, plug your guitar, then into a clean guitar amp, and play a few chords through each of them.
Please comment . An MP3 wouldn't hurt either.
Please see that this humble solution is the simplest way to do it, no fancy math, no elaborate Lab equipment.
It takes practically as much time and effort as the "magic numbers" version, and is *much* more consistent, not to mention that it has more gain and sounds closer to "the real thing".
Why I chose a 2.5Vp FET?.
Choosing among general purpose cheap FETs, that VP value is easily achieved by most; those few closer to 1.5Vp are higher quality ones (higher Gm) and reserved for higher gain stages, if needed; those 3.5Vp and above make bad preamps but excellent switchers.
Definitely you waste nothing, although you should order a few more than needed to make matching possible.
