Fet version of Marshall Superlead Plexi

[rowdy_riemer]

Absolutely agree. One of my favorite circuits, the Dr. Boogey, sounds pretty good, but I hate how it too uses a trim pot for the drain resistor to adjust the bias. This makes the problem with variation in FETs even worse. I've been toying with a variation using trimpots for the source resistor instead. Don't know if it will be much better, but doing so might give more consistent results. Obviously still not as cool as your biasing scheme, but with my time and resources, I've got to stick to simple.  Simple might be ok for the Dr. Boogie concept anyway. I don't know that it is as critical in that circuit that the jfets clip just like tubes.

Some time in the future, though, I might try to build one of your designs. The sound samples sound bad ass!!!

[J M Fahey]

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 !!"  :trouble 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.

[rowdy_riemer]

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 !!"  :trouble and so on.
The drain trimpot "solution" *is* being lazy, there's no other way to put it.

Ok, that's fair. Sorry for taking it the wrong way. As for the rest of the post, it's preaching to the choir.  :tu:

[rowdy_riemer]

Ok, I take back the "preaching to the choir" thing. You're retelling why the drain biasing scheme is lazy is still interesting, and you've added a little extra insight. Good stuff.

[KMG]

Several additional reamp samples at full power (PA overdrive).
http://milas.spb.ru/~kmg/files/projects/slp/reamp/fp/

[J M Fahey]

Congratulations. Very good sound.
Loved the raw ZZ sound and the chunk in Criminal.
Did you use a noise gate on Classic ? Or some equivalent software process?
The best, of course, is Almost.
Thanks for posting.

[KMG]

The only processing is volume normalization for all samples.

[J M Fahey]

OK, thanks. Keep posting if possible, your friend does play well and has a good tone.

[joecool85]

...your friend does play well and has a good tone.

I'd say so!  I wish I could play like that.  Maybe with time.

**edit**
Is "Classic" the beginning to "You Shook Me All Night Long" by AC/DC?

[KMG]

It's not yet my friend, reamp of DI samples downloaded from several Internet sites.
http://milas.spb.ru/~kmg/files/distr/DI%20Samles/
Additional 3 samples:
http://milas.spb.ru/~kmg/files/projects/slp/reamp/fp/Country%20Esque.mp3
http://milas.spb.ru/~kmg/files/projects/slp/reamp/fp/Kid%20Paul.mp3
http://milas.spb.ru/~kmg/files/projects/slp/reamp/fp/LP%20Delay.mp3

[gbono]

Sorry, joining this discussion late. Why the OT in the MOSFET PA? Gilding the lilly or does the bandpass like response and non-linear transfer function of the OT add to the "tube sound"?

I would love to see a FET (JFET that is) design for the preamp.

I have used this source for JFETs - very good process control, screening available and good product offering http://www.linearsystems.com/

[KMG]

Sorry, joining this discussion late. Why the OT in the MOSFET PA? Gilding the lilly or does the bandpass like response and non-linear transfer function of the OT add to the "tube sound"?

Some answers to your question you can find there.
http://www.ssguitar.com/index.php?topic=1981.msg12953#msg12953

Small test "Who is who". Reamp samples of the tube "Bogner Ecstasy" (plexi mode) and the FET Superlead.
http://milas.spb.ru/~kmg/files/projects/slp/reamp/Classic%20T.zip

[gbono]

Thanks for the links -

Low output PA impedance lead to hard speaker dumping, that doesn`t allow cabinet to "breathe".
Look at frequency response of transformer output fet PA loaded on active load and cabinet (voltage on PA output).

It seems like an OT is an expensive solution to solve the problem - damping factor of a tube vs SS design. What if the "soft clipping" of the signal occurs not in the PA but in the preamp stage?

BTW I have a very good friend who writes and speaks Russian language (he is from Moscow) if you have more papers/information.