FET and Tube conversion

[Steben]

Am I an complete fool or not regarding the tube-to-fet aproach?
As I see it, there is a slight difference in gain and headroom.

A single coil guitar signal can shoot around 200mV, with a "cruise" voltage of 100mV.

Tube (pre)amps often are powered by + 200 Volts (100 Volts mid).
12AX7's usually give a max gain of 100 to less, sometimes they are biased less.
With four stages full on you have
1= 0.1V in 10V out (then some loss) no clipping
2= 5V in 500V out (5 on 1 distortion) slight clipping
3= 100V in 10000V out (100 on 1 distortion) severe clipping
4= after tone controls around 100V in, yet usually master volume

Lots of FET emulation (pre)amps are powered by a brittle + 9 Volts, sometimes 12. Let us take 10 (5 volts mid) for the easy math (yet 9 is in reality less headroom).
FET's usually give a max gain of 100 (J201) to less, yet let's take 50 in account (which gives another reserve in my statement).
With four stages full on you have
1= 0.1V in 5V out (then some loss) on to clipping
2= 2.5V in 125V out (25 on 1 distortion) medium clipping
3= 5V in 250V out (25 on 1 distortion) medium clipping
4= after tone controls around 2.5V in, yet usually master volume

Are this differences taken into account when designing emulations?
I mean: at full blast all will sound similar. But when I look at those +6 stage emulations, I really wonder if it isn't plain overshoot, given the headroom difference.

[R.G.]

I keep telling people over and over: JFETs are not tubes, and will only sound reminiscent of the circuits done by tube-to-JFET substitution.

If someone likes the sound they get this way, fine. But no one should be deluded that they're the same as the original tube circuits sound.

[Dragonfly]

JFETs are not tubes...

Simplified for clarity.

[bancika]

I agree fets are no tubes (I'm tube guy myself), but I don't understand your analisys.
What 10000V are we talking about? Standard preamp tube could not possibly create voltage swing higher than 200V p-p, the same goes for 250V for fets.

[Steben]

I agree fets are no tubes (I'm tube guy myself), but I don't understand your analisys.
What 10000V are we talking about? Standard preamp tube could not possibly create voltage swing higher than 200V p-p, the same goes for 250V for fets.

I'm talking virtual amplitude.

If the gain is 100, you would have 500V with a 5V input. If the supply voltage is 10 V however, you get 500V cut by 10V treshold. It's like a clipping diode at 10V. 50V would be cut too, but the edges will be softer, you are hitting the 10V simply less hard.

It's like a punching bag.
If you hit it soft, it bounces less than when you hit it hard. In both cases however, the bag won't fall off.

What I'm getting to is that tubes have a much higher treshold, yet their gain is in the same region as the fet's. It seems to me you would have to "at least" scale the gains of the fet's or use a much higher supply voltage.

It works fine as long as you want high gain stuff. High gain distortion is in the first place sensitive to equalizing-shaping.
Low gain, soft compression, dynamics, .... I would say the BSIAB(II) gets there really close. The circuit on the other hand is not a copy of tube circuit at all!!

[Caferacernoc]

"I would say the BSIAB(II) gets there really close. The circuit on the other hand is not a copy of tube circuit at all!!"

That's not really true:

http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm

[d95err]

Are this differences taken into account when designing emulations?

It's been quite a while now since Runoffgroove abandoned the part-for-part copy for tube-to-FET circuits. The gain/headroom difference is exactly what they're working with in later designs like the Thor. They also use other means than FETs to emulate frequency response and harmonic content.

[B Tremblay]

Are this differences taken into account when designing emulations?

It's been quite a while now since Runoffgroove abandoned the part-for-part copy for tube-to-FET circuits. The gain/headroom difference is exactly what they're working with in later designs like the Thor. They also use other means than FETs to emulate frequency response and harmonic content.

I'm glad someone noticed!