Question on MOSFETS Vgs(th) biasing and headroom

[EricKnabe]

If I have two MOSFETS and one has a much lower threshold voltage, but they're both biased to their proper centrally fixed Q point, would the transistor with the higher threshold voltage allow for less voltage swing before cutoff?
In other words, would my guitar signal clip earlier with a higher Vgs(th) valued MOSFET?

[EricKnabe]

Not clip earlier but clip at lower input signal levels I should say. Have less headroom. You get the idea

[PRR]

No. Input overload is essentially proportional to Vgs(off) Gain isinversely proportional.

[EricKnabe]

I have a follow up question. BJTs that have lower impedance than FETs, does the lower impedance cause more signal attenuation?

[antonis]

Yes..!!

But that's recoverd in a high degree by BJTs much higher transconductance..
e.g. signal attenuation for 100k BJT (gm=40mS) and 1M for FET (gm=4mS) input impedance (bias circuits taken into account..) result into same recovery effect..

In general, for signal impedances up to 50k say, BJT Emitter followers are prefered..

[EricKnabe]

So that would explain why, I've been testing a transistor preamp circuit, and using MOSFETS provided me with more overdrive than BJTs. Is this why?

[antonis]

Can't say "why" without any circuit diagram..

MosFets & BJTs CE/CS amps doesn't exhibit "common" circuitry arrangement other than biasing architectural configuration..

[EricKnabe]

It is the bright channel of a Marshall 1959 preamp. Just the first two stages is what I'm testing. The 12ax7s were replaced with either MOSFETs or BJTs, and I'm biasing them with a voltage divider of 10.5M (two resistors in series)/1M (Using high values to prevent signal loss to ground) Both the MOS and BJT are center biased using this voltage divider but the MOSFET adds more gain and therefore distortion. They're running on a 20 volt supply.

[antonis]

BJTs don't like too much 10M-like bias resistors..

Although, in theory (ingoring base current), you biased Emitter on 1V (20x1/11.5 -0.7), you actually biased it at 1.7 - (I_E/h_FE X 10M/11.5M) - 0.7 V..
You had a voltage drop between voltage divider point theoretical value and Base voltage value of I_C/h_FE X 913k..
For a I_C of 1mA and h_FE of 150 say, you had a voltage drop of 6.08V..!!
Of course, the above voltage drop couldn't exist for a +20/0V supply, so BJT was almost out of proper bias, hence out of CE amp potential gain..

P.S.
1mA Collector current is intentionally taken for example, to show you that for voltage divider high values resistors you have to take into account Base current for proper Emitter voltage level bias..

[EricKnabe]

That's interesting. I'm running the tests in a sim software which is saying they are both properly biased. The mosfets just seem to overdrive much more. Strangely the BJTs are sounding closer to what the tube circuit should in terms of overdrive. This is all very frustrating since I'm such a novice and I bit off more than I can chew clearly. But I'm so close as well.

[antonis]

Sim softwares are good as far as you have properly arrange their library parameters..
(e.g. check, for instance, the value of forward current gain for circuit's specific BJT - don't be suprised by a value of 50 X 10³..