Jfet Mu amp cap questions?

[jcknowles89]

So checking out the schematic posted below. I noticed this jfet mu amp used to replicate a 12AX7 tube. My question is the cap that goes from the gate to the source on the top JFet circles on the pic, does that have any effect on tone or is it just to stabilize the jFet? Trying to figure out what different values here would do. Additionally a plate resistor bypass cap in a 12ax7 set up is a good way to round off high end. What would be the equivalent cap in this set up? Thanks guys

[Kipper4]

Hello and welcome
Put it on the breadboard and try it.
Or try changing c2  c4 c5.
I'm not quite sure if you want it to change the tone or not?

By the way it won't sound like a 12AX7.
SRPP

[amz-fx]

It affects tone. When you make it smaller, it rolls off bass response.

Here is a related circuit version...  delete R5 and R6 then put a jumper in place of C3. You get max freq response but the bias can now be off a little if Q1 and Q2 are not similar - they don't have to be matched but they need to be similar.

regards, Jack

[amptramp]

This is not a µ-amp or SRPP (shunt regulated push-pull) stage.  It is a grounded source amplifier with a current source drain load.  It is a useful circuit but it is not a µ-amp.  A µ-amp has a resistor connected between the lower drain and the upper source and the upper source is where the output is taken from.  The resistor between the FET's should have a value of 1/transconductance with reasonably matched FET's and the coupling cap to the upper gate should come from the lower drain.  See the discussion here:

www.tubecad.com/may2000/

Note that this is a power amp that must drive an impedance set by the current swing and voltage swing.  The same current flows through the lower and upper FET's. When the signal is applied, the upper and lower FET currents go in opposite directions.  The difference in current must go to the load.  If the load impedance is too high, the stage will limit at the upper and lower levels and you will have a fuzz, not an amp.

[jcknowles89]

That might be even mor useful...could this drive a speaker by chance? How much power could I get out of something like this?

Thanks!!!

[amptramp]

That might be even mor useful...could this drive a speaker by chance? How much power could I get out of something like this?

Thanks!!!

If you want to drive an 8 ohm speaker, you have to drive 125 mA for each volt going to the speaker and this is 1/8 watt.  You might be able to drive headphones, especially high-impedance ones, but the difference in upper and lower FET current has to be delivered to the load.  Since there are source resistors in series with the FET's, this is not an efficient way to drive a speaker because the FET current goes through them and dissipates power.