J202s in a Benson Preamp?

[Boba7]

Hi everyone,

Pretty sure it wouldn't work but since even smd J201s are somewhat hard to come by, would J202s work in a Benson preamp clone?
Alternatively, could I use a J202 in one spot of the circuit? Maybe last transistor? What do you guys thinks?

Thanks!

 

[temol]

They "would work" as other transistors. They would bias to proper drain voltages. But... expect lower gain than with J201.
Here's comparion (in Benson) - J201, J202, 2N5457, 2SK208R. As you can see - 2SK208R (2SK30A) looks like better alternative. At least 2SK208R I have measured.

 

[Locrian99]

Hi everyone,

Pretty sure it wouldn't work but since even smd J201s are somewhat hard to come by, would J202s work in a Benson preamp clone?
Alternatively, could I use a J202 in one spot of the circuit? Maybe last transistor? What do you guys thinks?

Thanks!

I just picked up pre soldered 201 smd’s from aion.

 

[Boba7]

They "would work" as other transistors. They would bias to proper drain voltages. But... expect lower gain than with J201.
Here's comparion (in Benson) - J201, J202, 2N5457, 2SK208R. As you can see - 2SK208R (2SK30A) looks like better alternative. At least 2SK208R I have measured.

View attachment 34534

Interesting! What am I looking at exactly on this graphic?

 

[Boba7]

I just picked up pre soldered 201 smd’s from aion.

thanks! I'll keep that in mind

 

[temol]

Interesting! What am I looking at exactly on this graphic?

It's a frequency response graph of the Benson with differen transistors (taken from ltspice simulation). In short - higher gain (higher amplification) = higher graph.

 

[Boba7]

It's a frequency response graph of the Benson with differen transistors (taken from ltspice simulation). In short - higher gain (higher amplification) = higher graph.

So thats the overall output of the pedal that’s being estimated?

 

[temol]

Visualisation of the gain between 20Hz and 20kHz. Taken from the VOL potentiometer.

Two more screens from ltspice.
Input - 660Hz, 0.2V p-p, decaying sinewave. Again - signal at the VOL potentiometer.



You can clearly see - with J201 you still have clipped signal (that's the distortion) while with J202 it's almost clean (you can tell this because of the rounded tops of the red trace).

 

[Boba7]

Visualisation of the gain between 20Hz and 20kHz. Taken from the VOL potentiometer.

Two more screens from ltspice.
Input - 660Hz, 0.2V p-p, decaying sinewave. Again - signal at the VOL potentiometer.

View attachment 34549

You can clearly see - with J201 you still have clipped signal (that's the distortion) while with J202 it's almost clean (you can tell this because of the rounded tops of the red trace).
View attachment 34550

Thats great ! Thanks a lot!

 

[MichaelW]

They "would work" as other transistors. They would bias to proper drain voltages. But... expect lower gain than with J201.
Here's comparion (in Benson) - J201, J202, 2N5457, 2SK208R. As you can see - 2SK208R (2SK30A) looks like better alternative. At least 2SK208R I have measured.

View attachment 34534

Whoa, that's interesting.....I socketed mine and used out of spec J201's...now this makes me want to go get some 2SK20R's.....those are currently production right? Having said that, I love my Benson the way it is.....

 

[temol]

My "out of spec" J201's (through hole) have parameters closer to the 2N5457 or J202. This means higher Idss and Vp than J201 (MMBFJ201).
All of the 2SK208R that I have measured have Vp/Idss within J201 specs.