Transistor Substitution question

[dfp123]

Hi,

I am planning my next build (only my second one so I am still trying to figure everything out). The build calls for the MPF4393 transistor, which I cannot purchase. I have a couple of substitutions in mind but I am not sure which is the best way to go.

I have looked up the data sheets for the MPF4393, and the potential subs: J201 and 2N5457.

MPF4393
Vds, Vdg, Vgs: 30, 30, 30
Ig: 50
Pd: 350

J201
Vdg, Vgs: 40, -40 (no Vds listed)
Ig: 50
Pd: 360

2N5457
Vds, Vdg, Vgs: 25, 25, -25
Ig: 10
Pd: 310

The values for the J201 are higher than the MPF4393.
The values for the 2N5457 are a little lower than the MPF4393.
Is it better to go a little higher or a little lower? But both have negative values for the Gate Source Voltage. Does that matter?
The 2N5457 has a much smaller Forward Gate Current (10 instead of 50).

Which values are most important when finding substitutions? Which way would be better to go with?

Thank you for the help!

[garcho]

The build calls for the MPF4393 transistor

how would anyone know how to determine a substitute without a schematic, or at least a description of what you want to do with your device? 

most likely, either substitute could work. i say 'could' because depending on the design, even a component with the same name, in this instance MPF4394, might be out-of-spec enough to through everything off. the JFETs with the specifications this particular circuit needs might have to be chosen after some testing.

[dfp123]

Sorry for not specifying. It is for the Sabbra Cadabra build, which calls for 5 MPF4393's.

http://1.bp.blogspot.com/-1BbOnDJyOsI/VQ8UQXiQxqI/AAAAAAAAGEs/RHBNhvkf3KE/s1600/Catalinbread-SabbraCadabra.png

I wasn't sure if there was a standard convention when choosing substitutes, or which values to pay most attention to.
Thanks for your thought.

[digi2t]

PN4393 seems to be the same part, and maybe cheaper too.

It's been reported over at Tagboard that J202 and 2N5457 worked in it as well. Maybe read through the build reports;

http://tagboardeffects.blogspot.ca/2015/03/catalinbread-sabbra-cadabra.html

[R.G.]

You need to look at the specs for Vgsoff, Yfs (also called transconductance) and Idss. The specs you quote for Vds, etc. are breakdown specs, and not all that useful as long as they're bigger than the power supply voltage to be applied. Likewise, Pd is the power dissipation, and if you have to worry about that, you're building the wrong circuit. Ig is useless for this.

2N5485 will probably work, as will **some** 2N5457 or J202.

[dfp123]

PN4393 seems to be the same part, and maybe cheaper too.

It's been reported over at Tagboard that J202 and 2N5457 worked in it as well. Maybe read through the build reports;

http://tagboardeffects.blogspot.ca/2015/03/catalinbread-sabbra-cadabra.html

Thanks digi2t, I had gone through the build report and there seemed to be some who had a bad build with the 2N5457 and I was curious if there was a better choice between the subs that were discussed in the report by looking in the data sheets.

I am very new to this and unfortunately don't even know what I don't know. Thanks for your thoughts!

[ubersam]

Sorry for not specifying. It is for the Sabbra Cadabra build, which calls for 5 MPF4393's.

http://1.bp.blogspot.com/-1BbOnDJyOsI/VQ8UQXiQxqI/AAAAAAAAGEs/RHBNhvkf3KE/s1600/Catalinbread-SabbraCadabra.png

That's what we'd call a "layout". A schematic would be a bunch of lines and symbols, like this: http://4.bp.blogspot.com/-S_pVliAdXiM/VPYDmAqg7tI/AAAAAAAAEIk/od6m1Y7WEK4/s1600/esquema.png

Anyway, the Sabbra Cadabra schem looks like a tweaked BSIAB. The 5 JFETs make up two mu-amps (the gain stages) and the output buffer.

You need to look at the specs for Vgsoff, Yfs (also called transconductance) and Idss.

That's what I'd look at to find drop-in subs, at least for circuits like the muamp/BSIAB/SabbraC. Personally though, I'd just drop a sub in and try it out to see if I like the sound. But just for fun:

The min/max V_GSoff of the MPF4393 is -0.5/-3 Vdc, close to a J202 @ -0.8/-4 or a 2N5457 @-0.5/-6
The min/max I_DSS of the MPF4393 is 5/30 mA, the J202 lower @ 0.9/4.5, a J309 will be closer @ 12/30
The typical Y_FS of the MPF4393 is 12 mmhos, the J202 has a min of 1 mmhos with no published max, the J309 goes from 10 to 30 mmhos

[dfp123]

Thank you for the information! I appreciate your time to help.