2N3904 hfe?

[Kummeli]

Hi,

I'm building a Prescription Electronics Clean Octave Blend: http://tagboardeffects.blogspot.fi/2012/02/prescription-electronics-clean-octave.html

It uses 4 2N3904 transistors. What hfe would be best suited and how to determine the right hfe?

[Elijah-Baley]

I never built that pedal, but about the 2N3904: https://www.sparkfun.com/datasheets/Components/2N3904.pdf. Search "hfe" in the document.

The hfe is a variable value, you can determine it with a multimeter, almost all with that function. https://www.youtube.com/watch?v=rIUl4TsPyQ4

If you meant the right hfe for that pedal in particular you can try several 2N3904 and see which of them make it sound better. If you want to try higher hfe there is 2N5088, or lower hfe 2N2222A. You have to search npn bipolar transistor. Sometimes the hfe make the difference.
When you try different transistors you have to read the datasheet and check the pinout.

You can read the page you linked, there's some information about the transistor you can use, but I guess in this case the 2N3904s are ok.

[Kummeli]

Thanks! Yeah I ment the 'right hfe for that pedal' and how to determine the right hfe for pedals in general.

[antonis]

the 'right hfe for that pedal'

Probably it's a new transistor spec term..

Pedals like COB are working well with a great variety of BJTs (those that are refered as "general purpose amplifiers"..)

They don't need any "special" transistor (like high gain and/or fast switching) because their gain is (shoud be) set "independent" (much lower) than their max hFE and their frequency responce is more than acceptable at the highest pedal's working frequency..

As a rule of thumb, you can use ANY transistor you have handy as long as your circuit doesn't involve digital operation and you aren't searching for a specific sound "colour"..

P.S.
In some cases, e.g. current mirroring, you have to match their characteristics but this is beyond your query..