Carbon Black with 2N3904s?

[BJG]

I suspect the answer is that I can't use them, but figured I'd ask. I have a bunch of 200+/- HFE 2N3904 transistors lying around and was going to use them in a Carbon Black Fuzz instead of the 2N3903's required. My understanding is 2N3903s have half the HFE, so would my 2N3904s be too much for this circuit?
Thanks!

 

[iamjackslackof]

I can give you the standard answer until someone smart shows up to give you the actual one: socket them and see!

 

[Aleph Null]

You absolutely can substitute them. The result will likely be a fuzzier fuzz. Because of the negative feedback between Q2 and Q3, it may be that the relative beta between them is more important than the beta itself, in which case the difference might be more subtle. If you end up with too much fuzz (is there such a thing?!), you could reduce R4, R6, or R8. I suspect R6 would have the biggest effect.

 

[beansell]

I just whipped one of these up on the breadboard! The 2N4123s worked a treat for this. MPSA42s also work just fine, similar gain range. Tried some higher gain transistors and like @Aleph Null says, it's a fuzzier time. Not a huge difference, but a difference nonetheless.

 

[Feral Feline]