transistor- two in place of one!??

[garfo]

Hello, simple doubt, not sure what the answer is...!!!So in a Fuyzzface, lets say I would put two trannys with lesser hfe to replace Q2, how would that work, is it possible?
I happen to say this because my best soundind trannys are mp39 all with the range of 50 hfe.

[midwayfair]

You can do a Darlington, but not just stick two in parallel.

You can, however, parallel FETs.

Transconductance (Gm) of BJT varies directly as current.

2 devices, each takes half current, gives half Gm, two halves is a whole, gain is unchanged. (Further: it is difficult to get two BJTs to split equally.)

Field-effect devices (tubes, JFETs, MOSFETs) in the saturation range (normal current): the Gm varies slower than the current. Assume square-root. Half current is 0.7 of Gm. Two 0.7s is 1.4. Gain goes up to 1.4.

You don't have the same problem with paralleling JFET's as you do with bipolar transistors.  With a bipolar transistor, one slightly stronger transistor will hog the current because the higher temperature from more current flow will reduce Vbe and therefore, devices that start with nearly equal current will shift entirely to the transistor with lower Vbe.  This can only be corrected by separate equal emitter resistors.  There is no such effect with a JFET or MOSFET - the channel is modelled as a resistance with a positive tempco of resistance and if one starts to heat up, its resistance increases and it tends to share the current without the need for source resistors.  This of course implies the Vgs on voltages are equal to start with.

http://www.diystompboxes.com/smfforum/index.php?topic=102208.msg904904
http://www.diystompboxes.com/smfforum/index.php?topic=101274.0

I built a very good sounding low-gain germanium fuzz face with hfe 40 in Q1 and hfe 60 in q2. If you can get close to that same ratio with the ones you have, you can have an excellent sounding box.

[garfo]

I think I might just do that instead then.I'm using the russian mp39's, I have a couple of them.I'm using a ggg similar and schematic; would I benefit from using the simpler schem from the madbean site?