How do you get low HFE transistors??

[Focalized]

With SI fuzzes. Using 2N2222, 3904, BC108, 109, etc.

I always read those who use something like 70-120 for Q1 and 120-170 for Q2 to get a good sound. All the transistors I have from above measure from 140 to over 300. The BC108,109s even higher. Up to 600+ with the 109. Using the hFE mode on my DMM. How do you get the lower ones for Q1? Is the reading on the DMM accurate?

[tehfunk]

Those sound like pretty accurate readings, if you're concerned with the accuracy of your DMM check the Hfe ratings on datasheets to make sure they correspond. If you want lower gain, you simply have to buy lower gain transistors. Also, just check through lists of peoples own silicon fuzzes, write down the transistors they used and check them out for their gains. Hope that helps!

[Focalized]

Well just as an example, i read about somebody using a BC108 with an hFE at 80 along with a 2N3904 of 150. All my 108s are way higher. Are there different BC108s?

[brett]

Hi
way back in the seventies, transistors had lower hFEs.  I can remember when (delete boring old man story).

Anyway, there's 3 ways to go.
1.  Pull apart old rodios, etc and get old BC107s, 108s, and other early Si transistors.
2. Buy high speed switches.  I suspect that to switch really fast, designers sacrifice gain.  The 2N2369A (hFE=70) is a classic example.
3. Piggyback two modern transistors to make one with low hFE.  (use the search function to find out how to do this)  You can rig up a wide range of hFEs from 10 to almost normal by piggybacking.

cheers

[MartyMart]

Yes, 2369A's are great and also 3903's - which don't get much of a mention ( Thanks Andy ! )
I got some low gain Si pnp's from Ebay which were great , can't seem to find 'em or remember
the number ..... small chrome "top hat" and gold plated legs !!
I'm sure they were from about 40 to 80 hfe.

MM.

[jayp5150]

I was wondering the same thing, because Dragonfly once mentioned he used 2N3904's in a particular circuit with an hfe of like 110 or something, and every one I have was up around 290...

I guess it's off to ebay next time I'm parts shopping...

[Gus]

Or you redo the circuit to use different hfe transistors.  Look around in schematics one of the  links up top.

  Look up the specs on your meter. What current is used to test for hfe?
  Then do the math and figure out what Ic the fuzz is working at.  Then read some books or google, ASK etc transistor .........

[jpm83]

Here is couple good alternatives 2N1893 and 2N2219. And 2N1711 is also worth Checking.

Janne

[brett]

Hey Gus
that's right. 
I must have been asleep when I replied above.
There are LOTS of low hFE transistors all around us.  Especially at low Ic.
Read this hint, learn, then look 'em up.

HINT : millions of 2N3055s have been made since the 1970s.  They have always been cheap and really useful.  What is their "typical" hFE?  Check out the Ic vs hFE graph on the datasheet.  What is the hFE at the Ic of a typical fuzz circuit?  What kind of device is it?  Is the modern, small-package version (MJE3055) much different?  Would it work, too?

[Gus]

And as you posted piggyback.  I like using ones like a tip29

And the 10K and 100 ohm setup of Q1. Q1 can be different part number Si transistors as the hfe goes down the you might want to reduce the 100 ohm.
http://www.diystompboxes.com/pedals/gusFuzzFace.gif

[Dragonfly]

Another thing I've found is that often times silicon fuzzes sound great with EXTREMELY low hfe transistors...for instance, using silicons with an hfe of 40 - 80 in a fuzz face sounds great IMO.

Also, as Gus mentioned, tweaking the circuit for the appropriate transistors is a great way to go. Lets say youre replacing germanium transistors with silicons for example....ask yourself "what are the different characteristics that a germ has to a silicon, and how would I go about compensating for that?"...also, "do I need to set up my biasing differently"? (i.e. Tonebender).

Piggybacking is quite effective as well. DEFINITELY work experimenting with.

[Dragonfly]

I was wondering the same thing, because Dragonfly once mentioned he used 2N3904's in a particular circuit with an hfe of like 110 or something, and every one I have was up around 290...

I guess it's off to ebay next time I'm parts shopping...

2N3903 tends to be in the 80-120 range.

2N3904 typically in the 150 + range (though NOS ones can be lower)

[DougH]

Feed them less fertilizer than you would a high hfe transistor...

[aron]

I've been looking for a simple low hFE transistor to have in the store. I gotta go back through the forum and check. I remember people were recommending certain ones.

[Joe Kramer]

FWIW, ditto on 2N3903s(NPN) as well as 2N3905s(PNP).  I have some oldie TO-92s with gold leads, some of which read as low as 20 Hfe on my meter.  I think they do sound good in FF circuits, as well as anywhere you want to "round off" an overly bright circuit.  The trade-off is more noise than a high-gain xstr like the 2N5089. . . .

Regards,
Joe

[mac]

2n3440
2n2193
2n1893
2n1613/1711
2n2102
2n2219A/2218A (in my FF)
2n3895A
BDxxx --> some have a sub index 10 or 16, BD139-10, BD237-16.

My choice is the 2n2369A but can get some RF. Then the 2n2218A/9A.

Look at
http://www.centralsemi.com
They have pages and pages with with si transistors and their key characteristics.

mac

[Derringer]

piggybacking

I did it for a SI tonebender that used 2N222 Q's

I was shooting for hfe's at 70 for Q1 and Q2 spots and 100 for the Q3 spot

Connect the bases and run a resistor between the emitters. I think I ran 18K to get the lower hfe and 22K for the higher.

You'll have to experiment though to get exactly what you want. I just breadboarded everything with a pot between the emitters of the Q's I was testing and hooked it all up to my DMM.

I haven't built a lot of fuzzes, but I really like the way this one turned out.

[Gus]

What kind of fertilizer?

[JDoyle]

What kind of fertilizer?

I think the peak to peak kind...

[Focalized]

You guys are so full of good answers. I'm going try some of these suggestions. Thanks..