Help trying to learn how to bias a transistor

Started by synthetic76, September 10, 2011, 01:23:12 AM

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synthetic76

Hi guys I am building the Simple Overdrive by Joe Davisson (http://www.freeinfosociety.com/electronics/schemview.php?id=926) I have ordered a 2N5089 and some 1N4001's as i didn't have any but built it anyway using 2 red LED's for the diodes as I'd always wanted to try that out and happened to have some close at hand and used a 2SC458D transistor I recently rescued from an old organ.  I built the circuit as specified and it doesn't really overdrive in any noticeable way, if anything the volume drops slightly.  I'm assuming that the bias needs to be set and I don't know how to work out what it should be, I have the datasheet for the 2SC458D transistor (http://www.audiolabga.com/pdf/2SC2308.pdf) the Hfe for it should be 250 to 500 which sounds okay to my limited knowledge.  I have found the Transistor Bias Calculator (http://diystompboxes.com/analogalchemy/emh/emh.html) but I'm not sure what the transistor is looking for as far as voltages. I have listed the voltages as they come out with the circuit as is.  I would appreciate it if someone could give me a hand and help me understand what I am looking for when I am trying to bias a transistor. 
E - 8
C - 0.04
B - 0

Thanks for your help with this,

Nate

LucifersTrip

#1
there's some good info on the geofex site regarding transistor amplification and loads of other stuff. ie:
http://geofex.com/fxdebug/bias_prob.htm

transistor voltages will be different in every circuit, so you will need to find out what they should be in this specific circuit you're working with. unfortunately, most schematics don't have the voltages on them.

hopefully some here has already built the Simple Overdrive and can give voltages

and there's stuff like this all around the www
http://www.youtube.com/watch?v=PSdHf6yozyc
always think outside the box

PRR

> I'm assuming that the bias needs to be set

No. It is totally self-biasing for any transistor hFE of 20 to 1,000.

> E - 8
> C - 0.04
> B - 0


That's so unlikely that there must be either a significant mis-wiring or mis-measuring.

Does this help? The "0.75V" could be 0.7V or 0.8V, but not 0.04V or 8V.


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synthetic76

Thanks for your help guys will try re-measuring incase I screwed that up, thanks for the voltages  and the self-biasing tip PRR.  Thanks LucifersTrip I must have missed the biasing problem stuff on Geofex when I was looking through.  Will give it a better read.  Thanks for your help guys, will let you know when I work out which specific variety of twit I am (I'm guessing one who can't read a multimeter for a start :icon_rolleyes: )

Nate