acidfuzz wrote:I'm sure I got 4.5v @ the collector. Did you also omit R5?
We built is by the schematic above...and it sound killer. the only thing I changed was the 500K pot to 50K, whcih gave me a far better sweep.
Did you check out the link above. I don't understand the calculations, but one of the really good guys figured it out. It would be nearly impossible to get 4.5v if wired as the schematic shows.
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"> where do i take the calculations from that point
It is a 430K:220K voltage divider, probably.
IF the amp has a bias-point (if it "works"), we know the tap is 0.55V.
What voltage do we need at the top of 430K:220K to get 0.55V?
0.55V * (430K+220K)/220K
I work it out to be 1.625V.
This assumes the Base draws ZERO current from the voltage divider.
There IS base current. How much? Do we have to account for it?
2N5232 spec is given as "beta: 250-500". This is similar to 2N3904 beta. But such numbers are usually at 1mA. Beta may be less at the low 0.025mA here. The 2N3904 data shows hFE "typical" very flat from 0.1mA to 10mA. It may be (must be) down some at low-low current (and the hard-data says so), but maybe not by much. Let us take "250" as the current gain. If collector current is 0.025mA then base current is 0.025mA/250= 0.000,1mA. This must flow in the 430K resistor. 0.000,1mA*430K is 0.043V added drop.
So now the collector voltage which makes the base happy is 1.625V+0.043V or 1.668V. If hFE were higher, it would be less. But even if hFE were infinite we'd be back to the 1.625V assumption. (Base current is much less than divider current.) So collector voltage CAN'T vary by much. It has got to be in the 1.6V-1.7V range.
(I musta counted too fast when I figured 1.5V-1.6V.)
OK if collector is near 1.6V and supply is 9V, then there is 9V-1.6V= 7.4V across the 180K collector load. This current must be 7.4V/18K= 0.04mA. That's somewhat higher than our "in the middle" estimate. Go back and squint/extrapolate Vbe for 0.04mA. Might be nearer 0.58V. Voltage-divider means to get 0.58V at base the collector must sit near 1.69V. The base current is higher but still small compared to divider current. We might pencil 1.7V as our expected voltage.
EDIT: Oh!! I forgot the 100K collector to ground! This is an odd feature (it makes the clipping less asymmetrical). There's a couple ways to account for its effect. Obviously if collector is anywhere near 1.6V, then there is 1.6V/100K or 0.016mA in the 100K. That means 0.016mA less in transistor. So the "0.04mA" is more like 0.024mA. Vbe is lower, maybe 0.056V. Collector may be 1.65V, or a wee bit more. No big difference. "================================================================================