Author Topic: Transistor Biasing Problems  (Read 297 times)

SpringbokUK

Transistor Biasing Problems
« on: February 11, 2019, 10:02:08 AM »
Ok guys here's what I got.

I have a simple overdrive circuit that I put together on breadboard that consists of 2 J201 transistors. The first J201 I'll bias until I think it sounds good using a 50k trim (as I do with the 2nd J201) but for some reason it'll just rebias itself. So say I set it at 4.5v, it sounds good and I'll give my guitar a few strums and it takes about a minute and it starts splattering then I'll measure the bias and it'll be 9v. So I change the trim pot for a 1m, which will take me down to about 5v but it's still just garbage. Then I'll try various troubleshooting things and then suddenly it'll go back to like 0.5v on the 1m trim, so I'll put the 50k back in, rebias, and it works again for about a minute. I've got about 10 of these J201s and it's happening with all of them. The 2nd J201 in the circuit stays as it is. Sometimes when I take the output from the output cap of the first transistor it'll bounce back but still the bias will be different.
 
If it makes it easier to help me, my signal starts off going through a 1m res to gnd, then a 1uf cap to the gate as well as a little cap going to gnd and gate. Trim pot is connected to the drain and output cap is coming from the drain also. Source is a resistor and cap in parallel to gnd. Output cap goes to a gain control, then to a little bit of filtering (resistor and cap in parallel) then into the gate of Q2 (same config as Q1) , into a tone control then to volume than out.

I can't for the life of me figure this out so hopefully one of you could assist!

Thanks!
Liam

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Re: Transistor Biasing Problems
« Reply #1 on: February 11, 2019, 10:37:22 AM »
Wow, that's WEIRD, Liam!!   "Draining off" brings to mind something like bad cap.  Is your source bypass cap oriented correctly?  What happens if you remove it?
What values are your source resistors and caps?

Do you have another 1M pulldown at the gate of Q2?
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SpringbokUK

Re: Transistor Biasing Problems
« Reply #2 on: February 11, 2019, 11:16:24 AM »
Hi mike

Yeah the orientation is correct. Its a 22uf and a 1k at the source. The cap going from gate to gnd is a 1nf, I've also tried a 100pf as well. I've replaced caps to be sure but still no luck.

When I remove the source cap it gets a little cleaner that's about it.

I also increased the pulldown resistor to a 2.2m.

Would me using active EMG pickups have anything to do with this?

There must be something wrong with my circuit. I split the 2 transistors into 2 identical circuits and the same problem exists in both.


diydave

Re: Transistor Biasing Problems
« Reply #3 on: February 11, 2019, 11:25:23 AM »
If it makes it easier to help me, my signal starts off going through a 1m res to gnd, then a 1uf cap to the gate as well as a little cap going to gnd and gate.

Try doing it the other way around:

signal in -> 1uf cap -> 1m resistor to gnd -> gate.
I'm suspecting your Gate is floating and doesn't have a reference to Ground. Thats the job of the 1m resistor.
Your second j201 has reference to ground: gaincontrol and a resistor in parallel.

SpringbokUK

Re: Transistor Biasing Problems
« Reply #4 on: February 11, 2019, 01:12:20 PM »
If it makes it easier to help me, my signal starts off going through a 1m res to gnd, then a 1uf cap to the gate as well as a little cap going to gnd and gate.

Try doing it the other way around:

signal in -> 1uf cap -> 1m resistor to gnd -> gate.
I'm suspecting your Gate is floating and doesn't have a reference to Ground. Thats the job of the 1m resistor.
Your second j201 has reference to ground: gaincontrol and a resistor in parallel.

Yep! This did it!
Hey thanks man.

The 2nd transistor needed a bigger trim pot in the end but yeah man I got a healthy distortion/overdrive.

BEAST!

pinkjimiphoton

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Re: Transistor Biasing Problems
« Reply #5 on: February 11, 2019, 04:26:17 PM »
glad ya got it sorted!

but to answer your question about emg's... no, they can't make that happen...

BUT
if they're active, and have a cap leaking dc, i think it could @#$% up your bias in unexpected ways.
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Re: Transistor Biasing Problems
« Reply #6 on: February 11, 2019, 09:38:17 PM »
> a 1m res to gnd, then a 1uf cap to the gate as well as a little cap going to gnd and gate.

As you learned, EVERY pin on tube/transistor HAS to have a DC path to some voltage.

You had a cap in and a cap drain, but nothing to set the DC. As diydave said, the Gate "floats", does not know what voltage to sit at. On JFETs and tubes, it *may* drift to near ground, but any big signal slapping it around will not go positive (gate diode) so the average goes negative and the tube/JFET cuts-off. Perhaps for amny-many seconds until it drifts back groundward.

diydave

Re: Transistor Biasing Problems
« Reply #7 on: February 12, 2019, 02:42:30 AM »
Glad to be of assistance  :D