Dr. Boogey Bias Problem

[Schappy]

Im building a Dr. Boogey from this vero layout
http://img195.imageshack.us/img195/4938/drboogey.gif

I successfully biased all JFETs except for Q2. As I turn the trimpot it quickly jumps from 2 to 9V and I cant get anywhere near 4.5-5V.

I know J201s can be tricky and Ive tried a few others I had and still no luck.

Should I try more j201s or debug elsewhere?

[Scruffie]

Did you do the fix I suggested in your other thread? Because the Layouts wrong... you need to move C8 left one space and place a cut between the positive leg of C8 & C9 on the third row down.

Otherwise, bad trim pot? Have you tried the debugging page?

[deadastronaut]

hmmmm..nice one...i built this version too...but i never really got round to sorting it out...maybe coz i biased by ear instead....but i'll try that fix..

i didnt realise it was wrong...lol...couldnt i just put a cut between c8 and 9 without moving it...?.

looks like it should be ok!...

[Schappy]

Yes I fixed it.

[Scruffie]

hmmmm..nice one...i built this version too...but i never really got round to sorting it out...maybe coz i biased by ear instead....but i'll try that fix..

i didnt realise it was wrong...lol...couldnt i just put a cut between c8 and 9 without moving it...?.

By ear is fine if you wanna spend extra time doing it, but having it in the ball park range with a multimeter simplifies things.

You can use a stanley blade to slit inbetween the 2 parts if you want yeah, you'd only need to move it if you were using a drill bit to make a clean hole cut, as long as the copper is broken between the 2 parts though, it'll be fine.

[deadastronaut]

glad u fixed it ...

i''ll have to try mine now..

...cheers scruffie...!

rob.

[Schappy]

I did the fix you said but still cant bias the Q2 drain.

[ralley]

I also had problems with biasing - exactly the same issue, 1 or more of the trimmers would cause an immediate jump from a low voltage to much higher.  By persevering I managed to turn the trimmer by truly tiny amounts and get the bias as close as possible (something like 6V in one case I think).  Sounds fine but quite hissy.  I never found an underlying cause for this problem.

Rob.

[Scruffie]

My Suggestions would be to try out different FETs in the circuit (they vary greatly) and if that doesn't work, some other good quality trimmers.

[Schappy]

Can someone give me the correct transistor voltages?

[PRR]

> an immediate jump from a low voltage to much higher.

First: power OFF, put the ohm meter across the pot and measure while turning (yes, you need three hands or jumper-clips). Be sure it turns smoothly from around 100K down to much less than 10K. Remember which end is "very low resistance".

If the pot is jumpy (bad contact), get a better pot.

> As I turn the trimpot it quickly jumps from 2 to 9V

Find that point.

Is the trim-pot near the middle? Or is it all the way to one end? To the low-resistance end?

If you have to turn the pot way-down below 10K resistance to get a good bias, this FET is a bad fit for this circuit. It is "too hot", passes too much current. You should grab another FET. It may be that all FETs in a bag are about equally hot, maybe get another bag of a different type number?

Alternatively you could replace the 100K trim pot with a 10K pot; but power consumption may be high. OR replace the source resistor (2.7K?) with a much larger value, 5K or 10K; but the gain may wind up low.

It's not easy to make FETs work the way you (the designer) wants when we only have 9 Volts to play with. As Scruffie says, it does indeed come down to selecting an FET which fits the circuit, within the trim-range of the 100K pot.

> correct transistor voltages?

The Drain should sit in the middle between 2V-3V and 9V. Anything 4V to 7V will work OK. The hard part is finding an FET which "can" sit in that zone with a reasonable trim-pot setting.

Once it DOES work OK, you can fiddle. In overdrive you get different flavors when you lean one way or the other or centered. But don't even fool with that until you can comfortably set it within the 4V-7V zone.

[Schappy]

Got it working. Has some noise but hopefully boxing it up will tame that a bit.

Scruffie,

Did you use shielded wire on the gain pot?
Have you built any other versions besides this one?

I see that the gaussmarkov version has some power supply filtering and miller caps. I was wondering if there was much of a difference between the two.

[Scruffie]

I used shielded wire everywhere I could fit it really, it was a bit difficult around some areas but the gain pot is a good one to put it on and the input & output of course.

There is some power supply filtering on that vero (the 100uF cap) you could enlarge it if you're using a power supply.

No I haven't built any other versions yet although I do have a Gaussmarkov Dr Boogey PCB waiting to be built... I did build that vero up with the modified Gaussmarkov ToneStack values though, I didn't really get a chance to compare the two versions though, they both sounded good.

Boxing it will certainly help though especially once the shielded wire has somewhere to properly ground.

[Schappy]

I did the modified tonestack as well.

I was thinking about squeezing in the miller caps from gaussmarkovs version but not sure what they are there for.

[Scruffie]

I did the modified tonestack as well.

I was thinking about squeezing in the miller caps from gaussmarkovs version but not sure what they are there for.

The Miller Caps aren't a brand they create an effect called Miller Capacitance http://en.wikipedia.org/wiki/Miller_effect

Basically... it rolls off some highs and makes the FETs Frequency Curve slightly more simmilar to a tube... it may well help with some high end noise.

[Schappy]

I understand that they are not a brand but wasnt sure the reason for putting them in there.

Have you tried them before?
It might be a pain to squeeze them on my vero so I was just wondering if it would be worth it.

[Scruffie]

I haven't tried them no but I see the reason for them, the Dr Boogey can have a bit of a nasty high end, try it stock and see what you think, if you find it unpleasent, try adding those caps in... it'll also approximate the FETs sound closer to a tube, if that's what you want, then try it, I think it'll be fairly easy to fit them in, you could always solder them to the track side of the board if you're out of room.