Blue Magic/Need To Get Voltage Down A Bit To The J201?

[97STRATDELUXER]

Hey all....I Like the tone and the overall feel of the "Blue Magic" pedal....I was measuring the voltage at my J201 FET and the lowest I can adjust it via the 50K trimmer is about 6.6 volts...Wondering what I can do(or what resistor do I Increase)to bring that voltage down to say about 5 volts or so? I think the pedal would benefit a bit at this lower voltage as it's boomy in the lows and less articulate with the volume and gain up...Otherwise,the pedal sounds great! The only change I've made to mine so far was to use an 820 pf ceramic cap for the mid boost so I can actually hear the boost...Below is the board.Maybe someone could show me what to change to get my trimmer pot voltages to the FET correct..Thanks all...

[davebungo]

If you reduce the source resistor (is it a 1K?) slightly then you will be able to adjust the drain voltage to lower levels, otherwise try a different device.

[ninoman123]

Maybe try putting in a 100k trim pot so you can have more resistance thus be able to lower the voltage more

[97STRATDELUXER]

Maybe try putting in a 100k trim pot so you can have more resistance thus be able to lower the voltage more

Thanks for posting so quick guys! I'm going to socket the J201 and try different FETs and see if the voltage comes down where we need it? I'll also try a 100k trimmer instead of the 50k...The pedal sounds really great already,but I don't like being at the extreme end of adjustment on the trimmer and being at over 6 volts..My battery is new and reading close to 9.6 volts and I'm sure my drain readings are in the ballpark already? I'm reading that the 4.7 volt readings were based on 9 volts...Thanks guys,I'll post my findings..

[97STRATDELUXER]

I switched the 50k trimmer to 100k and I now can get all the voltage range I need...While I was back inside the pedal,I Lowered C2 to 4.7 mfd and replaced the .001 mica input cap with an Orange Drop .001 100v poly instead...I also increased C11 to a .0022 Orange Drop poly cap and socketed the 22k resistor,though I've left a 20k in the socket for now..I feel it makes the pedal more natural(smoother)and less hi fi sounding...I also noticed an overall increase in the gain as I lowered the voltage to the drain of the J201...I was probably great right at my original setting,but I'll mess with the settings more tomorrow...Great pedal....I Like it better than my own tweaked TS808 and TS9s...

[ninoman123]

Awesome. Glad it works. Lets see the top of the box.

[97STRATDELUXER]

Awesome. Glad it works. Lets see the top of the box.

I'm kinda embarassed cause I haven't learned how to do graphics yet and I'm using the black and white little stick on letters and numbers for now.... The pedal rocks though and it's going to be around alot longer than I will be! 

[davebungo]

Bear in mind that if you increase the drain resistance you also increase the gain of that stage.  This might not be a problem but it's worth pointing out.  Reducing the source resistance slightly won't affect the voltage gain (as it is bypassed for AC signals).  So if you have too much gain and you want to lower the operating point of the circuit consider altering the source resistor.

[97STRATDELUXER]

Bear in mind that if you increase the drain resistance you also increase the gain of that stage.  This might not be a problem but it's worth pointing out.  Reducing the source resistance slightly won't affect the voltage gain (as it is bypassed for AC signals).  So if you have too much gain and you want to lower the operating point of the circuit consider altering the source resistor.

Thanks Dave....Not sure if I'm following you right,so excuse the dumb question? So by lowering the voltage to the drain,we are increasing the resistance and are saying it's best to stick with the 50k resistance trim pot?

John

[petemoore]

  IIUC the gain of a mosfet is set by the proportion of the source resistor to the drain resistor.
  Mosfets can be set up at various bias point voltages as set by the resistors, and these voltages may change from one Fet device to the other.
  I'd change the source resistor value or the device.

[97STRATDELUXER]

  IIUC the gain of a mosfet is set by the proportion of the source resistor to the drain resistor.
  Mosfets can be set up at various bias point voltages as set by the resistors, and these voltages may change from one Fet device to the other.
  I'd change the source resistor value or the device.

Ok....I'll go back to the 50k trimmer and I'll try different J201s since I have socketed for the 201....Thanks...Great advice!

[97STRATDELUXER]

I went back to the 50k trim pot and tryed a different J201 and got it dialed in perfectly now...I'm actually at around 5.5 volts on the drain with a battery reading almost exactly 9 volts....I then added a 25k pot to replace the 22k resistor...I Like this mod alot as it both tightens and smooths out the feel of the pedal...I'd call it the "feel" pot...Great pedal....Much better than a Tube Screamer!