Bias String...should this work for IRF510?

[petemoore]

  I'm trying to bias another IRF510.
  Offshoot of Gus's Mosfet Distortion.
  I used 150ohm source R, and 1k5 Drain R...this worked last time, and is proportional...
  This time for gate bias I decided to try noiseless biasing.
  I used a...from ground...10k > 10kl pot > 15k to V+, wiper of the pot goes through a 4m7 to gate.
  This provides a nice adjustment of the Drain voltage, the gate likes to sit at around .65v
  The source stays around .3v
  The IRF data on the package states a 2v - 4v Vt., I don't think it'll work with .65v on gate .3 on source, that's only .35v...
  Anyway, out in left field without a compass again..daft !
  There are no traces, just resistors and the IRF 510, and a battery clip.

[petemoore]

  Edit: who knows what to think about what I typed, none less than me...I'll post something else if I can figure out better what to type.

[Gus]

Are you measuring the gate volatge at the wiper end of the 4.7meg or the gate end.

Measure at the wiper end of the 4.7meg.   if you measure at the gate the DMM will load down the reading(the meter has a current path to ground caused by the meter loading)

also use a cap to ground on the wiper

9V?

bias  from 9V 15K then 10K pot then 10K resistor to ground?

9V/(15k+ 10k+10k)= .26ma

.26ma  x 15K = 3.9V  SO 9V - 3.9V= 5.1V gate to ground wiper at "top" of the pot

.26ma X 25k =6.5V  SO 9V-6.5 =2.5 V gate to ground wiper at "bottom" of the pot

[Sir H C]

FETs will work below their threshold voltage, "subthreshold" operation it is called, and they behave more like a bipolar device.  Still I agree with Gus, you are probably seeing measurement error and so everything is fine when not measuring (much like quantum physics).

[petemoore]

  well...we're all three agreed.
  I didn't have an expl for it 'till I thought it over last night...
  After a long satisfying play of the sound the circuit produces..of course, two of these is I think getting very close to the Mosfet Overdrive, a great sound, and chose to decide it has to be something wacky with the meter reading, the opposite of the sound of something that needs fixed.
  Still I went back and...reading throught the Mosfet Article RG generously provides, [again-igan] and these Mos items..well I'll be wiring and biasing another I think soon. Mosfet = Excellent performance.

[Gus]

petemoore

   Did you get a chance to measure the gate voltage?

[petemoore]

  Gate voltage read .84v [IIRC], but that made no sense [seeing on how it works] and I forged ahead, testing the circuit, great boost sound...so I figured what was said about the meter loading down and giving a faulty gate voltage has to be true.
  @@ rate, this being the third mosfet to show same and good performance, [bias-tuning the gate voltage by ear w/bias adjust pot], I think they're all the same and working.

[MartyMart]

Pete/Gus, perhaps I'm not running this with high enough voltage ( 9v ) but as yet
I have no output !
I used Gus's original bias setup of 2m2/1m gate, 150 ohm source and at first
a 20k trimpot at drain ( pinout G-D-S LtoR "bulge" facing me ) which I couldn't get
under 7.8v.
Input is a 47n output 100n and 10k vol pot
So .... tried a 1k2 from 9v to drain - voltages
Gate  1.48
Drain 8.33  !!  ?? WTF !
Source 0.00
Battery 8.54

Any ideas ?   

Marty.

[MartyMart]

Got the drain down to 6.7v with a 270k resistor !!
I have some "splatty" output now, so I must have something badly wrong.
I can't see a manufacturer stamp so perhaps my pinout is incorrect !

MM.

[petemoore]

I used Gus's original bias setup of 2m2/1m gate, 150 ohm source and at first
  See the GEO article.
  I used the noiseless biasing described there, lots of leeway in values on the voltage divider, 56k 50kpot 82k [something like that...so I used
  10k 10kpot 15k bias string, and that allows moving the gate voltage around.
  The S/D resistors are apportioned for gain, ITCase 10x [120ohmS / 1k2 drain], so those can be set [I'd measure the actuals...]
  Then moving the gate around [the source will 'follow' - 'some voltage' see article...something like 2v plus the VT of gate source..] moves the drain voltage around.
  Since I have the pot in the divider, the bias can be adjusted for volume drop [toward CCW], and gating out completely [toward CW], so bias adjust is pretty easy, and came out to about 50% setting on the bias pot.

[MartyMart]

Thanks Pete, found the article ( looong search ! ) and went for a bias of
100k-> gate ->50k pot & 47k r with the original Drain r of 1k2 and source 120 Ohm
no bypass cap .

I HAVE SOUND 

Seems like a boost, not earth shattering but it's a start !
( perhaps I'll stick to BS170's  )
My try with 18v tomorrow ...
MM.

[Gus]

Marty

  Note in the schematic the square by the top resistor of the bias string.  Thats the resistor I change and it will be different for different mosfets I use a BUZ71 in the schematic.

  The drain and source R can be changed to adjust the drain current up or down and they also control the gain somewhat because the source is not bypassed.  Its all about compromizes with simple circuits like this.  It is a textbook circuit.

  I set the drain R for a drain current I want to be around.   What I do is define the power supply voltage and then what part of the power supply voltage I want to have the mosfet work at 1/2 or 1/3 or 3/4 etc.  I subtract the operating  point(drain voltage) from the power supply take that volatge and divide by the drain current wanted V / I gives the drain R

So make the top resistor of the bias what you change.

  At the current I like power mosfets at a 9 v battery will drain fast.

[MartyMart]

Thanks Gus, taking all that into consideration, would it be "better" to
use a source bypass cap of say 1uf-4u7 with this setup rather than nothing ?
Voltage at drain 4.2v and gate 3.1v "sounds" good and bright, just not overly
loud.
Battery dropped from 8.53 to 8.12 in the last 30 minutes !

MM.

Edit : Wired a 1uf poly from lower bias "trimpot" wiper to ground by mistake
but sounded "clearer" if that makes any sense ? then added a 4u7 source bypass
cap which seems to have doubled the output - nice
Good quality is that it seems to "compress" a high output pickup quite nicely ..

[MartyMart]

Pete - sorry for derailing the thread a bit, but thought this was cool !
I wired up a second stage using a 50k pot between them.
Wiper of pot comes to a 100n before the gate of *2's bias setup etc.
Exact second stage as *1 and both bias up nice, however .....
the "crunch" sound is quite harsh and has nasty "farty" decay to it, when
backed off to clean it's all fine !
They dont seem to breakup in a very nice way at all .....
I wonder if a Joe D compression diode from gate *2 input would help ?

MM.

[Gus]

The schematic has a 5K vol if it is any bigger with power mosfets you can get a treble loss because of the gate capacitance.  Power mosfets have higher gate capacitance.  I also have the circuit at 12V

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