Selecting R source for a Mosfet Buffer..help!

[dschwartz]

Hi all!
I´m driving a low impedance tonestack (0 to ~50k Ohms) and i dont know wich R source to use, currently i´m using a 100K source resistor but the buffer distorts and sounds bad...

My B+ is 280 and preceded by a tube gain stage directly.. 
so wich resistor should i use, is there a calculation to be made? (of course there is..but i dont know it)
please help...

[petemoore]

  You want the source biased so it swings up and down without encountering a power rail.
  GEO updates to mosfet boosters, AmZ etc.
  I think I'd start with 4k7 or 10k, measure the source voltage.

[R.G.]

The general rule of thumb is that you want things that drive other things to be no more than 1/10th the impedance.

In your case, I would use no more than 10K for a source resistor, and look longingly at a 4.7K **if** the voltage on the MOSFET is low enough not to overheat it.

Distortion can also come from not having the source voltage near the middle of the power supply, as Pete says. Measure the voltages and rebias if you need to.

[dschwartz]

Thanks for you replies!

The general rule of thumb is that you want things that drive other things to be no more than 1/10th the impedance.

Hmm maybe that´s why i get that ugly gatey sound when i turn the master volume down, i´m lowering the impedance of the tonestack that way to maybe 10K or less (at least the output dc resistance is something like that)

In your case, I would use no more than 10K for a source resistor, and look longingly at a 4.7K **if** the voltage on the MOSFET is low enough not to overheat it.

the mosfet is rated over 400V and b+ is 280V so i dont think there will be a problem with that.. i think i´ll use a trimpot to tune it by ear..


Just one more thing...the preamps sounds good (altough with the follower problem) on the power amp input of my peavy bandit (SS)
but when i plug it into my tube amp, it just hums.. no sound, just hum like touching the tip all the time..checked for DC and there was no DC reading..i though that i reversed the output jack leads, but in that case it would not sound on the SS amp..or can it?

sorry if i ask so many questions..i feel like a rookie newbie 

[dschwartz]

i put a 50K trimpot and a 2.7kOhmresistor in series and tuned by ear...these are my findings:

B+=Vdrain=214V (kinda low)
gate Voltage=52V

the best sound was with 27K as source resistor, where V source= 54 (ish)

tried a 10K resitor, but the mosfet got hot pretty quickly

now it sounds so much better!!!

pd..i had the output jack wired wrong..thats why my tube amp didnt sound with it!! DUH!!!