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Messages - reroe

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Preamps and Effects / jfet emulator, buzzing - help me
« on: September 16, 2011, 06:30:52 PM »
i post to ask something about jfet emulation circuit.
many people says about tube to jfet conversion quality or difference but i'm not intending to discuss about it at this time.
 
i have a buzzing or oscillation problem.
i've just finished a carvin legacy jfet pedal (so called leegazzy in the web) and a laney gh100L preamp part(exactly same circuit with tube version)
and they have same phenomenon, that is, ~193hz buzzing  at high volume(or gain).  i write "193hz buzzing" because i think buzzing frequency is as same as with open g note and when i turn vol or gain pot, the pitch of oscillation changes
 
i googled to solve the problem and did something to fix it
0) trimpot biasing is around 4.5~4.7V for every JFET(i use 2sk117)
1) check ground scheme, jack isolation
2) 9V power (i use DC 9V 300mA adapter though) filtering with 100uf cap
3) add dropping resistor to gain stages like tube amp power supply chain
4) shorten gain and volume pot wiring
5) add 0.01uf (roll offcap?) to drain-gate, drain-gnd,gate-gnd
all of these above were not properly working.
 
could anyone suggest about my problem?
 
thanks.

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