jubal81
Well-known member
Chuck's thread on the Brownout OD finally motivated me to do some breadboarding with JEFTs and opamp servos for biasing.
So far, things are looking pretty neat. I put together a couple stages of the Wampler Black 65 for testing and sure enough, I'm able to swap in and out JFETs willy-nilly and have it bias up perfectly. I don't have any J201s, so I've been using 2SK170, 2Sk117 and the good old 2N5457. I've got it set up so I'm getting 5.8V on the drains with a 9V supply. When they begin to break up, the distortion looks identical on the scope, the only difference being that the higher |vp| transistors have less gain/more headroom.
As far as the Black 65 goes, I've read that they're set up with 4V on the drains and I've not been able to dial it that low with this setup and my JFETs. (Using a rail-rail JFET opamp). I've always done 5.8 or so on a 9V supply with my projects where top and bottom of the signal start distorting at the same time.
Anyway, here's the circuit snippet I've got worked up and some pics of scope readings with different JFETs.
2SK170:
2N5457:
So far, things are looking pretty neat. I put together a couple stages of the Wampler Black 65 for testing and sure enough, I'm able to swap in and out JFETs willy-nilly and have it bias up perfectly. I don't have any J201s, so I've been using 2SK170, 2Sk117 and the good old 2N5457. I've got it set up so I'm getting 5.8V on the drains with a 9V supply. When they begin to break up, the distortion looks identical on the scope, the only difference being that the higher |vp| transistors have less gain/more headroom.
As far as the Black 65 goes, I've read that they're set up with 4V on the drains and I've not been able to dial it that low with this setup and my JFETs. (Using a rail-rail JFET opamp). I've always done 5.8 or so on a 9V supply with my projects where top and bottom of the signal start distorting at the same time.
Anyway, here's the circuit snippet I've got worked up and some pics of scope readings with different JFETs.
2SK170:
2N5457: