Help Needed: Biasing a JFET Common Source

[jafo]

Hello, all, and happy pre-turkey day. I'm trying to build a preamp with a J201 in a common source config, and I've found that it really is as tricky and frustrating as I've heard.

I've measured my J201 via the ROG rig in the Fetzer Valve article, and found it to be pretty standard: with a 9v that measures at 9.61 V, the Vp is 1.096 and Idss seems to be 0.76 when measured directly, but when I measured voltage and current individually I got 0.089V and 0.167 ohms, which gives Idss 0.533. Either way, pretty normal for a J201, right?

I've tried several cookbook configs, and gotten no joy. Tillman's formula, Rd of 6k8 and Rs 2k2, seemed sorta ok but quiet and farty. I'd read in another thread here that Rd 15k and Rs 1k biases most J201s -- no joy. Way too quiet, and misbiased. The Fetzer Valve calculator told me that Rs 1k2 and Rd 8k would be great, but all I had convenient were 10k and 1k, which gave me pretty much exactly what 15k and 1k did. I also tried wiring it up as th einput stage to a Jfet Vulcan, which has Rd 100k and Rs 33k (and a diode and Rg of 10m, tried with and without), which also got me a really low volume. When I say "low volume" in this paragraph, I mean really really low -- a peak of about -41dB.

Edit: tried Rd 15k (I think -- brown green (blue?) orange) and Rs 2k2. Volume is decent, just barely needing a volume control to avoid overload, but it's still kinda farty, and just a bit gritty.

All in all, I'm frustrated. Great tones are there, but they're covered up by a lack of volume and bias. Kinda like a mouse fart, I guess.

Any advice? TIA

[DavenPaget]

Hello, all, and happy pre-turkey day. I'm trying to build a preamp with a J201 in a common source config, and I've found that it really is as tricky and frustrating as I've heard.

I've measured my J201 via the ROG rig in the Fetzer Valve article, and found it to be pretty standard: with a 9v that measures at 9.61 V, the Vp is 1.096 and Idss seems to be 0.76 when measured directly, but when I measured voltage and current individually I got 0.089V and 0.167 ohms, which gives Idss 0.533. Either way, pretty normal for a J201, right?

I've tried several cookbook configs, and gotten no joy. Tillman's formula, Rd of 6k8 and Rs 2k2, seemed sorta ok but quiet and farty. I'd read in another thread here that Rd 15k and Rs 1k biases most J201s -- no joy. Way too quiet, and misbiased. The Fetzer Valve calculator told me that Rs 1k2 and Rd 8k would be great, but all I had convenient were 10k and 1k, which gave me pretty much exactly what 15k and 1k did. I also tried wiring it up as th einput stage to a Jfet Vulcan, which has Rd 100k and Rs 33k (and a diode and Rg of 10m, tried with and without), which also got me a really low volume. When I say "low volume" in this paragraph, I mean really really low -- a peak of about -41dB.

All in all, I'm frustrated. Great tones are there, but they're covered up by a lack of volume and bias. Kinda like a mouse fart, I guess.

Any advice? TIA

Really depends what circuit you used , care to show your schem ?

[jafo]

nm, got it working -- just had to up Rs a bit. Added some diode compression going in and a voltage divider to the gate (which removed the fartiness), which makes it basically the first stage of a Jfet Vulcan, except that I used Rd 10k and Rs 2k2. The output goes through a high pass filter composed of a 1F cap and 100k resistor. Joy is had! I might cut the highs a teensy bit at the output, but it's fine for now.