JFET Shootout

Vgs,off is the gate voltage necessary to drive the drain current to zero. Since it is not possible to measure zero amps, the datasheet specifies a very small drain current, usually 10nA or 100nA, although I have seen Vgs,off spec'ed at Id = 1uA in one datasheet.

Vgs,off and Idss define the limits of JFET operation. The drain current can never be less than 0 or higher than Idss. It's just not physically possible.
Vgs = Vgs,off when Id = 0 and Vgs = 0 when Id = Idss.. Vgs can be more negative than Vgs,off or more positive than 0, but nothing useful happens when Vgs is outside the range from 0 to Vgs,off.

I did not use the most accurate method for measuring Vgs,off. I used the easiest method. I'm exploring a more accurate method and will revisit the data I published at the top of this thread.
 
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I hope @Chuck D. Bones doesn't mind hijacking his thread....
There's a recent thread about Sunn Concert Lead preamp. Schematic lists 2N4304 jfets. I've never heard of them before.
Looks like they have high Vp, up to -10V and low Idss. Any suggestions for possible replacements? BF245A? MPF102? 2N5458?

1670603557394.png
 
Can you point me to that thread?

The InterFET datasheet I looked at shows Idss can range from 0.5mA to 15mA. Vgs,off max is -10V, but no min is specified.

My best advice is to analyze the Sunn preamp circuit to determine what range of Idss & Vgs,off the circuit needs.
 
OK, looking at the sch I'd say you want a pretty high Vgs,off, like at least -8V and don't worry about Idss. They're all runnind Id around 400 to 500 μA. J111 could be a good option. MPF102, maybe if you find a real hot one. BF245 & 2N5458, no, too cold. Whatever you use, you will need to cherry-pick for high Vgs,off. Lower Vgs,off JFETs could work if you decrease the source resistors. Whatever you use, make sure it's rated for at least 25V.
 
Chuck D. Bones said:
Here's a comparison of most of the JFETs we use in pedal circuits.
I pulled 5 samples each of 15 different part numbers. I measured Vgs,off and Idss. From that, I used the JFET formula to calculate the Vgs and gm (transconductance) at Id = 200μA. I chose 200μA because that's a typical drain current for pedals. Some pedal circuits run the JFETs at a much lower or much higher drain current. It's easy enough to recalc the Vgs and gm for any drain current between 0 and Idss.

This is a small sample size and does not represent the full range of Vgs,off & Idss that we might experience for a given part number. All of these parts were in-spec, although a few were right on the edge. The test conditions for Vgs,off vary from one part number to the next. Some spec sheets measure at Id = 10nA, others at Id = 1μA. My measurement conditions were in the ballpark, typically around 150nA. The resulting error in my Vgs,off measurements are a few % at most. Close enough for Rock & Roll.

Some tips on reading the table below:
gm is negative because the JFET inverts the signal.
To calculate voltage gain, multiply gm (in mS) x drain resistor (in K). Example: Rd = 22K, gm = -1.691ms, then gain = 37.2 (31dB). This assumes the source resistor is bypassed by a large capacitor.
The source resistor can be calculated as Rs = -Vgs / Id. Example: 1.66V / 0.2mA = 8.3K.

View attachment 32190

View attachment 32193

View attachment 32194
Click to expand...

I'll use this occasion to ask: I've recently built a BD-2 and used J201s as that's what the BOM called for. There's something about the overdrive texture I just don't like as much as the original (I have the non-Waza original, SMD version, but wanted a narrower top jack version).

Having also tried with MPF102 (have a few of those in through holes), I noticed that gain doesn't change much (due to being use as a discrete opamp) but tone does change a little. The MPF102s were muddier than the J201.

So wondering if the JFETs are my problem. I see that a certain other PCB seller recommends 2SK209s as a modern day equivalent. They say "not far off from a J201 but different enough". They're fairly cheap on Mouser but I'm out of SMD adapters ("Cobalt" PCB hasn't been updated to have the SMD pads yet).

Is it in your opinion worth ordering new adapters and those JFETs or will the difference be minimal?

Thanks
 
Snicksound said:
I'll use this occasion to ask: I've recently built a BD-2 and used J201s as that's what the BOM called for. There's something about the overdrive texture I just don't like as much as the original (I have the non-Waza original, SMD version, but wanted a narrower top jack version).

Having also tried with MPF102 (have a few of those in through holes), I noticed that gain doesn't change much (due to being use as a discrete opamp) but tone does change a little. The MPF102s were muddier than the J201.

So wondering if the JFETs are my problem. I see that a certain other PCB seller recommends 2SK209s as a modern day equivalent. They say "not far off from a J201 but different enough". They're fairly cheap on Mouser but I'm out of SMD adapters ("Cobalt" PCB hasn't been updated to have the SMD pads yet).

Is it in your opinion worth ordering new adapters and those JFETs or will the difference be minimal?

Thanks
Click to expand...
More than part number, I’d be concerned about the specs the circuit requires.
 
Giovanni is correct. I wrote a long and boring 4-part article about the BD-2, starting here. Part 4 has a section on checking and setting the bias. Getting the bias right maximizes the headroom and is the key to best performance. If the bias is far off, the headroom will approach zero.

giovanni said:
I forgot to ask: what method did you use to measure the JFETs?
Click to expand...

I used the ROG method to measure Vp & Idss. The source resistor was 10M for the Vp measurement.
 
Here goes...

When the switch is in the up position, the gate is shorted to the source. That is the test condition for measuring Idss. The DMM is connected across the 100Ω resistor. We calculate the drain current from the voltage drop (V) across the 100Ω resistor.
Idss = V / 100Ω

When the switch is in the down position, the gate is connected to GND. The source is connected to GND thru the 1M-10M resistor. The DMM measures the voltage between gate (GND) and source. That's Vgs. The drain current, Id, flows thru the 1M-10M resistor, which causes the drain current to be very low, between a few hundred nA and a few μA, depending on the exact resistor value and Vp. That's the test condition for measuring Vp (Vgs,off). The DMM reads Vp directly. Note that Vp = -Vgs,off.
 
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