JFET Shootout

[giovanni]

Can you remind us what Vgs,off represent from a physical perspective?

 

[Chuck D. Bones]

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.

 

[fig]

I used the easiest method.

But you said you didn’t have a DCA75.

 

[fig]

I found my 2N5426 list...

 

[temol]

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?

 

[Chuck D. Bones]

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.

 

[temol]

Only two posts there, nothing technical.

Schematic here

 

[Chuck D. Bones]

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.

 

[giovanni]

I forgot to ask: what method did you use to measure the JFETs?

 

[Snicksound]

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.

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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

 

[giovanni]

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

More than part number, I’d be concerned about the specs the circuit requires.

 

[Chuck D. Bones]

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.

I forgot to ask: what method did you use to measure the JFETs?

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

 

[giovanni]

Is this the correct circuit?

 

[Chuck D. Bones]

You're gonna have to show me a schematic. I'm too lazy this morning to trace the Vero.

 

[giovanni]

You're gonna have to show me a schematic. I'm too lazy this morning to trace the Vero.

I know me too. Couldn’t find the damn schematic!

 

[Chuck D. Bones]

I used the one down at the bottom of this page:

https://runoffgroove.com/fetzervalve.html

The 100Ω resistor can be 1K if Idss is <7mA.

 

[giovanni]

I used the one down at the bottom of this page:

https://runoffgroove.com/fetzervalve.html

The 100Ω resistor can be 1K if Idss is <7mA.

That’s pretty nice! Thank you! And I do think it is the same circuit as that vero board.

 

[Chuck D. Bones]

That's for someone else to figure out. I have lost interest in Tagboardeffects. Too many Vero layouts with errors.

 

[giovanni]

That's for someone else to figure out. I have lost interest in Tagboardeffects. Too many Vero layouts with errors.

I’ll triple check before I use it then.

 

[giovanni]

Is there an explanation of how that circuit works? I can kinda make some educated guesses…