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Author Topic: Substitutes for J201 FETS for lowatt amps  (Read 18982 times)

xylix

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Substitutes for J201 FETS for lowatt amps
« on: April 10, 2007, 07:19:42 AM »
Hi,
I want to make a few low wattage guitar amps. Projects like the Little Gem Mk II and Ruby (http://www.runoffgroove.com/littlegem.html) use an FET in an input buffer. I built one Ruby with a J201 I ordered online which doesn't work - I think it is because I soldered the FET directly and didn't use a socket. (Live and learn.)
They suggest one of:
    MPF102
    J201
    2N5457

The thing is, I have found that I live really close to a large number of electronics parts stores (in Osaka, Japan) so it is easier and cheaper to buy parts locally rather than order from SmallBear et. al. I just asked about buying such an FET at 3 different stores. At one they had tons of FETS, but said they didn't sell any of those three. At another store a helpful guy told me they don't sell those, but suggested one of three others that are the same. They are:
    K170
    K246
    K30

I bought one of each to try out. I am wondering (1) if these ARE substitutes and (2) how one knows what can be substituted. Doing a web search has so far not given me much information. It seems like there are other choices for FETs around here, but I have no idea how they are different.

Also, can anyone explain simply what an "input buffer" is/ does? and what purpose an FET serves here? I have read the wikipedia entry on FETs but don't really get it.

Thanks.

teemuk

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Re: Substitutes for J201 FETS for lowatt amps
« Reply #1 on: April 10, 2007, 01:51:00 PM »
First, the basic explanation:
http://en.wikipedia.org/wiki/Buffer_amplifier

Practical applications:
http://www.muzique.com/lab/buffers.htm

And then, some theory:
http://www.colorado.edu/physics/phys3330/phys3330_fa05/pdfdocs/AN102FETbiasing.pdf

Since the FET is used as a buffer amplifier (source follower) the fact whether the FET is a substitute to some specific model is a minor concern. Besides, the substitutes recommended by component stores are selected on totally other principles than "tone" and you should be familiar with the parameters a FET in some specific circuit needs to have.

And a small "side trip": The reason why 99% of FET-based guitar effects use J201 is mostly "historical" and not related to parameters of the concerned FET at any way. ...But that's another story. My main point is to say that in most cases the J201 is a very poor performer because it's selected "out of habit".

IMO, the concerned FET is one of the worst ones to use in buffering applications. For the given application (buffer in low voltage circuitry) you could randomly pick almost any FET and expect it to operate pretty well. In most cases it will also surpass the performance of J201.

xylix

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Re: Substitutes for J201 FETS for lowatt amps
« Reply #2 on: April 10, 2007, 09:52:59 PM »
Besides, the substitutes recommended by component stores are selected on totally other principles than "tone" and you should be familiar with the parameters a FET in some specific circuit needs to have.

Thanks for all the information. How can I find out about the parameters of any FET? I have seen datasheet information for the J201 somewhere on the internet, but how about other FETs such as the ones recommended by the shop? Or, more importantly, if I do find datasheets for different FETs how to I compare them? What are the important parameters? (Perhaps I just have to spend more time with that theory document you linked to.)

R.G.

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Re: Substitutes for J201 FETS for lowatt amps
« Reply #3 on: April 11, 2007, 08:52:03 AM »
Quote
How can I find out about the parameters of any FET? I have seen datasheet information for the J201 somewhere on the internet, but how about other FETs such as the ones recommended by the shop?
Go to google.com and enter " (part number) datasheet". The (part number) should be the full part number of the FETs; what you were told is the shortened version. Those numbers should have "2S" in front of them, so look for 2SK170, 2SK246, and 2SK30.

The 2SK30 is a good choice for buffers in a 9V effects box.

Quote
Or, more importantly, if I do find datasheets for different FETs how to I compare them? What are the important parameters? (Perhaps I just have to spend more time with that theory document you linked to.)
Look for Vgsoff or "gate cutoff voltage", Idss, and Yds. Vgsoff is the voltage that when applied to the gate will cut off conduction completely. The J201 is unusual because it has a Vgsoff of only 0.1 to 0.5V, the smallest of any depletion mode JFET I've ever seen. The 2SK30 is 0.5 to 4.0 as I remember, and it's a good choice for self bias in a 9V environment.

The Idss is the biggest current the JFET will conduct without destructive voltages. It needs to be big enough to set up your buffer correctly. The J201 has a tiny Idss, really too small for good input buffer situations. The 201 is a good amplifier, less so a buffer. Again, the 30 is a good choice. I don't know about the others.

Yfs is the forward transconductance, the change in drain current per volt of change on the gate. It is the number which determines voltage gain in a gain circuit - which you're not doing - and how close to a gain of 1 you get in a source follower. It's not very critical in this application.

The parameters of JFETs vary about 5:1 to 10:1, so they are quite difficult to get exact biasing points on and exact gains. This is one reason they never got the wide application of bipolar transistors. In a source follower, though, the actual JFET is not as critical. You want a Vgsoff that's a significant fraction of the 9V battery, but not most of it or bigger than 9V. One around 4.5V is good for a 9V source follower.

Try the 2SK30A.

Conguito

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Re: Substitutes for J201 FETS for lowatt amps
« Reply #4 on: April 11, 2007, 10:48:15 AM »
The perfect substitutes for the J201 are:
BFW13 and 2n4338
But you have to pay attention to the position of the pins, they are not in the same position as in the J201.
Salu2.

teemuk

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Re: Substitutes for J201 FETS for lowatt amps
« Reply #5 on: April 11, 2007, 11:38:32 AM »
Yes. These are very good substitutes for J201 with almost identical performance. However, if you understood R.G.'s post you also understood that in the concerned application (buffer) you do not wish to substitute the FET with a one behaving similarly. The J201 was a bad choice in the first place. Try something like J308, J309, U310, 2N3458, 2N3459, 2N4339... Heck, pretty much anything. Like R.G. said: The J201 is "unusual".

xylix

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Re: Substitutes for J201 FETS for lowatt amps
« Reply #6 on: April 11, 2007, 12:23:37 PM »
Go to google.com and enter " (part number) datasheet". The (part number) should be the full part number of the FETs; what you were told is the shortened version. Those numbers should have "2S" in front of them, so look for 2SK170, 2SK246, and 2SK30.

Great post. Thanks for the info. When I asked about a J201 the guy had replied "oh we don't have 2SJ201" and I had wondered about that.

Quote
The 2SK30 is a good choice for buffers in a 9V effects box.

Good, I just bought a couple of those on the shop guy's recommendation.

Quote
Look for Vgsoff or "gate cutoff voltage", Idss, and Yds. Vgsoff is the voltage that when applied to the gate will cut off conduction completely. The J201 is unusual because it has a Vgsoff of only 0.1 to 0.5V, the smallest of any depletion mode JFET I've ever seen. The 2SK30 is 0.5 to 4.0 as I remember, and it's a good choice for self bias in a 9V environment.

I have already downloaded a few datasheets, but couldn't figure out what amongst all of that technical information was important. Again, thanks a lot for helping a newbie.

Tonight I was just looking over the schematics for the Ruby and the Little Gem Mk II (both of which use an FET as an input buffer) and a simple guitar preamp schematic which uses a FET as a preamp. (The latter is at: http://www.till.com/articles/GuitarPreamp/). As far as I now understand it a preamp boosts the signal by about 3 dB. But an input buffer has no effect on increasing the gain, but only increases the voltage (or is that current?). How is it that these three circuits are very similar (but none identical) but only one of them acts as a preamp? Still trying to wrap my head around how FETs are used in general terms.

teemuk

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Re: Substitutes for J201 FETS for lowatt amps
« Reply #7 on: April 11, 2007, 01:10:47 PM »
How is it that these three circuits are very similar (but none identical) but only one of them acts as a preamp?

The term "preamp" is just semantics: Consider the definition from Wikipedia: "A preamplifier (preamp) is an electronic amplifier which precedes another amplifier to prepare an electronic signal for further amplification or processing". That doesn't really tell much, does it.

What you are interested in in electrical terms is either voltage or current gain. The difference between common emitter (BJT) / common source (FET) and emitter follower (BJT) / source follower (FET) configurations is basic electronics. If you have trouble understanding these concepts then I dearly suggest you take a few courses in electronics or find a good book/website before bothering about vague terms like "preamplifier".

The answer to your "question" is: Both of those circuits are - sort of - preamps.

 

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