Alternative for 2SK30A for bypass?

[fogwolf]

Anyone suggest an alternative to the 2SK30A JFet for use in a bypass system in a circuit? Anything other than the NTE458, which is pretty pricey (at Mouser at least).

[PerroGrande]

You should be able to use a 2N5457 in place of the 2SK30A.

The range of Vgs-off is *close*.  The 2Sk30A claims a range of 0.4 - 5.0 volts (negative), while the 2N5457 claims 0.5 - 6.0 volts (negative).

I don't recall exactly how far negative the BOSS multivibrator swings.  It is *possible* that if you randomly end up with a -6.0v 5457 and the boss only swings to -5.2 volts, it won't shut off the FET... In which a case, you could try another FET -- or pick one with a lower Vgs-off.

**edit**

A J112 might also do the trick.  It is close to the original stats, too.

[fogwolf]

Thanks, I'll give those a shot!

EDIT: And actually, this is for a Boss redesign where the person who designed the circuit did not decide to include bypass, so I am going off the Boss schematic. If it helps as far as voltages and what not it will be coming from a N.O. momentary switch.

[Mark Hammer]

DOD likes to use the J113.

[fogwolf]

Thanks for the suggestions.

Not sure if you saw my edit above but this is for a Dim-C clone, similar to the one Scott Stites did and which there is a thread about here somewhere. The guy who did this particular redesign decided not to include any bypass circuitry but I would really like to have this so I am going off of the Boss schematic to mod the one I build. I'll be using a N.O. momentary switch connected through a TS jack.

Not sure if any of this information narrows down which JFET would be best.

Also, how could you test for the Vgs-off voltage for a transistor? Could you do this with a multimeter somehow?

Thanks again!

[PerroGrande]

Paul,

The thing to be careful of when it comes to using a JFET as a switch is to make sure you can get the device to shut off.

JFETs are depletion-mode devices, which means that the gate has to go negative with respect to the source (for N-channel devices) by at least the amount specified by the Vgs-off rating on the datasheet.   For the 2SK30A, this value is 0.4 - 5.0 (negative), and is similar for the devices I mentioned.

You'll need to make sure that you know the details of the circuit so that whatever you're using to switch the JFET can pull its gate low enough (with respect to the source) to cause it to pinch off.   If you're going to clone the Boss multivibrator circuit, then you're probably going to be okay with any of the parts mentioned.  However, you can't just use a momentary switch straight-up, unless you want the effect to be one that stays engaged only while the switch is depressed (like a sustain pedal on a piano or synth).  You'll need some associated logic to toggle the state of the JFET's gate in order to accomplish switching.

[fogwolf]

Thanks for the clarification.

Here's info from Scott's build:

Effect Bypass Circuit

When I decided to build the Dimension C, I chose to see if I could find as many of the original parts as possible. This really isn't necessary, as Mike Irwin has proven that effective substitute parts will work nicely, but, at the same time, I figured I'd be lucky enough to get the thing working with the original parts.

So, the core of the Dimension C has as many of the original parts that I could find. The exceptions are as follows:

For 3V zeners, I used 1N5225's, for 5.1V zeners, I used 1N5231B's, for the power protect diode, I plan to use a 1N4002, and for the bypass flipflop, I am using a CD4013 - I could not find a BA634 to save my life.

Debouncing the 4013 proved to be quite a challenge for me. My test of the switch is actually a foot operated normally open momentary switch, and a very noisy beast at that for the 'trial by fire'. After a lot of experimentation, I finally wound up with very effective de-bouncing.

Jeff and I decided to add a bypass input that would toggle the bypass with a positive trigger pulse rather than a foot operated switch, making it controllable with modules from our synthesizers. I added the circuitry to do that, and found that not only triggers would activate the bypass, but also LFO's and VCO's. This actually provides an additional means of modulation - modulating the bypass at audio rate frequencies introduces a sort of AM effect.

One additional comment - I have no idea what color the original 'Check' LED (LN-28RP) was, but Jeff and I decided to use red as the power LED and green as the 'active effect' LED, so the original LED has been replaced with a green LED.

Below is the schematic for the Dim C/TZF bypass circuit.

And here is a link to this off-circuit schematic:

http://mypeoplepc.com/members/scottnoanh/sitebuildercontent/sitebuilderfiles/dimc_bypass.jpg

So I'm not sure if the CD4013 is what you might have meant about not using just the NO momentary.

Thanks again.

[PerroGrande]

So -- is if fair to assume that you're going to use the output from point "E" to drive your switching/bypass circuit?

[fogwolf]

Yes - should this work with the suggested FETs above?

I was going to follow the Boss schematic here to add the bypass circuitry to my project. You can see where C63 & R93 are in this Boss schem is where the snippet of schematic above cuts off, where the exit point E of the CD4013 goes to the LED goes on to the 3 FET/diode junctions (1 for the input 2 for the outputs):

http://www.mif.pg.gda.pl/homepages/tom/files/bossDC2.jpg

The redesign I'm building has the same spots on its circuit - I was planning to put the small by-pass circuit with the CD4013 on a veroboard as well as the 3 FETs/diodes and wire those into the where they would need to connect on the board (since there are no pads for these transistors or diodes on the PCB at these points). Here is the schematic for what I'm building:

http://www.mif.pg.gda.pl/homepages/tom/files/bossDC2.jpg

Thanks.

[fogwolf]

To be more specific, I'd have the transistors and diodes on the veroboard but where the source & drain of each FET would need to connect to main circuit I would use wires to bridge them and share pads with components leads at pin 6 of IC12 & at R88, again at pin 2 of IC12 & R87 then finally at C7 & R9.

I'm pretty sure that when using transistors as an amp you want to keep the traces between the connected components as short as possible, but is that the case here - will there be a problem that these FETs will be off-board and connected with wires? Should I just give up and accept I won't be able to bypass this? Not building into a stompbox but into a 19-inch rack chassis.

D'oh - didn't send correct link to the schem for my build - here it is:

http://electro-music.com/forum/phpbb-files/lush_diy_schem_with_switch_pins_190.pdf

[R.G.]

2N5485 and 2N5292 have low Vgsoff too. 2N5457 runs a bit higher.

[fogwolf]

Thanks for the extra suggestions. Would the 2N5484 be even better? Looks like the Vgs off is even lower than the 2N5485.