JFET Replacements for 2N5952 in MXR Phasor

[chaoseverlasting]

Im building an MXR phasor, the thing is I can't find the JFET's 2N5952. What other JFETS can I replace them with?

 

[uart]

Whats an MXR phasor? It's hard to recommend a good replacement if we don't know that application. Is the jfet being used as an RF amplifier or an analog switch or something else?

Anyway you might give the Fairchild J309 or J310 a try. They're not an exact match for you part but they're readily available and somewhat similar to it.

 

[vk6kro]

Seems to be just a low powered device but with 30 volt breakdown ratings.

Radio Shack have MPF102 s which have a 25 volt rating while the 2N5932 has a 30 volt rating. If that matters, don't use the MPF102. They are $1.29.

The MPF102 has the G and D wires in opposite places to the 2N5932, so be careful if you use it.

There is always a risk that a substitute transistor may not work, so try at your own risk.

 

[chaoseverlasting]

Its being used as an RF amplifier I think... An MXR Phasor is a guitar effect pedal. Variable phase shift all pass filter (Im 99% sure that's the exact description)

 

[vk6kro]

If you have a circuit, maybe you could post it.

It is unlikely to be an RF amplifier, but if it is, the MPF102 is normally used as an RF amplifier. It is OK at audio, too, of course.

Or, just get one and try it, if you are sure the original type is unavailable.

 

[chaoseverlasting]

Here's a pdf of the diagram. I couldn't find a JPEG link, sorry.

 

[vk6kro]

The FETs seem to be being used as switches but the description says they have to be matched. This is because they are all being controlled together.

This might mean you would have to buy 10 of them and test them to get similar ones.

They don't say which way they should be tested but one way would be to test them in a multimeter that has a transistor tester in it. This only tests for gain, but it may be enough.

In this circuit, it might be better to test for cutoff voltage, but you would have to set up a bit of equipment to do this.
This is the negative voltage on the gate that just stops the current flowing between the drain and source.

 

[chaoseverlasting]

How would I go about it? What do I need to do?

 

[vk6kro]

You would need some equipment.

TWO 9 volt batteries
one potentiometer, about 10 K total resistance.
TWO multimeters, one on voltage and one on current.

See attachment. Be careful to get it right if you wire it up.

The FET gets negative voltage from the potentiometer across the left battery and it gets its normal positive voltage from the battery on the right.
Current is measured in the drain lead of the FET and voltage is measured across the Gate- source .

You need to check a few FETs for
(1) gate voltage to cut off drain current and
(2) drain current with zero voltage on the gate and
find 4 that are most alike.

Just to remind you of my earlier comment:
There is always a risk that a substitute transistor may not work, so try at your own risk.

 

[chaoseverlasting]

I can't even find those! What would be the most generic, out-dated but commonly available part? Something like the 741 of JFETs?

 

[vk6kro]

Try this:

http://www.radioshack.com/product/index.jsp?productId=2062616

The MPF102 is an old FET but still works just fine. Nice to see Radio Shack carry them.

That test setup is ideal for learning about how FETs work, too.

 

[chaoseverlasting]

Thanks a lot vk6kro, Ill give it a shot. If this works, youre a lifesaver man!