Building a JFET matcher

[iamjackslackof]

Hi all,

I built a Greatly Improved JFET Matcher II recently, to test JFETs for an Aion EP-3 preamp. Since then I did some reading here how some JFET matchers, possibly the Rullywow one, only measure Vp a certain way, or incompletely, or something... It's all still a bit over my head. If I want to measure JFETs both for something like an EP-3, which requires specific Vgs(off) and Idss values, as well as match JFETs for stuff like phasers, is the Vero one sufficient? I did order a PCB for the Rullywow, is it sufficient? Do I need to spring for a DCA75 to cover all my bases? I have the DCA55, but it only identifies JFETs. I want to have some way of handling all the common JFET measuring situations, ideally in 1 circuit, but 2+ is fine as long as I know each of their uses and shortcomings.

Thanks!

Edit: Related question: The Vero matcher I built works, but I'm not certain which switch position is Idss and which is Vp. Any hints?

Edit2: There's also the ROG version as well (fig. 10), how would it fare?

 

[Cybercow]

After getting inconsistent results from some of the various DIY JFET matchers, I broke down and got the DCA75. Inconsistencies gone.

 

[fig]

Edit: Related question: The Vero matcher I built works, but I'm not certain which switch position is Idss and which is Vp. Any hints?

When you are reading gate-drain you are reading IDSS, when reading gate-source, it's Vp (or Vgs-off)

 

[Chuck D. Bones]

Cheapest method to measure Idss and Vgs.off (Vp) is to use the ROG version referenced above. The switch positions are marked and the instructions are at the top of Fig 10.

The Keen matcher gives Vgs for a specific Id. It's fine for matching JFETs for a phase-shifter. It is not intended to measure Vp.

 

[iamjackslackof]

Cheapest method to measure Idss and Vgs.off (Vp) is to use the ROG version referenced above. The switch positions are marked and the instructions are at the top of Fig 10.

Thanks Chuck, I suspected as much. I can't quite tell from the schematic if the junction of the 100R/1M/JFET/switch is all connected. I assume so?

After getting inconsistent results from some of the various DIY JFET matchers, I broke down and got the DCA75. Inconsistencies gone.

I'm starting to lean that way. Do you know what other benefits it would have over the DCA55 other than measuring JFETs? Still trying to talk myself into dropping that kind of coin when I have a DCA55 already lol.

 

[Cybercow]

I'm starting to lean that way. Do you know what other benefits it would have over the DCA55 other than measuring JFETs? Still trying to talk myself into dropping that kind of coin when I have a DCA55 already lol.

As you know, the DCA55 will only identify a device as a JFET, if it is. The DCA75 will measure & display ALL the important aspects of a JFET. The DCA75 also comes with MS Windows software that will chart an active device's curves across a selectable spectrum of applied voltages.









I'm convinced the my DCA acquisition was worth the investment. I also invested in the LCR45 to more accurately measure coils, and capacitors.

 

[iamjackslackof]

Well shit. I think you talked me into it lol.

As an update, I did manage to use my Vero build to measure Q1 and Q2 for the EP-3, and it sounds simply killer! So thanks to everyone who responded.

 

[iamjackslackof]

Aaaand I pulled the trigger on one lol.

 

[Alan W]

Aaaand I pulled the trigger on one lol.

You won’t regret it. Fast and almost fool proof. I’m anal about matching everything, yet I find doing so to be mind numbingly boring, so anything that make it harder to make mistakes is a big plus for me.

 

[Cybercow]

Aaaand I pulled the trigger on one lol.

To prevent constantly working the clips on the DCA75, consider making a ZIF socket adapter for when testing bunches of JFETs & stuff.



And FWIW . . . you may want to consider the SMD interface for the DCA75. But they want another 25 zorkmids for one.

 

[iamjackslackof]

Thanks @Cybercow. Luckily I already built a ZIF adapter, didn't take long dealing with the 3 test hooks to want something better. And I saw the SMD interface, but thus far I haven't been doing much with SMD. That will probably change soon, so I'm glad I can get the adapter when I need it. Do you know of any other handy adapters like that? You'd think they'd make their own ZIF thing as well.

 

[Feral Feline]

You’d think by now they’d have software ported to Mac.

 

[iamjackslackof]

You’d think by now they’d have software ported to Mac.

I was hoping they did too. I'll end up trying to pass through the USB connection to a Windows VM, since I only have a Mac in my pedal area.