Speaking of SMD JFET's ...

[RobA]

I was just wondering if anyone has tried these out
http://www.nxp.com/documents/data_sheet/PMBFJ620.pdf

They are a pair of JFET's on a single SMD device. I picked some up at Mouser in my last order in the hope that they might be pretty closely matched as a pair. They are a pain to test because they are really small SOT-363 packages, so I've only tested two of them that I put on adapter boards. But, the two pairs I've tested have been dead on in both Idss and Vgs-off. I'm going to breadboard a couple of phase45 sections to test them in first, but I'd like to hear if anyone has any experience with them.

[midwayfair]

Wow, that's almost enough to make me go to SMD ...

I wonder how close they are chip to chip, though?

[pickdropper]

The dual parts are definitely cool.  I've been meaning to do a build with some of the dual clipper diode packages but haven't gotten around to it.

[RobA]

Wow, that's almost enough to make me go to SMD ...

I wonder how close they are chip to chip, though?

I've no idea really. The two sets I tested were really close, but they were also adjacent in the tape package -- if that means anything at all. Actually, the two different chips being so close in specs gave me less confidence in the match of the pair on the same chip being consistently good. I'm still hopeful though. A matched set of four on an IC would be outstanding, but I haven't come across that yet.

[kothoma]

I think we discussed the J310 some time ago and you pointed out the high Idss.
Well, these have the same. I haven't checked all the other values but maybe these are just two J310 in one package? (Would explain the name...)

Edit: http://www.madbeanpedals.com/forum/index.php?topic=12423.0

Edit2: Pardon, just realised it wasn't you.

[midwayfair]

I think we discussed the J310 some time ago and you pointed out the high Idss.
Well, these have the same. I haven't checked all the other values but maybe these are just two J310 in one package? (Would explain the name...)

Edit: http://www.madbeanpedals.com/forum/index.php?topic=12423.0

Ooh, good catch!

[RobA]

Looking at the spec sheet for the two, they do look to have pretty much the same specs. I'm guessing you are right about the name simply implying two J310's.

I hadn't been considering these for gain circuits or the  J310's either, but now I'm interested. The spec sheet lists them as high gain and low noise. So, is this just the specifics of how they are used in the Fetzer valve layout? But, the thing has me interested is, if they really do what the Fetzer valve calculator says, then would using them as the input valve stage help with the over the top gain of the JFET simulators? Would they maintain the harmonic character while allowing reduced gain into the following stages? 

[kothoma]

I like this reasoning. So -14dB gain (80% loss) could be a good thing after all? Alternating positive and negative gain stages?

Edit: But I still can't believe what the Fetzer valve calculator claims.

[RobA]

... Alternating positive and negative gain stages?

Yeah, that's what I was thinking. I'll have to give it a try on one of the ROG circuits to see how it works.

[RobA]

First report is that they work perfectly in a Phase 45, Smoothie PCB actually.  tried the two pairs that I have soldered to the breakout board and they both biased right up with the stock components on the PCB. Actually, I didn't even need to rebias when I went to the second set. They worked exactly with the settings for the first pair. I still have no idea how consistent that's going to be though.

I'll follow up with some breadboarding of other circuits.

[jubal81]

First report is that they work perfectly in a Phase 45, Smoothie PCB actually.  tried the two pairs that I have soldered to the breakout board and they both biased right up with the stock components on the PCB. Actually, I didn't even need to rebias when I went to the second set. They worked exactly with the settings for the first pair. I still have no idea how consistent that's going to be though.

I'll follow up with some breadboarding of other circuits.

Now that's some excellent news indeed.

[kothoma]

First report is that they work perfectly in a Phase 45, Smoothie PCB actually.  tried the two pairs that I have soldered to the breakout board and they both biased right up with the stock components on the PCB. Actually, I didn't even need to rebias when I went to the second set. They worked exactly with the settings for the first pair. I still have no idea how consistent that's going to be though.

I'll follow up with some breadboarding of other circuits.

Now that's some excellent news indeed.

Yeah! And thanks for sharing!

[RobA]

I got a chance to put together the ROG Eighteen on the breadboard and test the J620 in it. It works. I built it up with the J201's first and then changed the first position to a J620. It brings down the distortion a touch -- still plenty of drive left though and I thought it sounded good. I then put one in the 1st and 3rd positions. This cleans the circuit way up. It still overdrives, but much more into a light crunch sorta area. I could not get a J620 to bias correctly in the second position. Actually, it did bias to the rec'd 7V, but the audio was gone. Biasing to any other voltage didn't work either. Itnteresting, but I tried an MPF102 in this position and it wouldn't work either. So, I think it has something to do with the gain structure off of the source and this would need to be altered.

The conclusion so far is that they will work in distortion circuits. The drop in gain does not seem to be as much as would be predicted from the ROG calculator. All-in-all, I think the J310's would be worth checking out. I'm going to keep experimenting with the J620's and see what more they could be good for. I think I'll try a discrete op-amp input stage next.

[RobA]

Oh yeah, I forgot to mention that to bias the J620's it really helped to move to 25k or 10k trimmers. The 100k were just too touchy and the bias point was nowhere near that point.

[atreidesheir]

I am following your reports closely. 

...I then put one in the 1st and 3rd positions. This cleans the circuit way up. It still overdrives, but much more into a light crunch sorta area.

The conclusion so far is that they will work in distortion circuits. The drop in gain does not seem to be as much as would be predicted from the ROG calculator. All-in-all, I think the J310's would be worth checking out.

I have been brainstorming how to reduce the gain of the ROG jfet conversion circuits in an elegant way, not just a parallel resistor on the gain pot.  This is great.  and I have avoided phasers for the biasing headache.  You may be making this more accessible to us "cowboys" who are not quite "genius"es