Problem matching transistor outputs!

[steveyraff]

Hi all,

I've built a few HPF units for bass players recently. I believe the layout is an FDeck replica.

I've been having some issues with the transistors used.

It states Q1-Q2 are 2N4416. Firstly, I had issues with these. They have 4-legs, when I only require 3 of them. Which for the most part wasn't a problem, except I found the 2n4416 to be strangely inconsistent. Some simply wouldn't work at all, others were fine.

So I tried to find a more reliable replacement. I had read the comment section about this layout on Guitar FX Layouts and some people also tried experimenting. One guy said he had good results with J201's. I've now built a few with J201's, but anyone I built for using those, reported quite a big drop in output signal.

Another user ran a lot of tests and found best results with 2N3819s. Again, these seem really inconsistent for me. I can't figure out why, but some 2N3819s work for me, and others (most) do not work for me at all (zero signal). Are there different kinds of 2N3819 transistors?

Lastly, I once ordered J201's and accidentally received PF5102's, which seemed to work, but I think they might also have been slightly lower output like the J201's.

Anyone got any other suggestions?

Thanks.

http://tagboardeffects.blogspot.com/2015/01/hpf-technology-hpf-pre-2.html

[imJonWain]

I have no recommendations from experience but looking at the 2N4416 datasheet the spec for Vgs off is quite large and Transconductance is rather high but only spec'd as a min at 400 MHz.  You may need to sort, match and Bin these like a lot of jfets.  It's rather quite different then a J201.  You could follow the "when in doubt" try a 2n5485 or 2n5457.  Those are a bit more similar and commonly used as amplifiers.


Datasheets:
http://www.farnell.com/datasheets/39775.pdf

https://www.onsemi.com/pub/Collateral/2N5457-D.PDF

https://www.mouser.com/datasheet/2/308/2N5485-1118308.pdf


I find if you buy from mouser or digikey in strips you tend to get luckier with transistors being closer in spec to each other.  This may just be coincidence on my part though.

[duck_arse]

can we see a circuit diagram? it seems the right side jfet is relying on a roundabout gate biasing scheme, involving unseen pots. the left side one is biased by the 10M, yes?

as for the fourth leg, it should be a case connection only appearing on the metal can packages. check with your meter - one of the legs [datasheet!] will be short with the can. you can ignore the lead, cut it off, connect it to ground. its presence shouldn't have any effect on the circuit operation.

the J201 would be a long way off a replacment for the 4416's.

[Rob Strand]

suggested replacement is the 2N3819 JFET.

The 2N3819's can be quite variable but they should at least work.

Schematics:
https://github.com/bassistTech/HPF-Technology-LLC/tree/master/HPF-Pre-Schematics

Ideally the two stages required different JFET characteristics for optimal bias.
Things like 2N5457's might not turn out so great.   If the gates returned to some
positive bias instead of ground it would be a lot easier to use different JFETs.


A J112 might even work but I haven't checked how well it works.