AION Plexus Bias Problem

[johno9]

I've built the AION Plexus with NOS 2N5457 through hole JFETs. The unit requires a bias setting  for Q5. Bias should be 4.5V, half the supply.  I am unable to get lower than 5V with a 9.1V battery. The unit functions, but is "hissy", which I think may be do to the wrong bias.
Any suggestions on parts value replacements (R11, R10?) to be able to obtain the correct bias.
Thanks.

[antonis]

A 10% of 100k trimpot added in series should bring Q5 Drain down to 4.5V so a 15k for R11 should be OK.. 
(I'd make R11 larger than 22k say, for more trimming convenience..)

P.S.
I'm sceptical about Q5 transconductane for such a Drain low current but I presume that's another story..

[johno9]

Thanks Antonis

[PRR]

5.0V ought to be fine. I don't think "exact half" is magic with that big source resistor.

However, try cutting R10 in half. I'm sure I can't read that fuzz but simply tacking another of the same value across the existing R10 does the same thing.

[Rob Strand]

However, try cutting R10 in half. I'm sure I can't read that fuzz but simply tacking another of the same value across the existing R10 does the same thing.

+1 That's what I'd be doing too.

It's a while since I've looked at this one but I think the resistance of the 100k Q5 bias trimpot+ 5.6k(ie. R11) should end-up at around 30k.   You could set the trimpot 25k then reduce R10 until the Q5 drain voltages is about 4.5V, or 3.4V if you go by GGG,

http://www.generalguitargadgets.com/pdf/ggg_bsiab2_instruct.pdf

Once you get in the ball-park use the trimpot to find the sound you like.

If you get it going with the trimpot set to 100k then the large drain resistance will start to change how the tone control works.   That's why it's better to set it to around 25k (AFIK it's around there).

The thing which will vary on your unit compare to other builds is the Q5 source voltage, that needs to match your specific JFET.  (FWIW, there's a lot of low VGSoff JFETs around these days and that requires a smaller R10.)

[duck_arse]

we need voltages. of all the jfets, please. also photos of wotchu bilt, cause, y'no.