How to set bias of JFET

[embotone]

I have been on another board discussing replacing a JFET, D1, in this circuit: http://www.sdiy.org/oid/mics/MXL-V67.gif   using this JFET(the J305):  http://datasheet.octopart.com/J305-E3-Vishay-datasheet-42053.pdf.  Several people are wondering how the bias can be set, there are some there who know, but are rather condescending about the matter and will not share what I imagine is not that big of a deal.  I have been looking on the internet for practical info about this but am confused.  I think I could inject a signal at the gate and adjust the source resistor R7 for max clean signal at the drain.  Is this correct?  Also, is there a way to adjust bias for those without a scope?  For example, adjust R7 for ideal voltage drop across the drain resistor, R8?  What would the ideal idle current be?  4mA?
Everyone one here has always been very helpful without treating people like idiots.  Can someone advise me on this?
Thanks

[PRR]

> what I imagine is not that big of a deal

I know this sounds condescending, but... can YOU do it? Can you even rough-out the key voltages inside this phantom-powered mike?

> What would the ideal idle current be?  4mA?

4mA flowing in R8 20K would be 4*20= 80 Volt drop in the one resistor. Phantom is only 48 Volts total. Does not compute.

I think it would be a good chewy problem for a 4th-year EE student's final exam. Or even a hiring interview for a medium-experienced designer.

You do know the 2SK170 is a significantly "better" (for _this_ use) JFET than the J305? With significantly different Vto and Idss? Is the 2SK170 truly unavailable?
http://ampslab.com/trans_2sk170bl.htm (20 for $20)
http://diyparadise.com/shop/index.php?main_page=product_info&products_id=95 (about US $0.80)

Here's a brief. The top of C12 is probably near +20V. (A "B" student or applicant should have this in 10 minutes.) The gain of "D1" is (obviously) 20K||180K on top, 3.9K+(1/Gm) on bottom. We know the Gm of a good JFET varies slowly with current. It is well-known that 1/Gm is often less than 1K, "small" compared to 3.9K.

So gain is mostly set by R8/R6, hardly depends on JFET bias IF the JFET is not crammed-up at zero or the full power voltage.

R7 affects JFET bias but does not affect gain.

Adjust R7 so that you get about 10 Volts at JFET Drain (pin 1 on that drawing). If easier, aim for 2V at JFET Source  pin 3.

NOT critical!

Using J305, expect R7 to be 1K-5K.

[embotone]

Thank you, PRR, for taking the time to explain this to me.  As to your question "can YOU do it? Can you even rough-out the key voltages inside this phantom-powered mike?", absolutely not.  That is why, after hours of searching the internet about how to do this, understand this, I come here begging for answers.  It was just above my head.  I have absolutely no formal electronics training.  What I have is what I can garner from forums like this, from people like you who are willing to take the time to explain things, and from a few books on tube amps I have read.  I started out as a guitar player, not an electronics engineer.

When I asked "What would the ideal idle current be?  4mA?", that is because I don't even know how the read the spec sheet for a jfet.  Terms like Idss are things I have never encountered before.  I saw "Saturation Drain Current" being 8mA and wondered if the bias current should be half of that.  Your right, it does not compute.  None of this does for me, but I am trying to figure it out.

You ask whether I "know the 2SK170 is a significantly "better" (for _this_ use) JFET than the J305? With significantly different Vto and Idss?"  No, I don't know.  However I have been  reading several threads on the subject of modding this mic, and several people who do this for a living, who appear to know what they are doing, who have a good reputation in the community, think this is a good idea, as it allows for a faster transient response.  I personally don't know how to evaluate other than listening.  It is an inexpensive part, and I am going to  be in the mic anyway.  To me, it would be like tube rolling, I am just going to have to listen.
And again, as for the calculations, thanks a million.  I really do appreciate your time.  This is not the first time  you have helped me to understand things that  are beyond my knowledge.