Adjusting trimpots

[MikeH]

I was breadboarding up a ROG eighteen today; I set the trimpots to give the approptiate drains as suggested on the schematic (4.5v on Q1 and Q3, 7v on Q2).  I had them dead on, and when I tried the circuit, it sounded ok, but the effect level was a little less than line level.  So I just went back and dialed them in by ear, stopping when they were the loudest.  Is that what you're supposed to do, or is it possible I have sommething hooked up wrong and the circuit is not functioning properly?  Are the voltages just suggested starting points?  I used all of the exact same resistor values, within 10ohms, which was within the margin of error for the tolerance anyway.

Here's the schem:  http://www.runoffgroove.com/eighteen.html

[markm]

In this particular case, I mostly adjust by ear and what sounds good to me.
Don't know if it's right or wrong but, being that I'm "old-school", it seemed like the right thing to do!

[petemoore]

  I set the trimpots to give the approptiate drains as suggested on the schematic (4.5v on Q1 and Q3, 7v on Q2).
  '1/2v = 4.5v' only if using a 9v PS.
  if the battery is 8v then 1/2v = 4v
  setting it is a matter of opinion, if you have good control of say 40% - 60% is nice [often times the too big a pot needed to accomodate wide tolerances needed to bias jfets are simply too touchy to easily dial from 4.5v to 4.6v, 'jumping past the fine tune spot].
  just trim by ear, measure, set to 1/2v, listen...this should give you an idea about setting bias.
  Substituting Jfets allows the drain resistor value to 'move'...this may influence the sound also, Jfet swapping is mentioned @ Rog...somewhere...

[markm]

  Substituting Jfets allows the drain resistor value to 'move'...this may influence the sound also, Jfet swapping is mentioned @ Rog...somewhere...

Yes, very true.
Jfets can vary quite a bit indeed.

[MetalGod]

I've got a question on this whole 'biasing to half supply voltage' with mosfets... if I'm using an 18v supply, do I still bias to around 4.5v or to 9v ??

[ildar]

If you're biasing to 1/2 supply voltage, it's self explanatory-1/2 of 9v is 4.5v, 1/2 of 12v is 6v, etc.
Just check your other components and make sure they'll handle the higher voltage.

[MetalGod]

If you're biasing to 1/2 supply voltage, it's self explanatory-1/2 of 9v is 4.5v, 1/2 of 12v is 6v, etc.
Just check your other components and make sure they'll handle the higher voltage.

I realise that - what I meant was... is 4.5v the 'magic number' or do you bias to half of whatever the supply voltage may be.  I think I have my question answered now, thanks.