JFET bias question

[tungngruv]

I'm going to build a Thundercheif and was wondering if this would work. I don't want to buy a bunch of trim pots so is it possible that I could leave the trim pots out and bias each JFET one at a time using a 100K pot "tacked into" the circuit, then replace it with the same value resistor that gets the 4.5 volts? I'm thinking I could do this one at a time and the resistance would not change as each JFET is biased. I understand I could not adjust them after using a fixed resistor but I'm thinking I wouldn't need to if I got the right volts to the JFET. Is this possible? Thanks in advance for any help.

[petemoore]

  Tha'ts what I've done, I ran a 100kB's wiper lug to wire to clip, and an outside lug to wire to V+...then biased each jfet with the pot, pulled the clipped end to measure the pot only, replace with resistor or resistors to match the R found in the pot.
  Remove pot when 'done'...then see the Jfets somehow 'drifted' and do it again...just me...it should work and remain biased though.

[tungngruv]

Thanks Pete. I might just use a couple 100K pots and do two or three JFETS at a time. I want to stay away from the "drifting part". Thanks for your input.

[markm]

Do the JFETs interact with each other?
Is this what some of the drift is??

[petemoore]

  "Drift' is probably a misnomer the way I used it.
  Somehow I got bias change, and had to re-adjust to get back to optimal bias.
  The DC blocking caps in the signal path between stages allow each stage to be separately biased.

[Noplasticrobots]

I use resistors instead of trims too. I don't even use a pot initially. I just take random guesses at resistor values and to be honest, I've never really been off. I just did a Peppermill and took a stab at a 10k resistor instead of a trim and sho' 'nuff, I read 4.37 volts. Good enough for me.

I think the only downside to this is not being able to precisely dial in a sweet spot...I don't know.

[Stephen]

The SWEET spot....
That is really a very important point... as I have had jfets work well from 2.5 -6.8 volts...In fact the boards I built would not work unless I could readajust the trims I had installed......

A important point as I have always thought that 4.5 was the holy grail of performance but I have found with trims MAN you can really change the quality and the etc.....of the scheme you are building....

MY 2 CENTS !!!

[theman]

For amp emulations with multiple JFET gain stages, the trim pots are critical to getting that sweet sound, or many different variations. For high gain circuits, for example, the first JFET can be biased for lower gain to remove oscillations. But I've found that you can also adjust later stages. There are some unique trim pot settings across the design which enable you to get sweet spots -- whether it be roaring juicy gain, or milder, crunchier and cleaner rhythms. Having trim pots also allow you to substitute other JFETs for different tones (MPF102/2N5457/J201 swapping).

The downside of trim pots are minimal: cost and board space. Cost for a typically-used Vishay 100K trim pot is about 75 cents each. Board space is slightly more of course than a resistor, but the advantages far outweigh the disadvantages. If board space is critical, there are trim pots that are slightly larger than a resistor (laterally) which can be used.

Regarding voltage readings -- I've never followed them. Use your ears instead! One key thing I have noticed in biasing .... you can adjust all the trims to get a good sound, but when a chord decays to low levels, you might hear sputtering. That's because the trims are optimized for higher input signals. You need to fine tune the trim: turn down the gain and potentially your guitar volume knob, and re-bias again -- esp the first and 2nd stages. Get a good clean sound with no sputtering or splattiness. Then turn the gain and volume up and you should be close to what you had before, but there's no sputtering.

[tungngruv]

Cool, lot's of great advice. Thanks

[aron]

One thing I used to do is use a socket, then trim with a pot, replace with a resistor. Later on when I would "fix" the circuit, I would solder the resistors onto the sockets.