Thundercheif: Q3 won't bias properly. HELP!!!

[saultime]

Arrrrggghhhh!

Yes, I tried the fixes on the runoffgroove site, as well as what was discussed in various threads on this board.

I dropped the 4.7k reisistor to 1K. Still, the gate and souce pins hovered around 2 and 3 volts. I dropped to 470 ohm resistor--100 ohms...I even took the resistor out of the circuit all together--the gate and source pins on Q3 wouldn't get anywhere near the reqired teritory. (Removing the resistor also made it impossible to properly bias the drain on Q4).

Will this just not work with the J201 FET's I have, or what? I know the "fix" has worked for some, but I've found several threads on this board where Thunderchief builders are just unable to fix the problem, period. It seems common.

If this is the case--that certain sets of FET's just cause this much of a problem, then maybe the folks at runoffgroove should advise builders to buy several J201s and 4 sockets, so they can switch them out if things get hairy. Or maybe this is obvious to people who have been doing this longer than me. I would MUCH rather spend a couple extra dollars on parts than have to endure this kind of headache.

So should I start over, or what?

[Mark F]

As you've found out Q3 is the most sensitive Q in the circuit. If you don't have any more J201's, which it sounds like you don't, you could try swapping Q3 with the other Q's and seeing if one of them are easier to bias in Q3's position. If that doesn't work I'd suggest getting more J201's to try. Or you could try a different FET altogether if you have any. I breadboarded the Thunderchief and had no problems with J201's, however, I tried different FET's and had problems with Q3. Hope this helps. Some individual FET's just won't work in certain circuits. Sorry. :wink: