Busy Weekend...Supreaux bias problem...

[mydementia]

Hi guys.
I had quite a busy weekend... built a Noisy Cricket (awesome dano!), my Dual CreamPie w/switchable Ge diode pair (nice...very nice Andrew!), and a pair of 112 extension cabinets (one has an Eminence Black Powder... Swamp Thang on its way for cab 2).  I also built an ROG Supreaux...

The Supreaux J201's biased just fine...but the MPF102 drain is a toughy.  Goes from mV values to 9V in a VERY narrow portion of the rotation.  I was able to coax it up to about 2.5V after futzing with it for 15 minutes...so I plugged it in - man does this thing sound great at low gain settings.  I really want to get the MPF102 bias up to maximize the tone in this thing... anyone have any ideas?

Thanks for looking.
Mike

[petemoore]

  Assuming everything else is ok...
  Different MPF102
  Different value trimpot [smaller value...maybe stop resistor/pot]...or tapering resistor on a 'big' trimpot...are you using 100k?...this value can be very touchy to bias 'certain Jfet Drains.
 

[dano12]

Congrats on the productive build weekend! Glad you tried the Noisy Cricket. How do you like it?

Regarding the jfet biasing issue, I'd try a few different MPF102's in there.

[MartyMart]

Try other MPF102's as suggested and possibly a 1k trimmer at Q3's gate instead of the 330ohm
I seem to remember a similar problem with mine, which a 470 ohm cured ! ( Q3 drain 4.9v )
Also check all solder points around Q3 with a mag glass ....

MM

[mydementia]

Thanks for the replies guys...Yeah - I tried several different MPF102's - none worked...

Marty - I'll give replacing the 330ohm resistor on the Q3 with something a little bigge a try as soon as I get back to soldering (design review at work this week...all I want to do when I get home is lounge on the couch and drink beer...)

dano - Your Noisy Cricket is pretty cool.  The tone control is a nice feature - as is the 'grit' switch...  Through my new Eminence Black Powder, it was tough to clean up... but sure sounded good with the 'grit' on, volume and level cranked!! 
It was only clean at VERY low volume...but I didn't dwell on it much... I had to go test the cab with my main rig!!!   I'm going to get started on the tube cricket soon...should be fun!

Mike

[krister]

I found this same issue last night with my Supreax. The 100k bias pot is extremely touchy 0-9 volt in veryy little movement of the pot. I had to use my fingers instead of a screwdriver to get the bias even close to the mid point. Problem is, even the slightest bump can throw it off.

[Ge_Whiz]

Use a much smaller value trimmer - 10k may suffice. I'm not sure about MPF102s, but 2N5457s need this low a value.