Jfet Gain Stage, gate current ?

[petemoore]

  A "Simple Stratoblaster, GGG. Built 'em before...
  I'm tryin' to figure out why the same resistor values change, must be the DMM, there's nothing else around these two seriesed 470k's, they measured .95 megohms today, same test w/same battery showed 305k and with the transistor in...11.7k to ground from gate.
  With transistor out the sources socket wiper on the board measures 0.0v, as does the gate wiper.
  Put any of 3 J201's in and the gate and source both move up to about 2.5vdc. [2.42 and 2.45 respectively].
  Pretty hard to figure why this gate wants to float up so high, it seems to be following the source.

[oliphaunt]

Looking at the GGG parts list and schematic for the "Stratoblaster Boost" I see nothing about 470K resistors.  http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=64&Itemid=26.  Is this what you are working on?

I have to say, many of your posts are incomprensible to me.  Perhaps there is a language barrier. I'm not trying to be rude, and I wish I had more to offer to help, I just don't understand the question.

A JFET should not have any voltage on the gate if understand correctly.  I would think that even with the JFET removed you should measure voltage going to the drain's socket.  Perhaps the problem is with the PC board, such as broken traces.



In an unrelated question, I have always wanted to put together a Stratoblaster.  Does anyone know if the tantalum caps make any tonal difference compared with other types?

[petemoore]

Looking at the GGG parts list and schematic for the "Stratoblaster Boost" I see nothing about 470K resistors.   
  I tied two of them in a row to make a base/V+ bias resistor of about 1meg.
http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=64&Itemid=26. 
Is this what you are working on?
  Yepp, the basic Stratoblaster.
  A JFET should not have any voltage on the gate if understand correctly.  
  That's my point, why does the gate supply voltage to the gate socket pin? The only things connected is a 390ohm resistor to the gate bias [large resistor to ground]
  I would think that even with the JFET removed you should measure voltage going to the drain's socket.
  Yupp, there's voltage on the drain's socket pin. 
  Perhaps the problem is with the PC board, such as broken traces.
  Always a chance, thing is I use 'distant point' testing, so broken traces that fell through the cracks there would be picked up by my 'dogmatic testing'...measuring the continuity of every trace, resistance values taken from points where the schematic shows a resistor, other checks that strongly tend to rule out that kinda thing, not sayin' it can't happen, just sayin' I'm dern good at findin' that kinda thing.
  I can't figure out what circuit is causing the gate to provide current through it's socket pin.
  In an unrelated question, I have always wanted to put together a Stratoblaster.  Does anyone know if the tantalum caps make any tonal difference compared with other types?
  No more than any other circuit would, or, not so I could notice.

[JDoyle]

Pete - do you have the pinout correct of the JFET?

If you had the JFET "upside down" with the Source to V+ and Drain to ground, what you say could make sense.

The Source-Gate diode would forward bias, but the 1M gate resistor would limit the current and therefore the voltage drop through the G-S diode as well.

Just a guess though...

[petemoore]

Pete - do you have the pinout correct of the JFET?
  Now I do ! DSG as looking at print, legs down.
If you had the JFET "upside down" with the Source to V+ and Drain to ground, what you say could make sense.
  Oh, that was it.
  The Source-Gate diode would forward bias, but the 1M gate resistor would limit the current and therefore the voltage drop through the G-S diode as well.
  I thought I had them Jfet's pinouts down [J201's / MPF102 / 2n5457's at least], nope..wrong !
  Just a guess though...
  Good guess !
  Wiring was clean, nice, fine, correcting the pinout made it work.
 

[JDoyle]

That may make me happier than you! 

It's been well over a year since I've gotten one right!!!

Take care!