Alembic Stratoblaster Biasing

[Sparky]

Nice simple build!  I used a 2n5457 on my first one and am trying a 2n5458 on a second one.  I noticed the drain voltage is listed as 6.9 v  on the General Guitar Gadgets site but I saw it posted somewhere that they biased it to 4.5 v.  I'm guessing that a higher value drain resistor would lower the voltage.   Sound differences ?

[antonis]

I'm guessing that a higher value drain resistor would lower the voltage.

Or a lower value Source resistor..

By raising Drain resistor value, you raise Q1 output impedance hence lowering out HPF corner frequency..

(in practice, you shouldn't here any difference.. )

[Sparky]

I'm guessing that a higher value drain resistor would lower the voltage.

Or a lower value Source resistor..

By raising Drain resistor value, you raise Q1 output impedance hence lowering out HPF corner frequency..

(in practice, you shouldn't here any difference.. )

Hhhmmm....the thought of a variable resistor comes to mind.......

[antonis]

Hhhmmm....the thought of a variable resistor comes to mind.......

All time classic thought..

For you particular build, a 10k trimmer in series with a 2k2 resistor should be fine..

[Sparky]

Hhhmmm....the thought of a variable resistor comes to mind.......

All time classic thought..

For you particular build, a 10k trimmer in series with a 2k2 resistor should be fine..

I'm wondering if an adjustable pot in that circuit would be really worth it as to the change in response.   It might make the sound a bit "softer"  around the 4.5 v  setting and a bit cleaner around the 7 v setting. 

[anotherjim]

I think Alembic meant the original to be fitted into the guitar so cannot have any hot pedals in front of it. Therefore the source voltage didn't have to bias halfway as there would still be plenty of headroom with whatever the JFET settled at. I don't know how old the design is either so superhot pickups might not have been around then.

I would also expect it was installed in the guitar with its input permanently connected and a toggle switch selected between its output or input to the guitar jack which is why the JFET gate has no bias reference resistor on the board, it relies on the guitar volume pot/s and pickups for 0v reference.

[Sparky]

I think Alembic meant the original to be fitted into the guitar so cannot have any hot pedals in front of it. Therefore the source voltage didn't have to bias halfway as there would still be plenty of headroom with whatever the JFET settled at. I don't know how old the design is either so superhot pickups might not have been around then.

I would also expect it was installed in the guitar with its input permanently connected and a toggle switch selected between its output or input to the guitar jack which is why the JFET gate has no bias reference resistor on the board, it relies on the guitar volume pot/s and pickups for 0v reference.

Here's an original one from the 70's.    General Guitar Gadgets has the drain at  6.9 v  [approx.]      I thought  *maybe* the 4.5 v gate would boost the volume a bit.

[antonis]

Gate or Drain..??

[pacealot]

I would also expect it was installed in the guitar with its input permanently connected and a toggle switch selected between its output or input to the guitar jack which is why the JFET gate has no bias reference resistor on the board, it relies on the guitar volume pot/s and pickups for 0v reference.

Based on visual evidence of the original '70s versions, which had SPDT toggles, you're absolutely correct.

I don't know how old the design is either so superhot pickups might not have been around then.

The Stratoblaster was specifically created to address the issue of Strat pickups not perceptually driving amps as hard as humbuckers, so it was originally intended to be used with lower-output pickups by design — also corroborating your theory that the precise bias point of the originals probably wasn't considered super critical at the time.

Edit: also, the original early/mid-'70s version under discussion had a slightly different schemo to what's been going around the interwebs since then — it's missing the 68K input resistor, and the actual JFETs found in the units were most likely something of a grab-bag (much as JFETs ever have been and ever shall be ); many seem to have had white-and-black "glob tops" of some persuasion or other:

[Sparky]

Gate or Drain..??

Oooops....."drain".   I corrected it.

[antonis]

it's missing the 68K input resistor,

No tube - no Grid - no stopper resistor..

[pacealot]

Well, I never said it needed it, just that most schematics show it...

[anotherjim]

A tube grid stopper resistor works with the tube input capacitance to low-pass filter out RFI. Does a JFET gate not have input capacitance? Can a Strat pickup radio?
I would think twice before using the basic circuit as it is in a stompbox for ordinary boost duty. The input needs attention but there are ordinary JFET boost pedal schemes to copy.

[pacealot]

Well, for what it's worth, the GGG "improved" or "modernised" version subs in a 100Ω input resistor instead, for, as they put it, a "less intrusive RF filter," so there's that.

[antonis]

Well, for what it's worth, the GGG "improved" or "modernised" version subs in a 100Ω input resistor instead, for, as they put it, a "less intrusive RF filter,"

Form 500kHz to 340MHz (for less than 5pF typical CS input capacitance..)

Pas mal..

[anotherjim]

FWIW, I'd fit 10k with a 100p filter cap to the JFET gate for RF protection.

[Sparky]

My "test version"  as per the Guitar Gadgets schematic is done.  I chose not to use a variable res. for the drain.   It's set at 6v and sounds good.  Right now it's in a box but I haven't installed a switch yet.  Since it was designed for a Strat I may put one in one of mine. 
Anybody prefer other similar boost circuits  [transistor or J-fet] ?

[antonis]

[transistor or J-fet] ?

Even if you don't like it (me neither..), JFets DO belong into transistors..

If you insist to mess with FETs, any simple MosFet booster (EQD Black Eye, Zvex Super Hard On, AMZ Mosfet booster, to name a few) should be fine..

[GibsonGM]

AMZ Mosfet Boost. 

[antonis]

AMZ Mosfet Boost.