Stratoblaster Observations

[petemoore]

I built a Jfet Stratoblaster today, and decided to re-try putting a small capacitor at the input, just in case.
 This substantially increased noise and general disliked characteristics [some kind of wierd mild distortion] , and producing rather loud humming when I got anywhere near it [like within 2=3 inches].
 So, I used a .1uf on the output of this one, and got 12k actually from ground to source after a kwik debug...
 I just found it interesting, but can only speculate why: actually I have no speculative theories except something to do with the super low current at Jfet Gate...
 Also I thought it wasn't working for about ten seconds after I pulled the battery...the cap was powering it even with the gain all the way up and input signal for about ten full seconds before fade out!!! Must be very low current consumption.

[Boofhead]

No offence, but I don't have much of an idea of what you actually did!

I built a Jfet Stratoblaster today, and decided to re-try putting a small capacitor at the input, just in case.

So was that a cap to ground or a cap in series with the input?  What size cap?

Note: That schematic has no bias resistor, from the gate to ground, on the JFET - it relies on DC path through the pickup coils bias the JFET.  A cap in series with the input will cause bias problems on the JFET - the gate won't be at 0V.

So, I used a .1uf on the output of this one,

Did you just sub a 0.1uF cap in place of the 1.5uF cap on the schematic?  

and got 12k actually from ground to source after a kwik debug...

Not sure what the problem is here.  The schematic shows 12k.

[petemoore]

No problem, a little trouble with the 12k for a minute.
 Very small...I don't know the exact value of the input cap, it just seemed that the Jfet's don't respond to bass rolloff of smaller input caps liek bipolars do [??]...cap at input in signal path

[Boofhead]

I don't know the exact value of the input cap, it just seemed that the Jfet's don't respond to bass rolloff of smaller input caps liek bipolars do [??]...cap at input in signal path

The roll-off problem and the noise problem all originate from the fact there is no resistor from the gate to ground on that circuit.  Checkout some other JFET designs and you will see a resistor from the gate to ground - say around 1M.

With this resistor in you will not get the weird noise problem you experienced when you add a cap in series with the input (for reasons mentioned previously).  Secondly, you *will* be able to roll-off the bass with the same input cap like you do with a bipolar.   With 1M to ground you will need to cap values below 10nF before you get any roll-off at all.  If you want to get roll-off then it could be an advantage to decrease the 1M resistor to say 470k and use a cap below 20n.  The reason for doing this is less noise - high impedances and small caps for bass roll-off can introduce noise problems.

[petemoore]

Way to word it Boofhead
 I couldn't have done so well myself !!! Thanks!

[ESPm2M]

Is the GGG Stratoblaster a mids booster or a line booster (full range)? Can it be modded as a mid booster?