JFET Vulcan is cool!

[mac]

1. About biasing a germanium vulcan...
I noted that in the 5089 version the current flowing through the 4.7Mohm is only twice the base current, so the other half goes to the 1Mohm, and that Ve = 10%Vc = 0.45V, supposing hfe=500 & Vbe=0.6 with no diode.
Using Ge's with hfe=200 and Vbe=0.1V the divider bias should be 330K and 2.2M to have the same "results".
Should you use this bias or try a different arrangement?
What do you suggest for  a starting point?

2. I like both the 5089 & 201 version. I plan to put many joe's pedals together in the same enclosure as in a multifx box.

3. I can't get 201s in my country, tried with BF245 with nice result. I read 2SK117 have a lot of gain and low Vgsoff.
Any replacement worldwide available?

mac

[MartyMart]

You should get decent gains from MPF102's and 2N5457's ... like used in the BSIAB
I've tried MPF102's in lots of "J201" circuits and though slightly lower gain, they work
very well on the whole ...
MPF102 is pin for pin same as a J201 and you could make small adjustments to the
circuit for more gain, with Joe's "high gainers" I dont think that you'll notice much !!

Marty.

[WGTP]

Also, increasing or decreasing the resistors between stages can boost or cut the overall gain/distortion level.  Mixing the different value by-pass caps is a good way to tweak.  Removing the the second or third stage one all together increases bass and lowers gain. 

[mac]

You should get decent gains from MPF102's and 2N5457's ... like used in the BSIAB
I've tried MPF102's in lots of "J201" circuits and though slightly lower gain, they work
very well on the whole ...
MPF102 is pin for pin same as a J201 and you could make small adjustments to the
circuit for more gain, with Joe's "high gainers" I dont think that you'll notice much !!
Marty.

I was testing one of the vulcan's stages to find the right resistors for a MPF102 and others fets, when I realized that one stage makes a good treble booster tweaking the bypass cap, ie, around 0.1uF. The diode adds some nice compression.
So I am thinking  about building a RangeMaster with the compressor diode (ge, si & led) when I have the vulcan built. Maybe a switch to turn the compression feature on/off.

Anyone tried the vulcan with other diodes like Ge & leds?

mac

[jafo]

I worked up two stages in LTSpice, but apparently it just transforms signals into square waves?! Clearly I've done something wrong, but I lack the skills to determine my error.

Here's the original schematic:


And here's what LTSpice shows:


Sorry for the thread necromancy, but I considered it appropriate, given the rocky terrain...

[jafo]

Ah, it's got to do with the source bypass caps, right?

[FiveseveN]

it just transforms signals into square waves?!

Square-ish. That's what distortion pedals do.
What do you mean about the caps?

[antonis]

Source bypass caps set JFETs stages maximun gain..
Which is g_m X R_Drain, for caps big values..
(capacitive reactance much lower than 1/g_m at frequencies of interest..)

[PRR]

it just transforms signals into square waves?!

You are feeding 300mV Peak which is a hot guitar. Try various levels to below 10mV. The squares un-square and round out as the signal fades, and vice versa too.

It's all in the wrist, Bob.

[Elijah-Baley]

Are there around some demo of the BJT and JFET version of this pedal?
I heard one, but it seemed harsh and fuzzish. I'm not sure if that's the real thing.

[jafo]

Square-ish. That's what distortion pedals do.

Some do, some don't. It's just as possible to get sawtoothy waves, by clipping only one phase.

What do you mean about the caps?

The caps bypassing Rs on each stage; this allows more/higher frequencies to pass, thereby increasing overall gain.