FET VULCAN mods?...

[DDD]

Well, to achieve the ACTUAL and stable limiting of the signal voltage on the FET gates, it's necessary to shunt gates to the ground with the 2 MOhm (or so) resistors.
Obsreving signal form on the gate with the real scope shows the limited signal because input impedance of the scope works like such a shunt. But when you take the scope probe away from the gate, the gate voltage becomes not limited at all. 
This effect have been caused with the diodes' and other leakages togetger with very high impedance of the gate curcuit.

[Davelectro]

Change the coupling cap before the Gain pot to 1n.

[deadastronaut]

^ yep thats better..cheers,..its more toppy, but i'm sure that low end will be there if i whack a tone on it...

this is what i have now...still not right, but a lot more defined/better...(with a crappy 1 pot tone on it)

http://dl.dropbox.com/u/7464107/vulcantest2.mp3

[Joe]

Sounds good man. 

Have been working on a four-stage, channel-switching version of the circuit for the past few weeks. The breadboarded versions were completely unusable but fine when soldered together.

FETs are ultra-sensitive and can pick up signals through the air from across the room. Even though the breadboard contacts are insulated they are very close together and the FETs can still pick up signals they shouldn't. A soldered-together build is probably going to perform better.

Might try raising the source bypass cap values if you can't get enough low end from your amp controls. Another idea, the uppermost bias resistor (10M) doesn't necessarily have to go to 9v. You can use a lower value (say 2.2M or even 1M) as long as it goes to a lower voltage source. Possibly less noisy, unsure.

[deadastronaut]

Sounds good man.  

Have been working on a four-stage, channel-switching version of the circuit for the past few weeks. The breadboarded versions were completely unusable but fine when soldered together.

FETs are ultra-sensitive and can pick up signals through the air from across the room. Even though the breadboard contacts are insulated they are very close together and the FETs can still pick up signals they shouldn't. A soldered-together build is probably going to perform better.

Might try raising the source bypass cap values if you can't get enough low end from your amp controls. Another idea, the uppermost bias resistor (10M) doesn't necessarily have to go to 9v. You can use a lower value (say 2.2M or even 1M) as long as it goes to a lower voltage source. Possibly less noisy, unsure.

hi joe: 4 stage one .nice.... dr boogie-ish is it?..
yeah i was looking at those 10M's seem a bit excessive in value, i'll try it, and the caps....
its funny i was going to pull this off my breadboard today, but i'll give it one more shot i think....cheers rob.


edit:  ive changed so many bits on this now i'm going to draw up a revised schematic..and post it up..

i tried changing those 10M - 4.7-1M  much much quieter even on breadboard...its really getting there..this will be a totally different beast to the original...a lot more bite and 'defined' chug...

[DDD]

A little bit modded version just with 2,2 MOhm resistors - pls take a look here:
http://imageshack.us/f/839/jfetvulcan1.jpg/
With the low cut-off voltage FETs (J201 for example, Ucut-off = 0.5V) one can get about 80-90 dB gain with three stages.
So, is it necessary to add the fourth stage?  

[deadastronaut]

^ thats a heavily modified one eh...: i'd rather keep it to 3 stages too if i can..

anyway heres where i am now..schematic and clip...its getting there!.. no tone control and recorded out of my bread into my mixer / no amp.

i let the note die off , its pretty damn quiet for being on a breadboard...i went with 2n5485's in the end and put the 10M'S back in.., less flabby that the j201's imo...



http://dl.dropbox.com/u/7464107/vulcantest3.mp3

[Davelectro]

I don't think C4 is really needed.

the uppermost bias resistor (10M) doesn't necessarily have to go to 9v. You can use a lower value (say 2.2M or even 1M) as long as it goes to a lower voltage source. Possibly less noisy, unsure.

Hey Joe (  ) can you further explain what you mean by "lower voltage source"? Thanks.

[deadastronaut]

i'll try it cheers...

[Joe]

I don't think C4 is really needed.

Hey Joe (  ) can you further explain what you mean by "lower voltage source"? Thanks.

C4 is needed or the gain pot would interfere with the following stage's bias point.
For the lower voltage refer to the schematic DDD just posted, just a voltage divider filtered with a cap as seen on many opamp circuits.

hi joe: 4 stage one .nice.... dr boogie-ish is it?..

It's similar to the Dr. Boogey. Tried to cop the Orange and Red channel EQ's, and rigged up an fx loop for each channel. Switching is pretty basic, just a single DPDT relay to select gain controls and fx loop returns. Kinda noisy and trebly but working ok, will post circuit when I have time.