FET VULCAN mods?...

deadastronaut · August 31, 2011, 06:07:49 AM

ok i have the fet vulcan on breadboard, ive read through a few threads on it, that were pretty old, but still relevant.

http://diystompboxes.com/analogalchemy/sch/jfetvulcan.html

i was just wondering if there were any updated mods/tone control/ etc etc....its a pretty good hi gainer for a 3 fet (j201) jobby...

just in need of a few tweaks i think....ideas?...have you built one?...what were your findings?..preferences?...cheers rob.

petemoore · August 31, 2011, 07:02:18 AM

IT's super high gain ! !
Tone shaping is subtractive, and requires a certain amount of current drive to work right, a gain=recovery stage post TC can bring the signal strength back up to normal or above normal output/volume.
Calculating the impedance is another matter, if needed add a buffer prior to the TC...a 'lighter load' TC might be another option, 'good' voicing + just a bit of treble control has served well on a number of other occasions [basically gets the 1 sound to be treble compensated as volume changes to help match amp>room>speaker settings/responses.

maarten · August 31, 2011, 07:54:59 AM

Hi Rob,

Maybe change the 100 pF right in the middle of the schematic?

Perhaps you like Davissons's BlackFire better. Listen to it at Paul Marossy's site:
http://www.zeta-sound.se/effects_eng.html

You can find the schem at the same place as where you found the Vulcan ( ...analogalchemy/sch)
Maarten

WGTP · August 31, 2011, 01:37:59 PM

Yes, definitely tweak the 2.2M/100pf values. They have a large effect on the "voicing". For bass changes, the 1uf electro's can be reduced. I like a BMP tone control at the end of these type circuits. There is plenty of output.

Try this: use the 470k/470pf part as suggested and remove the last 1uf electro.

Also measure the bias of the J201's. I found they varied a lot when I tried it.

deadastronaut · September 01, 2011, 05:51:23 AM

cheers guys:

this has a heap of output, probably get away with a tone without a buffer i would think...(compared to bypass at the moment anyway)

i'm not a great fan of the bmp tone..its way to bright for my tastes, but i'll have a tinker with it as i have a seperate plugin vero board bmp with socketed caps/resistors.....i'll mess with values etc...

@wgtp: what were your bias voltages?...or rather what should they be?..

i'll mess with those 470k/470pf, and electro's too, it needs tightening up on the low E, definately, to get more of a punchy chug, it definately has great potential to be a
nice heavy and lead sound...

cheers guys!...

deadastronaut · September 01, 2011, 12:02:45 PM

ok, been messing around with this a little more....

i'm getting dreadful hum on this..hmmmmmmm....not matter what i do. any ideas...

my bread connections are all ok, using a 9v psu which i have used for all my builds and is fine ,

i think i'll put 100k trimmers instead of the 100k resistors , to get a better bias/sound..

it sounds really flubby on the low end ...with no bite/definition....hmmmmm, time to get those trimmers on there i think....

WGTP · September 01, 2011, 12:08:17 PM

Bias should be "around" the typical 4.5v with a 9 volt battery. Not sure about the humming. Try a .01uf input cap for tighter bass and reduced hum, experiment with it.

Using a resister between the output and the BMP tone control will reduce the highs in relation to the lows. Start with 10k and go to 100k if needed.

Good luck.

deadastronaut · September 01, 2011, 12:15:34 PM

cheers wgtp:

yep!, i just put trimmers on for the 100k's, hum has gone...so ive got to treat it as per dr boogey then really...4.5v, with a bit of tweaking by ear too...

cheers man, i'll let ya know how i get on... thanks again.

harmonic · September 01, 2011, 02:44:11 PM

I breadboarded this the other night. Ended up changing all the .1 caps to .01s and the middle transistor needed far more than 100k to bias right: I think I ended up at around 220k or so. In the end, I skipped the middle stage altogether! Less gain, but a more useable circuit for me. YMMV and probably will! :-)

deadastronaut · September 01, 2011, 02:50:34 PM

Quote from: harmonic on September 01, 2011, 02:44:11 PM
I breadboarded this the other night. Ended up changing all the .1 caps to .01s and the middle transistor needed far more than 100k to bias right: I think I ended up at around 220k or so. In the end, I skipped the middle stage altogether! Less gain, but a more useable circuit for me. YMMV and probably will! :-)

hi cam: yeah gotcha, that middle fet needs way more to bias....god knows how it has 100k on there!...

this should be trimmers for sure!... 100k/200-250k/100k's...

anyway, i'll try a few other fets in that middle stage and try the .01's and see how it goes....cheers rob.

edit:, ok , ive changed the first 2 100nf's to 10nf's..and left the last 100nf in there.....much better but still lacking definition on the low 'E'

its ok on lead up to the 24th fret, but just gets muddy as you get lower, especially on the neck pickup...

harmonic · September 01, 2011, 04:59:06 PM

The only demo I've heard of it was Aron in another thread using what sounded like a (very tasty) strat -- single coils anyway. Sounded lovely on that. I have a two humbucker 'super-strat' with a coil-tap on the bridge pickup. It sounded really undefined on the lower two strings, but much better when I went to the single coil. What guitar are you using to test with?

deadastronaut · September 01, 2011, 05:02:00 PM

hmmm...i'm not a strat guy...

ibanez rg470...humbuckers....

aron · September 01, 2011, 05:04:43 PM

I can't remember. I know I loved the FET Vulcan. I usually just put in the Shaka Tone Control or for extreme control, a modified VOX tone control.

harmonic · September 01, 2011, 05:09:29 PM

This was the thread I was referencing:
http://www.diystompboxes.com/smfforum/index.php?topic=67677.0

deadastronaut · September 01, 2011, 05:16:08 PM

^ yep thats the clip i heard....it does sound fendery..and it sounds ok on the low end of things...hmmmm..... $:-\$

edit: heres a clip of what i have at the moment...as you can hear it gets real muddy stiill on the lower stuff......(just into a mixer no amp)..

http://dl.dropbox.com/u/7464107/vulcantest1.mp3

harmonic · September 01, 2011, 07:15:12 PM

Intense! :-)

Are you using J201s? Whatever, maybe looking through your stock for lower hfe transistors to tone the madness down a little?

WGTP · September 01, 2011, 10:13:54 PM

Try a .001uf input cap.

Joe · September 04, 2011, 08:38:59 PM

Have had a lot of trouble breadboarding high gain circuits (especially with FETs), too much hum and oscillation. Even more so with 4 stages. The actual build should be quieter.

As far as the low-end goes, lowering the cap values would probably help, can always add the bass back in at the end with a tone control. Again the breadboard might be the actual culprit here, only one way to find out unfortunately

deadastronaut · September 05, 2011, 02:17:59 AM

cheers guys:

that clip is with a 10nf input cap $:-\$ , i'll get back on this today... thanks for your advice..

i have made allowances for un-shielded breadboard, i'll try some different fets today n see how that goes...

deadastronaut · September 05, 2011, 07:10:40 AM

right just to report, i changed out the fets for 5485's/mpf102's , sounded a bit better, but its still fluffy.

rebiased all the fet's swapped n changed em round, changed the 100pf/470k/ to other values,

put all 10nf's in there..trimmers on all fets...........hmmmmm....still lacks that bite definition,on the low E, still muddy, its like the sound is 'around the note', rather than

a good pinchy punchy chug defined, ....i'm starting to believe this is just not for me!........i had the same with bsiab2 too.....bad low end.......

i guess that's a hazard of hearing and going by clips, too many variables, guitar/recording/amp/playing etc...........oh well.....boo hoo..thanks anyway guys!...

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