JFet based schematic for more dynamic response?

iandy4 · April 14, 2011, 12:18:40 PM

Hey Everyone,

I was hoping someone knows of a JFet based schematic for getting a bit of tube-ish growl when you pick the guitar a little harder. Something that responds to the way you play, like breaking up a little when picked hard. Anyone have a schematic that fits this description?

Steben · April 14, 2011, 12:40:05 PM

Mini booster?
watch this:

http://www.youtube.com/watch?v=JHTJbjogcRA

Paul Marossy · April 14, 2011, 01:04:41 PM

Tillman preamp circuit with a bypass cap added.

ayayay! · April 14, 2011, 02:12:26 PM

Quote from: Paul Marossy on April 14, 2011, 01:04:41 PM
Tillman preamp circuit with a bypass cap added.

Added where Paul?

Paul Marossy · April 14, 2011, 02:15:29 PM

In the guitar, or as a standalone booster. It will give you a pretty decent boost if you use a big enough bypass cap.

iandy4 · April 14, 2011, 02:30:24 PM

Paul,
I think he means where would you add the bypass cap in the circuit. At least that is what I was about to ask. As for the Tillman, I built it before but wasn't too impressed. However I'll have to give it another shot and try a few different FET's.

Steben,
Thanks for the link I may try that one.

iandy4 · April 14, 2011, 02:58:40 PM

O by the way Paul, your site is really great. I've come across it many many times.

Paul Marossy · April 14, 2011, 03:07:18 PM

Quote from: iandy4 on April 14, 2011, 02:30:24 PM
Paul,
I think he means where would you add the bypass cap in the circuit. At least that is what I was about to ask. As for the Tillman, I built it before but wasn't too impressed. However I'll have to give it another shot and try a few different FET's.

Oh! In parallel with the source resistor.

If you use a 10uF cap, it'll have a pretty healthy boost IMO.

Quote from: iandy4 on April 14, 2011, 02:58:40 PM
O by the way Paul, your site is really great. I've come across it many many times.

Thanks, glad you like it.

ayayay! · April 14, 2011, 04:41:58 PM

Quote from: Paul Marossy on April 14, 2011, 03:07:18 PM
Quote from: iandy4 on April 14, 2011, 02:30:24 PM
Paul,
I think he means where would you add the bypass cap in the circuit. At least that is what I was about to ask. As for the Tillman, I built it before but wasn't too impressed. However I'll have to give it another shot and try a few different FET's.

Oh! In parallel with the source resistor. If you use a 10uF cap, it'll have a pretty healthy boost IMO.

Quote from: iandy4 on April 14, 2011, 02:58:40 PM
O by the way Paul, your site is really great. I've come across it many many times.

Thanks, glad you like it.

Thank you, athank you, and thanks, you!

iandy4 · April 14, 2011, 07:47:32 PM

Quote from: Paul Marossy on April 14, 2011, 01:04:41 PM
Tillman preamp circuit with a bypass cap added.

Hey Paul,

I just rebuilt the Tillman Preamp and it does improve the tone. Much better than the last time I built it. Maybe I got the pins mixed up on the Fet or just used a better Fet this time. Either way, thanks! I'll be popping this into my strat as a boost at some point. I also liked the 10mf cap boost that you suggested. Do you by any chance know of some other ways to modify the circuit? Possibly for more gain? Either way it sounds great.

petemoore · April 14, 2011, 07:52:40 PM

Memory memory...
Does Doug know of the one I'm thinking of ?
Reportedly the harmonics jump out and it sings and has amazing dynamic-distortion tone, IIRC the name starts with an S.
Maybe I'll think of it...I wouldn't embarass myself so but I have excellent memory that those who built it remarked it was harmonically rich/dynamically-wide-ranged-distorter.

sault · April 15, 2011, 08:03:04 PM

QuoteDo you by any chance know of some other ways to modify the circuit? Possibly for more gain? Either way it sounds great.

One option is to use a higher power supply - double up your batteries and get 18v. Raise your Rd to 12-15kΩ to take advantage of that extra headroom. Not really more gain per se, but you do get a bigger voltage swing, meaning a hotter signal, all the better to saturate the next gain stage with.

You can lower Rs and get more gain that way. Look up the Fetzer Valve... same basic idea as the Tillman, but uses a lower value for Rs (ie 1500-ish Ω) to change the Jfet's bias point, which introduces more clipping.

Saul T

Zipslack · April 15, 2011, 10:12:00 PM

I like the Tillman as a clean low-boost, but I think you may be interested in some of the RunOffGroove stuff...take a look at this

http://runoffgroove.com/professor.html

ROG does FET-based emulations of amps. The Prof. Tweed is a FET emulation of a 5F2-A Princeton...there are lots to choose from with soundclips.

arawn · April 15, 2011, 11:00:22 PM

I highly agree with the professor as a recomendation also look up mods for it as well

Paul Marossy · April 16, 2011, 11:47:40 AM

Quote from: iandy4 on April 14, 2011, 07:47:32 PM
Quote from: Paul Marossy on April 14, 2011, 01:04:41 PM
Tillman preamp circuit with a bypass cap added.

Hey Paul,

I just rebuilt the Tillman Preamp and it does improve the tone. Much better than the last time I built it. Maybe I got the pins mixed up on the Fet or just used a better Fet this time. Either way, thanks! I'll be popping this into my strat as a boost at some point. I also liked the 10mf cap boost that you suggested. Do you by any chance know of some other ways to modify the circuit? Possibly for more gain? Either way it sounds great.

As sault suggested, you could try running it at a higher voltage.

iandy4 · April 16, 2011, 12:54:54 PM

Niice, Thanks guys!

mistahead · April 18, 2011, 08:11:35 AM

In the case of using a cap parallel to the source resistor - would the amount of boost be increased with increasing cap value or decreased as the cap value rises? Yes I know I need to get back to basics (I was green when I moved and lost my workbench)... that is why I'm looking at this little query/experiment.

Plan is to drop a tillman as buffer/boost stage one for a Harmonic Jerkulator core circuit (which I paint-by-numbered last year and love but find... a touch lacking) and then run that into the Guv'nor tonestack... thinking of putting bypassable clipping section after the H.J. too... then the plan will get bigger and the clipping section will end up with sym / a.sym switching too... then I'll... you know what - baby steps back into this lol.

So far I've got a couple of basic sucesses but more knobs means better right lol

petemoore · April 18, 2011, 08:43:38 AM

http://www.diystompboxes.com/smfforum/index.php?action=search2

There it is...Doug's Meteor ! !
The OP catch phrases [dynamic/harmonics/..] reminded me of Doug's Meteor...it just took me a couple days to jog my memory back to the title.

mistahead · April 19, 2011, 12:53:24 AM

I went back to the actually Tillman article and got the answer to my question as well - knew I'd start picking up where I left off on the theory sooner or later.

Also - to the OP - have you had a look at Dragonfly's Sparkleboost?? It isn't (from bad memory) JFET but it does sound like it may be along the lines of what you're after...

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JFet based schematic for more dynamic response?