Tillman preamp with adjustable gain

[esl1313]

Hello, I'm new to the forum and hope this is in the right place, as it's technically not a stompbox per se, but I've seen a lot of posts about this particular circuit on here.  That being said I have not found any posts regarding the specific question I have.  I want to build the well-know Tillman FET preamp, (http://www.till.com/articles/GuitarPreamp/) as on onboard boost on a guitar.  I wish to wire the battery to a TRS jack so it disconnects when the cable is unplugged.  I also plan to wire it to a push/pull pot so that I can bypass it entirely and keep my guitar's stock sound as well. I think I would wire this circuit in after the volume and tone controls, before the output jack, correct me if I'm wrong.  My question is this:  I've seen it mentioned before that high output pickups could cause the circuit to clip, how would I go about installing a trim pot to adjust the gain or boost of the unit, and could it even be setup to possibly increase the gain from the stock circuit (if the clipping is in any way a desirable sound, like a build in overdrive)? So could it be made to range from slightly less boost to slightly more boost than the original? This way if I make a few of them for different guitars, each could be adjusted to suit the different pickups.
Thank you.

[blackieNYC]

The Tilman can be clipped by humbuckers.
It only has a little gain to begin with.  you could get more boost from another circuit - muzique.com

[samhay]

Hi,

Welcome to the forum.

The Tillman preamp:


The gain is largely set by the ratio of R3:R2. You will probably have to adjust the value of one of these to get it to bias correctly, so you don't have much, or much control, over the gain.
If you want more gain, connect a capacitor between the source and ground. If this is too much gain, add some variable resistance (a pot or trimmer) in series with this new cap - more resistance = less gain.

>I think I would wire this circuit in after the volume and tone controls, before the output jack.

That sounds reasonable. It won't load down the volume control and if it clips, you can turn the volume down.
Bypass as per any other effect, but if you are short of switch throws, you can probably leave the input connected without any problem (just switch output).

You might find this useful too:
http://runoffgroove.com/fetzervalve.html

[GibsonGM]

Hi,

Welcome to the forum.

The Tillman preamp:


If you want more gain, connect a capacitor between the source and ground. If this is too much gain, add some variable resistance (a pot or trimmer) in series with this new cap - more resistance = less gain.

The size of this cap (source bypass cap) can also be used to obtain some frequency tailoring.  A smaller cap such as .1u would emphasize highs/tighten up bass frequencies, while a larger one, 22u, 100u...would allow a full-frequency boost.    This can be used to advantage if you're following the boost with a distortion, which may not like having all that low end jammed in.    So the value you choose may be worth experimenting with a bit. 

[amptramp]

You do have a simple method of avoiding the battery switching issue - leave R1 (gate resistor), R2 (source resistor) and the FET plus any additional series R-C shelving network across R2 (as suggested by samhay) in the guitar and build an outboard box for R3, R4, C1, C2 and the power supply, whether battery or regulated DC.

If you don't want to modify your guitar, there is space in some cable plugs to put the preamp inside the plug hood at the guitar so if you want, you can switch over to a plain cable.  You could put it at either end, but at the guitar end, the cable capacitance is driven by R3 in parallel with R4 so the cable could be sized for a stadium stage.  Of course, this means there is only one correct direction for the cable - it now has an input and an output so you have a 50% chance of getting it right.  You would use this with the same outboard box.

[blackieNYC]

I have one in a little 1590A box with a short cable for its input, and a jack for its output. It dangles off the guitar strap. The output jack switches battery power. Same battery can last a year.

[Mark Hammer]

I've had several on-board preamps over the years, and been voicing my opinions about them for almost 40.

Their usefulness depends on what you want or need them to do.

For instance, as shown, the Tillman circuit provides a bit of boost, but more importantly provides a nice simple buffer for the signal so that it that what arrives at the end of that first cable is pretty much what the pickups delivered, and nothing less.  A little bit of gain can provide optimum signal-to-noise ratio.

But here's my apocryphal tale.  Well over 20 years back, I made myself a Tube Screamer to TS-808 specs.  I honestly couldn't see what any of the fuss was about, and thought it sounded terrible.  Then, I bought myself a new guitar.  Nothing special, but it didn't have an onboard preamp, such that I was driving the TS with only pickup-level signal, and not something that had a gain of 4x applied to it.  All of a sudden, the TS came alive, because I wasn't pushing it so hard.

That is neither a criticism of onboard preamps, OR the TS.  Rather, I mention it to illustrate tat onboard preamps have to be thought about strategically, in terms of what you expect to be feeding, and what sort of gain makes sense within the context of your rig.  I will note that the same 4x preamp on the same guitar made my little tweed Princeton sound fabulous...because I was driving it directly.  Where it caused problems was with the pedals between the guitar and amp.

So, in that spirit, if you can bypass the preamp (whatever circuit you settle on in the end), or stick a volume pot on the end of it to tame your output level OR push an amp hard, that would be good.

As an aside, a 9V battery should be able to provide clean output, provided the gain is very modest.  If you are worried about clipping in the preamp itself, or simply have space concerns, there are small 12V batteries commercially available, the size of a short AA.  They don't provide much current, but then the Tillman circuit doesn't need very much.

[esl1313]

Thanks for the advise. I just go straight into the amp, so I don't really need to worry about it messing with other effects. Honestly I just want a subtle boost to use my neck pickup for soloing. It has a mellow warm tone that I love, i just want a hair more output. I just read somewhere that it can overload and distort easily with humbuckers. I'm thinking of I make r3 and r2 adjustable, with trim pots, I could fine tune the circuit. Sorry for the newb questions, but if the spec calls for 2.2k resister for R2, what trim pot would I use? Something slightly higher? Thanks again for your help.

[GibsonGM]

You'll probably find that this 'muli-trim' approach doesn't work too well   JFETs are notoriously variable...can't really predict what values you'll need in the end.   You CAN make R3 a 10k pot, of course (that's how it is often done), but if they are BOTH trims, they will fight each other.

The best solution I've seen offered on this post is to place a cap from source to ground, with a pot between (maybe 5k or 10k, and a cap anywhere from .1uF to 10uF or more...smaller caps give a treble boost).    Just try this out; after you build this simple circuit and play for a while, you'll see what's up.

The cap and pot go from source to ground but you still need R2....they go in parallel with it. 

Another option is to visit AMZ Labs and look for the Mosfet boost. I love that, it's what I use for a transparent boost for solos.  May be exactly what you need!  http://www.muzique.com/schem/mosfet.htm

I find the Mosfet to have a better range from clean to slightly OD, and to be extremely versatile.

[antonis]

I wonder if there is any thread relative to boost, OD, gain, distortion where Sir Mike doesn't propose a MosFet booster.... 

[GibsonGM]

I wonder if there is any thread relative to boost, OD, gain, distortion where Sir Mike doesn't propose a MosFet booster.... 

NO, there is NOT.  It is a DECREE!!!   If you are new, why mess with those difficult JFETs...those Tillman's, and Fetzer's.....we say we don't want color in our sound, just more volume - so we pick circuits that will color our sound AND clip before giving us a large boost!!   NOoooooooo!   

Of course, I have MY version of the Mosfet Boost, but since I first came upon Jack's at AMZ, I give his the nod.  Mine was approved by the royal court.  It uses 2N7000's rather than BS170   

I tried to find the boosts on your page, Antonis, but only came up with this (?)   
And he calls ME royalty???!   

[GibsonGM]

In all seriousness, it seems to me that the Tillman, etc. do color your sound too much for the OP.  I haven't found a cleaner boost thru most all of its range than the AMZ...the only other thing to do would be to add another battery and run on 9V, no?

[samhay]

^I haven't found a cleaner boost thru most all of its range than the AMZ

I see your MOSFET, and raise you an op-amp.
I also prefer BJTs over FETs, but let's not muddy the water further.

[GibsonGM]

^I haven't found a cleaner boost thru most all of its range than the AMZ

I see your MOSFET, and raise you an op-amp.
I also prefer BJTs over FETs, but let's not muddy the water further.

I don't generally like the sound of opamp boosts (subjective thing)...yeah, I know...the Mosfet may be distorting to some degree at high drive levels - I also didn't want to muddy the water!  Each thing has its own limitations, some of which may be beneficial in actual use!
Don't want to go down another clipping path, here, ha ha.  I do not wish to fall on my own sword...

[antonis]

Obviously it should go somewhere like here: http://www.diystompboxes.com/smfforum/index.php?topic=117215.0 but I want to muddy the water of MosFet booster (I'm sure Sir Mike adores me..  ) so I post it here..

Once upon a time I was dealing with a Buffer_Booster pedal so Sam proposed me the circuit below:




Nothing sneaky or mojo (and IC1B may be omited - but it doesn't save any space/part) but the pedal is permanently connected at the end of my son's pedalboard for 3 years with absolutely excellent comments..
(no colouring, adjustable gain all down to unity - verified by scope measurement, absence of electro cap on V_B, e.t.c.)

Proof of plain and clean design effectiveness..
Thx again, Sam.. 

[samhay]

I'd forgotten about that.
Glad it's found some use.

I can't take too much credit though, as the basic circuit topology is older than I am.

[GibsonGM]

It is a good boost, Antonis, of course.   There are several ways to achieve the same end - for some reason, we all seem to have our own preference...but if it sounds good, that is all that matters!!       

That may work very well for the OP...