FET fans..read this...

[dschwartz]

I finished a new DR boogey monday night..IT had tons of gain,  but alike the previous dr boogies i´ve built, it had some fuzzy low end..almost farty, lost of definition and dynamics at higher gain settings..

so i started reading and analyzing the schem....and found out that triodes have lower imput impedance than fets, and higher output impedance..

triodes and fets have very high input impedances, but as i researched, triodes have around ten times less impedance than fets, so i fugured ot that the highpass filter produced by the coupling cap and input impedance of the next stage has the wrong corner freq...

correct me if the values are wrong:

triode input Z ~ 100k Ohms
Fet triode Input Z ~ 1M Ohms

if coupling cap= 22n then the corner freq with a triode is 72hz and with a fet 7.2hz (meaning good old FARTY bass)

so, i replaced the first coupling cap with a 2.2nF and YESSS!!, the buzzy low end was gone!!! the circuit gained articulation and dynamics and sounded pretty damn better!!!

so..all of you fet fans, check your builds changing the coupling caps to a lower value...i´m sure this will heal my extremely farty proff tweed!!!

[foxfire]

nice find. i wish i had the patience to learn why things work or don't work.

[dschwartz]

foxfire, just take 10 minutes of your day to look at schematics..  if you dont understand it now..eventually you will..i´m not a professional electronic engineer, but reading, doing and hearing  teached me a lot..

not to mention this forum, RG, Paul, Mark, Gaussmarkov, Frank clarke, etc etc this place is full of living effect legends!!!

if i have any doubt or curiosity, i search this forum and 90% of the times i find the answer, if not, 100% i learn something new!!

[Steben]

Well actually, lowering caps is a trick used with bipolar transistors too.

[Auke Haarsma]

Great find! Thanks!

I'll implement this in my Dr. Boogey.

The coupling caps you refer to are:
C2
C7
C8

Right?

[dschwartz]

yep, those ones...

[Pushtone]

Gee, I think my DR B has extreme highend response and tight bass.

Not hiss in the highend mind you but a upper frequency limit that is higher than most other dist pedals.

I tend to roll off the treble to tame it because it's a beast.

[antojado]

So would it be possible (or even make sense) to install some sort of tone control to adjust the amount of farting bass? Maybe a pot with two caps in parallel?

[dschwartz]

easier than that is to put a variable impedance resistor (look at the omega on ROG)

[slideman82]

Strange thing... I had to rise the first FET drain's output cap to 82n, but I finally lowered it. Oh, I didn't put the tonestack, my boogie is free as a fuzz face! But... I choose the right FET's just to make it sound better, it's similar to Samba pa ti now, but less fuzzy. Try lower gain JFETS too! And maybe a bipolar at last (buffer section) will make thins clearer, if I wasn't wrong. For more highs, lower the source caps, this is to the taste of the builder (I don't think this phrase sound good in English, it does in Spanish). Be careful, lower the source caps also reduce distortion.

[dschwartz]

Gee, I think my DR B has extreme highend response and tight bass.

Not hiss in the highend mind you but a upper frequency limit that is higher than most other dist pedals.

I tend to roll off the treble to tame it because it's a beast.

Pushtone:
the high end response is given mostly due to the tonestack, of course we have to tame down the highs, sometimes i use LPF at the output, and never use the presence knob and treble knob at full..that would be ear-bleeding....

the point here has to do with the grain of the distortion..it´s a fact that too much bass content in the clipping stages will yield fuzzy blurry clipping, maybe the tight bottom of yours has to do with your amp...

the improvement here is not to add highs, is to eliminate excesive bass, without killing the low end...and resulting in a more chimmy, sparkly cleaned up sound...

[dschwartz]

Strange thing... I had to rise the first FET drain's output cap to 82n, but I finally lowered it. Oh, I didn't put the tonestack, my boogie is free as a fuzz face! But... I choose the right FET's just to make it sound better, it's similar to Samba pa ti now, but less fuzzy. Try lower gain JFETS too! And maybe a bipolar at last (buffer section) will make thins clearer, if I wasn't wrong. For more highs, lower the source caps, this is to the taste of the builder (I don't think this phrase sound good in English, it does in Spanish). Be careful, lower the source caps also reduce distortion.

como va compadre!!
well i did put a mpf102 at the first stage (this thing has tooooooo much gain) and a switchable source cap there..with the cap off it has lower gain and a bit more fuzz due to the broadband amplification, remids me to a cranked marshall..engaged, boosts higher freqs and enters metal territory.   

choosing the right fet helps a LOT..this pedal needs to be tested with at least 20 fets till u´re satisfied

[slideman82]

Vas a tener que elegir entre BF245A, 2N3819, J201 o similar, quizas algun MPF102. EL Fet que hasta ahora mas me gusta es el 2SK30. Hasta si queres le pones un Ge como utimo transistor, ahi quedaria mas boutique que nunca!

[John Lyons]

Each stage of the DB has a 1uf cap at the source of the FET which is treble shelving. If you listen to each stage at the output of the coupling cap (.02) with an audio probe (build one!) the sound you hear isn't very full it's pretty thin sounding.
Making the coupling caps smaller (limiting bass) in early stages of an amp or pedal will give you a clearer more defined sound in general but usually this is made up for by cutting treble later in the circuit to make the sound fuller.
The Fender marshal tone stack does a good job of cutting some mids and treble anyway..then we turn down the treble and presence even more most of the time...

I never noticed a "farty" bass in my 3 boogie builds but if lowering the coupling caps made you like it even more then cool!
I would start with lowering the first or second ones but all of them shouldn't be necessary.

John

[dschwartz]

jajaja  chantarle un Ge? naaahhh ni cagando.. me gusta como esta con J201´s y MPF102 en la primera..

el 2SK30A tiene muy poco gain y otro patillaje...

[dschwartz]

Each stage of the DB has a 1uf cap at the source of the FET which is treble shelving. If you listen to each stage at the output of the coupling cap (.02) with an audio probe (build one!) the sound you hear isn't very full it's pretty thin sounding.
Making the coupling caps smaller (limiting bass) in early stages of an amp or pedal will give you a clearer more defined sound in general but usually this is made up for by cutting treble later in the circuit to make the sound fuller.
The Fender marshal tone stack does a good job of cutting some mids and treble anyway..then we turn down the treble and presence even more most of the time...

I never noticed a "farty" bass in my 3 boogie builds but if lowering the coupling caps made you like it even more then cool!
I would start with lowering the first or second ones but all of them shouldn't be necessary.

John

Hi john!
I´ve heard some dr boogie´s demos (i think yours too) and yes, they dont sound farty. but the problem is not farts...is lack of clarity, definition...ulgy farts are clipped off over stages so is not a problem..

maybe yor builds are lower gain than mine are or your guitar has less bass content than mine.. but in all my builds, at high gain (i mean over 50%) if you play a lead the 6th string over the 12 fret, neck position, it sounds like a buzzaw..too dirty and square...

if you probe the signal after the coupling cap you´re bypassing the effect of the second stage impedance, wich is the case i´m talking bout...

anyway, i know that lowering the caps is an old trick to clean up the sound..but the thing i wanted to inform is that technically, fets and triodes are different, different input impedances, then..different coupling caps...so we have a more accurate "emu"

[snap]

triode input Z ~ 100k Ohms

how to find out about this?

[markm]

I may be completely wrong about this because I haven't even looked to check but,
didn't ROG cover some of this in the Fetzer Valve info?

[Gus]

the input R is set more by the grid or gate to ground resistor. 

Now in tube or fet condenser microphones it is common to have gate or grid to ground at 1 giga ohm.  Now not all tubes will allow this and you have to do somethings to let this work better.   I build tube microphones for fun.

Now most jfets and small signal tubes  both work fine at 1meg to ground.

For most guitar stuff 1meg is fine.  If you look close a guitar to tube amp often has no cap to the input grid.

As others have posted it is more about limiting the bass, remember the lower strings often have more  metal moving over the pickups.

You can also reduce the gate grid to ground resistor ot move the frequency up but if you drop to much you start to load the pickups harder can be good can be not so good.

[John Lyons]

Good points Gus.
Changing the input impedance is done with the gate to ground resistor (and also the series resistor if one is used)

dschwartz
The main thing you are doing is setting the roll off point of the drain by lowering the coupling cap.
This is limiting the bass signal through that stage. Since the impedance is higher the cap value is different to achieve the same roll of as a tube triode. That is a good thing to know. I'll have to try that. Thanks for posting this news.
With so many stages and lowering all the caps you will be reducing the overall bass response though. You can either add some filtering at the end of the circuit like a 10K and .002 cap to ground (or a pair of them) to get the treble down to a good level. Maybe even just a small cap to ground tacked on the volume pot to ground.

I still need to bread board Martymarts Tornado circuit and get to the bottom of the noise issue (hiss) . That circuit sounds amazing and is very responsive even with the neck pickup. It actually uses a small value coupling cap here and there!! Maybe your on to something here...

John