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Danelectro Fab Distortion - Bass mod

P

pyrohaz

Apr 28, 2009
102
3
Cheshire, UK
#1
This is one of my first posts so go easy guys :)

A couple of years back, my friend finished his guitar playing times and I managed to acquire his Fab Distortion pedal. Me being a bass player, always interested in a distortion, I gave it a try!

The distortion was very thin sounding and was a complete tone suck, I still used it but not for any major recording or such.

After opening the back cover (I hadn't used a battery once!) I had a quick look at the circuitry and by 4 screws, I could unscrew the rear board which held the input and output jack, along with I/P and O/P capacitors. After looking at these capacitors, the I/P cap was numbered "473" (meaning 0.047uF) and the output capacitor "474" (meaning 0.47uF). Now obviously, the builders of this danelectro pedal will have tailored these values to guitar. Most probably having the input and output filters designed to have a -3dB point around 80hz (Lowest E on a guitar) which is double the frequency of the lowest E on a bass guitar! For example, a 0.047uF capacitor forming a high pass filter with a 47k resistor will have a -3dB cut off point of 72Hz, well above the lowest note of a bass guitar.

Luckily, if the capacitor value is doubled, the cut off frequency is halved. Therefore, if I double the capacitor value, I should have a cut off point of 36.02Hz (if the high pass filter does use a 47k resistor!) Just to ensure that the pedal can let enough "bass" through, I have added a 1uF capacitor in parallel with the original 0.047uF capacitor. This to me has made a noticeable improvement to the tone and I will include some tone samples :)

I have also done the same on the output side of the pedal, changing the capacitance from 0.47uF to a total of 1.47uF. For my capacitors, I used Axial 1uF 63v Philips capacitors, many people are against the use of electrolytic capacitors but these are of high quality and produce no noticeable flavouring to the tone of the pedal in my opinion.

Below, I have included some sound samples in the order:
Clean
Full Distortion, Tone at 0, Level near full
Full Distortion, Tone at ~1/3, Level near full
Quarter Distortion, Tone at ~2/3, Level near full
Quarter Distortion, Tone at ~1/3, Level near full (My favourite setting!)
[SOUNDCLOUD]http://soundcloud.com/pyrohaz/sets/bass-tracks-danelectro-bass[/SOUNDCLOUD]

DSCN1284r.jpg
Above: The innards of the pedal (Rear side of first PCB)

DSCN1285r.jpg
Above: The innards of the PCB above, flipped over so you can see the I/P and O/P capacitors. Note my blue caps!
 
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#6
I used to have one of these.
When I bought it, the guy at the shop said that I had to turn the input gain on my amp almost all the way down because the pedal had so much gain. :rollno:

Of course I believed him, because I was 12 or so at the time, and well it made sense to me then :rolleyes:

I let my 'friend' borrow the pedal a couple of years ago and when I asked for it back, it was 'missing'.
 
#7
pyrohaz said:
Note my blue caps!
Click to expand...

Awesome. I'm a DIY kinda guy, so I dig this sort of thing. I just wish I knew more about electronics ... I'm working on that :p

And with that in mind, I hope this isn't too much of a hijack ...

I plan on doing the exact same thing to a Behringer VT999, which is a clone of the Ibanez Tube King. I have a schematic for the Ibanez, and when I receive the Behringer in a few days, I'll compare to see how closely it follows the Ibanez design.

One thing I'd like to figure out is where the Bass, Mid, and Treble EQ centers are, and if they can be easily and predictably modified by swapping out a component. Can this sort of thing be determined from the schematic? How? :help: (I can post the schematic here if it's a good place for the conversation.)

Anyway, it's good to know I'm not the only one finding this sort of thing fun, interesting, informative and useful. Thanks for the pics and sound clips - very cool :cool:
 
#8
pyrohaz said:
Cheers guys! Yeh i'm with you, for the price, its a pretty darn easy mod and makes the pedal much more useable for bass! Btw, I was using a Dean Q5 for all the clips, its a gorgeous bass!
Click to expand...

Seems like a super fun (and cheap!) project. It might be worth trying since wiring the caps in parallel is so darn simple and accessible. I tried fooling around with the surface mount components on one of those FAB pedals before, and immediately realized that, despite dozens of pedal mods over the years, I need to limit my soldering attempts to through-mount components. That poor fab echo didn't stand a chance :(

I do like your clear explanation of the input/output cap filters. I'm an electronics noob and elbow deep in a text book, diy gear sites, and educational projects at the moment. Could you explain (here or PM) or send a link that gets down to the basic properties and math of how those filters work? I'm tired of just using suggested bass values floating around on the internet instead of thinking through it on my own and figuring it out.
 
#9
ulynch said:
Awesome. I'm a DIY kinda guy, so I dig this sort of thing. I just wish I knew more about electronics ... I'm working on that :p

And with that in mind, I hope this isn't too much of a hijack ...

I plan on doing the exact same thing to a Behringer VT999, which is a clone of the Ibanez Tube King. I have a schematic for the Ibanez, and when I receive the Behringer in a few days, I'll compare to see how closely it follows the Ibanez design.

One thing I'd like to figure out is where the Bass, Mid, and Treble EQ centers are, and if they can be easily and predictably modified by swapping out a component. Can this sort of thing be determined from the schematic? How? :help: (I can post the schematic here if it's a good place for the conversation.)

Anyway, it's good to know I'm not the only one finding this sort of thing fun, interesting, informative and useful. Thanks for the pics and sound clips - very cool :cool:
Click to expand...

After having a quick internet search of the Ibanez Tube King, I found that it uses 3 active op-amp filters for each section for the Bass, Middle and Treble, the responses of these filters are determined by the values of resistors and capacitors used. As an extremely rough rule, you can generally double the value of all the tone capacitors to make it sometimes soudn "slightly" better with bass (if you double the capacitor values, the main points of frequency half e.g. if you have a high pass filter with a capacitance of 220nF and resistance of 10k, this will have a cut off point of 72.3Hz, on the other hand if I follow the loose rule and double the capacitor value, this cut off point then moves down to 36Hz.

Unfortunately, this rule really doesn't work in some cases. For example, increasing some capacitor values can produce drastic changes in tone and can mess with some of the devices dependant on attenuating certain frequencies.

I had simulated the tone circuit of the ibanez tube king and have found out, the bass section has a -3dB point at around 300Hz but it seems to be a form of shelving bass EQ meaning this section of the tone should be fine for use with bass! On the mid section, it ranges from ~100Hz to 1.1KHz, a very wide mid section I believe as I would class the mid section of bass generally from 150Hz to 800Hz (personal opinion!). The treble section extends to from ~1k+ I personally don't see any point in changing these values so if you're going to mod your pedal, you will probably be able to get away with leaving the tone section alone!

If the behringer pedal does follow the Tube King schematic, the input filter is equivelant to a high pass filter consisting of a 47nF capacitor and a 511k resistor. This has a very low cut off at about 6Hz so this should not post a major problem. On the output side, it has an output filter of 1uF and a 500k Pot (The output filter will be dependant on the load impedance!) if the load impedance is very high, it will be 0.32Hz. If the pedal is connected to a pedal of 10k Input impedance, the cut off point will be 15.9Hz. This doesn't pose a major problem to this pedal and if this schematic is correct, it shouldn't even need majorly modding!

To dullsilver_mike: I completely agree, my limit comes to surface mount devices too! It also takes me a bit of time with double sided PCB's. If you are quite good at following the math of electronics, you can go to: Invalid Link Removed But if you find that a bit too complex, i'd happily PM you an easier way to understand it :)
 
#10
sorry to resurrect an oldie...but I had one of these early edition models laying around (bigger, all metal build). It did rob the low-end from bass. I had to look up a schematic on some other forums to figure out what were the input and output cap's on this model, but I think I got it. I put in 1.0uf caps on C4 & C9 in addition to the current ones and hooked it up. what a difference! I might need to run it in an FX loop though, as the best bass output comes with the volume up about half way, and it is far louder than bypassed signal. I turn the fab all the way down, max the bass, with a hint of treble. It goes over the top with the fab up past 8:00.....like a chainsaw through sheet metal. a usable pedal now, if I can find the spot for it. ;)

also, I didn't really check for "tone-suck" before, but I did after the mod, and I can't notice any difference at all with it in-or-out of the effects chain.

2h58ec5.jpg

5ea9t1.jpg
 
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#11
is there an easy way to decrease the output level on this pedal so that I can more effectively use the volume and fab controls? i am forced to keep them pretty low so that this pedal can be used in the chain and keep the signal somewhat even.

any and all help would be appreciated. I think this will make a nice fuzz/raging distortion pedal once I get the levels worked out.
 
#12
Thanks for sharing! I love working on cheaper pedals too. I just got a new cheap project in the mail; a digitech death metal. My plan is to install an internal trim pot instead of the resistor in one of the gain stages so that I'll be able to tweak the amount of gain this pedal has and make it hopefully more usable as a bass distortion.

In my guitar setup, I use a behringer to800 as my tubescreamer derivative; people are surprised when they see it on a board with pedals that cost ten times as much.
 

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