Add a resistor bewteen output pot and ground for boost?

[bigjonny]

I'm thinking about adding some functionality to my TS808 and BSIAB II pedals by having a secondary stompswitch that gives a "boost" functionality for soloing.  I want it on the pedal (rather than just a booster pedal), so I can choose the boosted option when the pedal is bypassed, so I can go from bypassed mode into boosted distortion w/o stomping on two switches simultaneously.  Essentially like the Fulldrive 2 has...  


My thought is to put a resistor in series between the standard connection from the 100kΩ output pot to ground.  Use one section of a DPDT stompswitch as a SPST to jumper around the resistor (bypassing the resistor), and you can toggle between "stock" 100kΩs and "boosted" (250kΩ?).  Use the other side of the DPDT for an LED. Therefore, the 100kΩ output pot would look more like a 250kΩ, for instance, in boosted mode.

Has anyone tried this with success, particularly with the TS808 and/or BSIAB II pedals?  I have read about people subbing in different pots for higher output, or bypassing the tone stack for a boost, but nothing like I am suggesting.  Unfortunately the search terms I could think of are so generic, I couldn't find anything in the archives...

Thanks, all!

[petemoore]

Yes.
 I did this on a Blackfire, works just fine as long as you can get what you want from it by tuning the resistances.
 It is general enough that typing 'particulars' as to TS or what have you...all have volume setups like this apply.
 I belive I went with a smaller output pot value...ya know I forget which of the ways I ended up doing it, but there was a second pot involved that preset the second volume setting.
 Just by figure 'more' resistance to ground makes less output, and choose resistor value for how much 'more'.
 Works great, few parts, changes volume. Sound is changed as minimally as is possible.

[cd]

NO no no no no.  #1, do you want to increase the gain, or increase the volume?  By GAIN, I mean if you could keep turning the DRIVE knob another rotation.  If it's gain (drive/distortion/whatever) you want, you have to figure out how does the circuit create the gain/distortion?

In the TS808, the amount of distortion is set by varying the gain of an opamp, which then causes the signal to be clipped by the clipping diodes.  In the TS808, the DRIVE pot does this - as you turn up the DRIVE pot, the resistance increases.  The DRIVE pot is wired as a rheostat, in other words think of it as a variable, single resistor.  At the low end, it's a 0k resistor, at halfway, it's a 50k resistor, at full up, it's a 500k resistor.  So if you want more distortion, all you have to do is wire another resistor IN SERIES with the DRIVE pot and have it switchable - I would use a 1M resistor.  So this switchable "boost" instantly goes from wherever you have the DRIVE pot to a very high resistance (1M) which results in lots of distortion.

In the BSIAB2, the amount of distortion is set by overloading the second gain stage; in between the two stages is the DRIVE control.  The DRIVE control in the BSIAB2 is wired as a voltage divider; this control takes the signal and makes it smaller and smaller from its maximum possible value.  So if you turn up the DRIVE control all the way, it's already at maximum distortion; you can't get any more without changing other parts of the circuit.  You can get the "illusion" of a boost by putting a resistor at the INPUT of the DRIVE pot and making that switchable.  With the resistor "active" (say a 470k resistor) the available distortion will be cut in half because even if the DRIVE pot is turned up all the way, the signal is still being divided in 2.  Switch the resistor out and you have the full range as before.

If you want more gain with the BSIAB2 (though I don't know why you would - it's pretty gained out already) change the value of R3 and C3a/b - decrease R3 and increase C3.  This might result in howling/oscillation.

[bigjonny]

Hrm.  I was debating if I wanted a distortion boost, or a volume boost.  I just figured a volume boost would be easier...

cd, excellent advice, though.  Now, I am tempted to do the distortion boost for both, as they are easier mods, and have the potential for more grit.

[petemoore]

I just put a gain/bass boost in the Rocket, it switches between 22uf and 55uf, by adding another 33uf capacitor to the fixed one with a spst.
 I stay away from running long gained up wires, preferring to use a ground wire if it needs to be long [often when adding a switch the wiring of the circuit gets longer]. I don't know how much of a factor noise really is when running switches for gain control. Yes I SeZZ!!!
 To test this out, and see what hapens, read 100k pot made into 500k.
 cd made some interesting responses, and taking a pot and some resistors and trying to taper it is informative.

[bigjonny]

ah just thinking about this a bit more... My buddy has the TS-7, which is not renowned for its greatness, but has a nifty feature in that you can have it function in "stock" mode, or flip a switch and have it in "more distorted" mode.  It must be a simple toggling of a resitor in series with the DRIVE pot (or not).  On the TS-7, it's a finger-select switch, but my pal's always wishing he could "stomp" that option.  Think I *will* go with the gain boost for the TS808 clone.

Regarding BSIAB II:

So if you turn up the DRIVE control all the way, it's already at maximum distortion; you can't get any more without changing other parts of the circuit.  You can get the "illusion" of a boost by putting a resistor at the INPUT of the DRIVE pot and making that switchable. With the resistor "active" (say a 470k resistor) the available distortion will be cut in half because even if the DRIVE pot is turned up all the way, the signal is still being divided in 2. Switch the resistor out and you have the full range as before.

cd, I certainly dig this idea for the dodging of the squeal factor, but couldn't I just do like I originally suggested for the VOLUME boost (putting a switchable resistor in SERIES between lug 1 of the pot and ground) to increase the "overdrivedness"?

At any rate, I think with BSIAB II, I will do this procedure on the 100kΩ VOLUME pot, for a simple output volume boost.  The 500kΩ DRIVE pot I got is slightly more than 500kΩ, even if you dial in the trim pot "just so", the BSIAB II squeals at high treble settings (Although, using cd's suggested BSIAB II DRIVE mod, I could cure that).

[cd]

On the TS-7, the switch does two things; puts in a larger tone control cap to cut more highs, and decreases the gain set resistor on the - input to quadruple the amount of gain available.  It's like turning the Drive control around 4 more times, but that range is now in 1 pot rotation.

Regarding the BSIAB2, if I'm reading you correctly (from the input of the pot to ground) what you're suggesting will actually lower the volume slightly.  If you mean putting a resistor from the pot lug that is grounded, lifting that and putting a resistor there - that will work (you'll get a bit more volume when the DRIVE control is maxed) however if you have that switched on (resistor active) you won't be able to turn the DRIVE all the way "off" (no signal).  That's how it works in a Big Muff Pi.

Have fun experimenting!

[ryanscissorhands]

I know you said that you don't want to do another nooster pedal. However, you could house a booster like the beginner in with your TS (if you were cool with a new enclosure) and use it as a front end. This would boost both gain and volume a little, I believe. Now, you would then make the bypass stomp for the TS to bypass the entire circuit.

[bigjonny]

you could house a booster like the beginner in with your TS (if you were cool with a new enclosure) and use it as a front end.

Yes â€" good, practical suggestion, ryanscissorhands.  This was my 1st thought, as I have a couple booster circuits all soldered up and waiting for a home.  However, my actual grand scheme, here, is to fit the TS808 & BSIAB II into a 1590DD so I can have a 2-in-1 distortion box with three stomp switches:

Distortion effect: on/true bypassed
Distortion type: TS808/BSIAB II
Distortion boost: On/Off (I was debating this until this online discussion happened â€" now its a "for sure" thing to add!)[/list:u]
Plus a couple finger-toggles:

TS mode: TS808/TS9
TS clipping: symmetric/asymmetric
BSIAB II bass boost (increase input capacitance): on/off (this one's a maybe)[/list:u]
Ultimately I decided that its a waste of components, space, and money (to me) to use a booster circuit with all those components, when I can toggle in a resistor or two, and BLAMMO: instant boost!  Furthermore, the 1590DD is only about twice the width as a 1590BB (all other things being roughly equal):  it's already a tight fit with all the controls, jacks, one battery and the two PCBs from

GGG.

If you mean putting a resistor from the pot lug that is grounded, lifting that and putting a resistor there - that will work (you'll get a bit more volume when the DRIVE control is maxed)

Yes, that's what I mean.  I also did realize that the volume cannot be turned completely quiet at that point, but honestly who ever wants that?  Plus, depending on where I do the mod on the BSIAB II (DRIVE or VOLUME), the control I don't mod will make the thing totally quiet.

On the TS-7, the switch does two things; puts in a larger tone control cap to cut more highs, and decreases the gain set resistor on the - input to quadruple the amount of gain available.  It's like turning the Drive control around 4 more times, but that range is now in 1 pot rotation.

Hrm.  Switching in a low-pass cap on the BSIAB II for a DRIVE boost would be the ticket!  Then I can avoid an undesired trip to Squeal City.  Don't know why that didn't occur to me before!  Also, I don't know how much more boost I'd get, but looking at the Mini Booster, upon which the BSIAB II is based, it's plain to see that can I jumper a (switchable) short around R10 (see the BSIAB II's schematic) for a little more grit.

Anyhow, thus far this has been a very useful discussion.  Thanks all!

[dosmun]

How do you delete a post anyway. I posted on the wrong topic and can't get rid of it :x