'The Crank' boost pedal, with possible mods -- your feedback?

[bipedal]

After successfully completing a TS9-to-TS808 mod, a Tweako, and a Tremulus Lune, I'm now itching to get started on my next build.

The Crank boost (Mark Hammer: http://ampage.org/hammer/files/The_Crank.gif) looks like an interesting project, as I'd like something to add a little more oomph and grit to my clean signal.  Anyone out there built one of these lately and willing to share their thoughts?  How does the gain compare to a Tube Screamer?

A couple of mods I'm thinking of:

1)  The original design calls for a 10K dual-ganged pot to simultaneously control both gain stages.  I can't find a dealer who carries this pot (recommondations?)...  I might just use two separate 10K pots to separately control each gain stage.  Makes it a bit more complicated from the user-side, but also more fine-tunable.  Do you see any problems with this approach?

2)  As a preamp/boost meant to push the signal going into my amp, I could see this being a pedal that I would always leave on.  Therefore, I'm thinking of building in two volume pots, using a stomp switch to route the signal to volume 1 or volume 2.  One volume would control level of the "regular" boost, while the second could be dialed a bit higher to act as a "solo" boost.

If I go with this dual volume approach, it would be nice to include an indicator LED to show when the "solo" boost is engaged, but I'm not sure how I would incorporate this.  Your suggestions?

Thanks –

[Mark Hammer]

Because it is intended to provide only very modest clipping, and because of the use of a 2+2 diode arrangement in the second stage, much of gain it adds (somewhere around x40, IIRC) actually comes through in the output.  So, to respond to your Crank-vs-TS9 query, it is both louder and cleaner.

Since neither half of the dual-ganged pot is used as a voltage divider, you can use just about any value dual-ganged pot you can put your hands on (25k, 50k, 100k, etc) and simply tack on a parallel fixed resistor to the appropriate lugs so that the critical resistance in each stage does not exceed 10k (or at least by very much).  For example, if you used a dual-ganged 100k pot,strap on a 12k resistor in parallel with each half for a max resistance of 10.7k.  You should be able to get a dual-ganged pot of some value from somewhere.

The optimal strategy for having two preset volumes would largely depend on what else you have between the pedal and amp, or between the guitar and this particular pedal.  A number of choices to pick from.

[Stompin Tom]

I'm pretty sure I got a dual-ganged 10k pot from futurlec. Hard to imagine mouser wouldn't have it, too.

[Mark Hammer]

Mine were bought from Radio Shack, I think.

[snoof]

I know i've gotten some from Mouser b4...

[bipedal]

Thanks for your replies -- louder and cleaner is exactly what I'm aiming for.  I find I rarely turn my Screamer's gain knob past about 10:00 o'clock.

Appreciate the futurlec tip too -- hadn't heard of them before.

[boogietube]

Having had the pleasure of borrowing The Crank from Mark, I'd say it's what you're looking for. With my Strat or Hamer it sounded wonderful. That loud very slightly overdriven sound that I was after. I wouldn't mod it at all. It's perfect the way it is.

Futurlec: Read posts on the forum and make an informed decision. I have had no trouble, but some have.

[bipedal]

Thoughts on adding a Tone control at the end of the Crank circuit?  From what I can see, it looks like the basic BMP tone stack before the volume should work fine...

[Mark Hammer]

Just about any passive tonestack/control should work.  The larger question is whether the amount of passive loss incurred by the addition of that circuit still lets you use the available boost in the way you want.  I should point out that this is true of adding a tone control onto ANY sort of booster circuit.

If what you hope to achieve is beating your amp's input stage into submission, then you'll want to retain every little db as much as you can.  If what you hope to achieve is simply a somewhat louder tone for solos with a hint of dirt in it, then mod away.  There is more than enough overall gain in the pedal to tolerate losing as much as 1/3 - 1/2 of the output level to passive loss in a tone control.

Personally, I would recommend against a BMP-style tone control.  There is really nothing you'd want from this pedal that the nasal side of that tone control would get you.  My gut sense is that either a Fender/Marshall type tonestack or a simple affair like the SWTC would be more helpful.

BTW, you CAN increase the gain of the pedal.  It's just an op-amp booster.  The trick will be in maintaining the relative gain of stage 1 vs stage 2, and in reproducing the gradual treble cut as gain goes up.  It's not impossible but there'll be some math involved.

[dachshund]

Thanks for the info, Mark & bipedal  
I have to add this one to the list.
... so many circuits, so little time........... 

[bipedal]

Thanks again for your speedy feedback and suggestions, Mark -- much appreciated.

As is probably obvious, I'm just brainstorming at this point -- I suspect adding a Tone is probably more trouble than it's worth for me, especially if the Crank's boosted signal is fairly transparent.  (Besides, I've got perfectly functional tone pots on my guitar and my amp -- do I really need another one in my pedal chain too?!?   )

Cheers -

[Mark Hammer]

I didn't suggest the SWTC lightly.  Fact of the matter is that whenever you push a subsequent stage harder, you WILL get additional harmonic content unless that subsequent stage has ridiculous headroom.  The quality of that additional harmonic content will always depend on the harmonic content of what is pushing it.  That is, in fact, why the Crank trims back the treble as gain is increased.  Whether that is "enough" trimming back is a question of taste, tonal needs, and the sensitivity and prone-ness to clipping of the subsequent stages.

In a sense, one always needs to approach booster pedals as if they and the amp were a sort double clipper pedal.  Perhaps you do have plenty of available tone/EQ controls situated elsewhere in your signal chain, but it is not a given that the sort of tone generated downstream by a "tonally untamed" booster situated upstream would not require any additional tonal adjustment the moment you hit the stompswitch.  You want the degree of "bite" elicitted to be measured and predictable.  It's not a robust sort of control like the BMP tone control, but the SWTC *will* allow you to adjust the Crank in that manner, so that the amp it pushes sings instead of yells.  That's what you want out of any booster, I imagine.

[bipedal]

Point taken about the tone control.

I just revisited your SWTC info, and ran the frequency range formula numbers to make sure I understood what's going on.  (Was able to duplicate your high and low Hz values with your example resistor and cap #s, which is a good sign for me.)

It took me a couple of glances at the SWTC schematic and discussion to realize that P2 refers to the volume pot in the main circuit, not a second tone control pot.  Once that clicked, this control suddenly seemed pretty easy to build into my project, and it seems like (based on the Crank discussion) there should still be plenty of gain coming out of the pedal even with this tone built in.

Thanks again for the recommendation.

Am working on a fuzz face build for a friend now, but the Crank (now with tone control) is #2 on my project to-do list.