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HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by jedijunk on 2012-07-25 15:34:53

So I had a Ernie Ball VP JR volume pedal (the one with the 250k pot) that I wasn't using and I came across a brief description of a way to mod it to allow it to also be used as an expression pedal (toward the bottom of this thread):

http://www.thegearpage.net/board/archive/index.php/t-652140.html

Basically">http://www.thegearpage.net/board/archive/index.php/t-652140.html">http://www.thegearpage.net/board/archive/index.php/t-652140.html

Basicallyit involves adding a 27k resistor in parallel to the pot in the pedal to bring the resistance down to 24k (like the Ernie Ball VP JR 25k pedal that many use as an expression pedal). I followed this guy's idea of putting the resistor in a switched circuit so it can be switched off, allowing the pedal to still be used as an unmodded volume pedal as well.

All went well with the mod, and I confirmed that the resistance of the pedal now sweeps from 0k to 24k with the resistor switched on. But when I hooked it up to my HD500 (with a standard instrument cable from the HD500's MOD2 jack to the "Out" jack on the Ernie Ball pedal), the assigned value swept from 0 to 100 in only one-fourth of the pedals travel. And the HD500 doesn't have a calibration setting for the MOD2 jack. So I replaced the 27k resistor with a 10k resistor (resulting in a 9k max resistance for the pedal). That resulted in the assigned value sweeping from 0 to 100 in half the pedal's travel - better but still not good enough. I measured the resistance of the pedal at the point in the sweep where the assigned value on the HD500 maxed out, and it was 6.5k.

I can easily further reduce the resistance of the pedal to a max of 6.5k by swapping in an even smaller resistor. But I don't understand why that should be necessary. Multiple people say the 25k Ernie Ball pedal works fine with their HD, and the Line6 and Mission Control pedals are both 10k pedals. So why should I have to go even lower than 9k to get full travel out of my pedal?

Can anyone give me an idea what's going on here?



Re: HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by jimsreynolds on 2012-07-26 03:06:29

My understanding is that the standard requirement is for a 10K Log pot (http://line6.com/support/docs/DOC-1472)">http://line6.com/support/docs/DOC-1472">http://line6.com/support/docs/DOC-1472)so 6.5K dpes seem a little low.   I cannot account for why you would need to go that way.

My suggestion would be a soldering in a pot that gets you into an adjustment range around 5-10K and then manually calibrate the pedal using that.



Re: HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by jedijunk on 2012-07-26 03:38:04

Good idea about soldering in a second pot... adding in, say, a 25k mini-pot would allow adjustment for different effect unit's requirements.

I still don't understand WHY it was necessary to reduce the resistance of the pedal this far, but figured I'd go ahead and report what I ended up doing. I only plan to use this pedal with my HD500, so I just replaced the 27k resistor in the mod description I linked to above with a 6.8k resistor. In parallel with the 250k pot in the pedal, this results in a 0k to 6.7k sweep on the volume pedal (the Ernie Ball pot is an audio taper pot though (i.e. it doesn't sweep/increase at a linear rate), so when the pedal is physically about half swept you'll be at 85% or so on your parameter sweep). But on my HD500 the result is a 0% to 100% parameter sweep with a full pedal travel movement.



Re: HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by jimsreynolds on 2012-07-26 05:58:34

Always good to get feedback on people's ideas and experimentation so thanks for posting



Re: HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by spaceatl on 2012-07-26 08:23:08

I am pretty sure that the taper is why you are reducing...basically, you are flattening out the taper of that log pot you have...I am not sure if you taper would follow the same as a linear taper or not...



Re: HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by phil_m on 2012-07-26 08:35:42

The EX-1 doesn't use a log pot - it uses a linear taper pot. Actually, it uses two 20k ohm pots wired in parallel. But, anyway, I think Space it right with his comment. I think the reason the original poster is seeing a difference is because he's using a log pot.



Re: HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by jimsreynolds on 2012-07-26 13:47:14

Ach !  Sorry, yes Phil is right.  It's linear.  Read it too fast.



Re: HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by jedijunk on 2012-07-27 12:49:10

That still doesn't make sense to me, Space and Phil. Not trying to argue, just to understand. As I understand it (with my Wikipedia edumacation and all!), the difference between a log pot and a linear pot is in the resistance curve used as they travel from point A (0k resistance) to point B (max resistance value of the pot). But a 10k log pot and a 10k linear pot share the same maximum resistance end point, which seems to me is the only really relevant point in the resistance curve when trying to get full travel out of this pedal. So the fact that I'm using an audio-taper log pot still doesn't help me understand why I had to reduce the max resistance of the pedal to 6.6k to get use of the full range of motion.

At any rate, the end result is a pedal that's working great as an expression pedal with my HD500 (albeit with a log pot curve as I sweep through the motion), AND with the flip of a switch I can use it as a passive volume pedal instead. Mystified but satisfied!



Re: HD500: issue modding Ernie Ball VP JR 250k volume pedal to use as MOD2 pedal
by phil_m on 2012-07-27 14:06:23

I understand what you're saying, and technically, you're correct. The starting and ending values of the two type of pots would be the same. But with a logarithmic pot, when you're halfway through the pedal sweep, the resistance could will already be near something like 85% or more of the rated value. So on the last half of the sweep, you'd be making up the remaining 15%. Once you get to 3/4 of the way down, you're above 95% of the rated value (these are approximate values, the exact curves of pots can vary quite a bit). So even though you have more to travel, there's just not a lot of range left in that distance. At that point, what happens depends on the sensitivity of the expression pedal input on the HD. I really don't know how sensitive it is, but I suspect that's why this is happening.




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