Left Handed Pots

I'm a left handed guitarist with 20 years experience. I've never used the tone or volume knobs while playing. I never saw the benefit and it never felt like I could do decent volume swells. Turns out there was a good reason. Most left handed guitars don't have the right pots in them.

At this point I should recommend a little reading, The Secret Life of Pots

Basically there are three main types of pot, linear taper, logarithmic taper, and reverse log taper.

A linear taper pot has a consistent sweep ie. at the halfway point you get half the value of the pot in terms of resistance. This is the most common type of pot.

A logarithmic pot increases in resistance quite slowly and then increases a lot more towards the end of the sweep. This is much better suited to things like volume knobs because we perceive sound in a logarithmic fashion. This way you could sort of get the volume to be 'half' as loud when the pot is at halfway.

A reverse log pot does the opposite, a sharp increase to begin with and less so towards the end. This can be emulated by putting a resistor across lugs 1 and 2 of a linear pot, but the maths behind it means that you need two very different values to get the best results, and even then the final maximum value is different to either of the two values you started with.

What's this got to do with left handed guitars? We'll most good right handed guitars will have log volume pots in them. Cheaper ones will have linear, which isn't so bad in the bedroom, but will be less useful in a live situation.

Most Left handed guitars will either come:

• Wired as right handed ie. clockwise turn to turn up volume, this is not a bad solution but it's counter intuitive for a lefty, because ideally you want the guitar to be at full volume when you pull up on the volume knob with your little finger ie. full anticlockwise
• Wired as left handed but with a linear pot, not the end of the world
• Wired as left handed but with a log pot, cannot emphasise how bad this is, the volume almost sounds like an on off switch.

That is why I've managed to find some reverse log pots for my guitar. C250k. The site was Bitsbox. I might actually find the volume knob useful yet. If you're left handed, check the pot values in your guitar. A is audio, B linear, and C reverse log. 

Why my Peavey Classic 30 kept dying

This is among many reasons why I haven't updated this blog in a while. My amp (Peavey Classic 30 Combo) occasionally started cutting out after a while. A slow fade out to nothing. My first thought was 'that pint of cider that fell on it a few months back'. But after looking inside I don't think liquid damage was the problem.

So I bought a matched set of valves from Hotrox UK . They worked for a bit, sounded ok, but ran a lot hotter and eventually the same problem returned. So then I took it to a tech, who left it on a dummy load for two days and found no problem.

So, I brought it back home, and set out again to look inside, as this had to be a thermal and mechanical problem. I think I got to the bottom of it:

Can't see it? Neither could I. Inside the amp the pcb is in 3 parts, kept together by a vast number of jumpers. Turns out one of them had come loose:

It took about 3 hours to get the amp apart, and two minutes to fix. So far no problems, fingers crossed. 

Mix and match your tubes

As much as I liked the matched valves, they seemed to amplify their character by all being the same, so the clean and OD channels seemed very similar, despite having different gain. I much preferred the stock valves for the clean channel, but the new valves for the OD channel. The power tubes didn't really make a difference although I think the matched ones were a bit quieter than stock when pushed. They definitely ran hotter.

At the moment I have all stock tubes with a matched valve in the OD slot (The middle preamp tube). I'm much happier with the OD sound now.

Back to pedal modding!

How to mod a Crybaby Wah pedal

I went to see Steve Vai the other day. As well as feeling like I knew nothing about the guitar, I also felt like I needed to use some of the techniques and gear he uses. I can live without a whammy bar to be honest, but his use of a wah pedal in so many different ways reminded me that I had one at home, and I needed to use it.

There's only one problem, it's a Jim Dunlop Crybaby wah, and while I can get some half decent sounds out of it at home, it gets lost in a live situation, and the sweet spot of the wah is in such a small part of the pedal sweep it's just not usable. So how do people get such good sounds out of it? Does it prefer humbuckers and high gain? Should I just buy a Vox?

I've modded the pedal already in the past but not to the point where I was 100% happy, so I'm going to go all out and do almost all the mods to see if I can improve on it. The only two I won't do is to replace the pot and the inductor. In my opinion if you have to go that far you might as well buy a different wah because they're the most expensive parts, after the shell.

The essential resource for these mods is wah-wah.co.uk. I also like the instructions for the Build Your Own Clone wah pedal as that has all the mods built in, so it's easy to get an idea for component values and how to mod. So here are the mods:

Adjust the pot/pinion

The simplest mod. I tried this a few times, never really got the sweet spot I was looking for. But definitely worth remembering when adjusting the other mods. 

True bypass / add LED

The existing footswitches in these wah's don't take the circuit completely out of the signal path, instead they use a buffer to bring back some of the high end when in 'bypass' mode. I found this mod does clean up the pedal when bypassed and in a chain of other pedals. The LED isn't entirely necessary but it's good to know for sure when the effect is on/off. I had to file the bezel down for the LED so it wouldn't physically interfere with the pedal. Some people like to put the LED in the side of the pedal but that seems silly to me.

Remove the input buffer

With the pedal true bypassed you don't really need this buffer any more, although it won't do any harm to leave it in. I read something about it interfering with fuzz pedals because of the impedance, so I decided to take it out to be sure. Besides I like taking away as much as possible from a circuit without compromising the sound. 

Trimpot mods

I already tried the 'Q' mod with a pot, very usable, gives the wah a lot more of a quack. But now I'm going to bring all the mods onto a bit of veroboard and have it all inside the pedal. It's easy enough to open up, and having knobs outside the wah shell just takes up even more pedalboard real estate. The other mods are 'volume', 'bass', and 'mids'. I tried a fixed resistor for one of these already, I think it made the sound worse so I look forward to being able to dial it out again. All I'm looking for is a consistent sweep without the bass-treble jump. 

Capacitor mod

This will adjust the frequency range the wah effects, which will give you the difference between a regular wah and a bass wah etc. I plan on having two socketed areas on the board for two capacitors in parallel so I can either just use the one, or use two, which will be the sum of the capacitance for extra choice. I fixed a rotary switch to the side of the wah once but never got round to hooking anything up. Again I'd prefer to just set and forget inside the pedal now. 

The daughterboard

The plan is to sit this beneath the battery compartment, so it's a tight fit. Also I need to leave room to put the standoffs through it. The trimpots are pretty big considering, 5x3 rows on the vero. The wipers are jumpered to the right hand lugs in case any of them fail (extremely unlikely). This means you may get a crazy wah sound all of a sudden, but it won't cut out completely. The caps can sit in the 3 rows that the trimpots don't use, which saves some space.  I'll put some sockets in so I can change them at will.

Verdict

• True bypass/buffer/LED mods help to clean up the signal when not in use and lets you know when the pedal is on
• I ended up moving the pinion so the pot made it to full sweep in the toe position
• The volume mod gave quite a boost but also meant the lower end sweep sounded distorted, so I left it at a moderate level
• The Q mod is essential for a quackier sound, I just left it maxed out
• The bass mod was interesting, it also affects the gain and resonance a bit, but the best feature is you can tune what note the wah resonates at in the heel position. I tune it to F Sharp because you can really get that Bulls on Parade sound. Too much bass caused a lot of honkiness when sweeping the pedal, even on high notes
• The mids mod interacts with the bass a lot. I thought I wanted to smooth the bass/treble transition but it turned out is sounds better doing the opposite
• The caps mod: This nailed the sound I wanted. Rather than the stock 10n, or 0.01uF, I put in two 4n7's which gives 9.4n. Very slight difference you might say, but this just tipped the wah into the frequency I'd been looking for. I had to lower the Q value because it sounded a bit synthy and extreme. I tried the bass wah mod for a laugh, it knocked the other mods out of place so I gues having a rotary switch for different values wouldn't get you far because I had to adjust bass and Q a fair bit each time I changed caps. 
• p.s. the caps made the daughterboard too high for the enclosure, so I had to put the standoffs in the other side and clip them a bit. 

Summary

No wonder so many of these end up for sale. The slightest change in values give you all sorts of different sounds, more bad than good. I doubt two wahs sound the same what with component tolerances etc. I'm not done tuning this thing, but for now I'm very happy, and I definitely recommend at least swapping the cap and adjusting the pot if you're unhappy with your wah.

How to convert a volume pedal to an expression pedal

Or to be more specific, how to convert a passive stereo volume pedal to an expression pedal that works with my particular pedals.

First let's take a look at the Harrier stereo volume pedal that I bought cheaply on eBay:

Considering it's plastic, it's very nicely made. It must be a template for other pedals like wah's etc because there's a battery holder inside and a place to mount a pcb. As I suspected there's a dual gang pot in there (to operate the stereo volume) as well as a dual gang pot connected to a knob under the treadle so you can set a minimum value of volume. I'm going to be ignoring this for this exercise as you can essentially dial it out of the circuit.

Now I wasn't really in the market for a stereo volume pedal. I don't run a stereo rig (yet) and even if I did, I get the feeling I'd need something more durable and higher spec than this. I would however like to know it's there if I need it, but also to be able to use it as an expression pedal.

Express yourself

I plugged a stereo cable into Out1 and ran it it to my Eventide Timefactor pedal. It worked straight away, but once you get the pedal half way up the expression is at 100%. That's no good.

Looking at the dual gang pots they're both 50k linear (odd that a volume pedal has linear pots and not logarithmic..)

Reading up on the Eventide Timefactor FAQ's, I find that the Timefactor only likes expression pedals up to 25k, which explains the maxing out at halfway point.

Why didn't you use the insert cable trick?

Some pedals will happily accept a volume pedal as an expression pedal using this trick. I tried it, but it still gave me 50k at the extreme end so it won't work here.

So what do I do? Put in a dual gang 25k pot? That's going to affect the volume capability of the pedal, and finding an identical shaped pot in the UK will be a headache. After much deliberation, I came up with this idea:

Run both outputs in parallel. The simple rule about resistors in parallel tells us that:

 1/R = 1/R1+1/R2+....

So in this case if we run two 50k pots in parallel, the maximum resistance will be:

1/((1/50k)+(1/50k)) = 25k.

Perfect! The taper of the pots will still be linear too which is nice.

Now my next issue, permanently wiring the output pots in parallel will ruin any volume capability of the pedal. I still want that option, either in mono or stereo. Enter this little fella:

Similar to the jacks currently in the pedal, but these are switched on the tip and ring lugs. Basically when you insert a jack socket, a connection breaks between the tip and a lug, and the ring and a lug. My solution is to reroute output 1 to output 2, but only if there's no jack in output 1. The benefits of this are:

• Can still use the pedal as a mono volume pedal
• Can still use the pedal as a stereo volume pedal
• Can use the pedal as a 25k expression pedal (when using Out2 alone)

The '1' signal goes to Out1 as normal, but then the switched lug on the tip of Out1 goes to the tip lug on Out2. So when there's no jack in Out1, both signals pass into Out2. I say signals, this will just be a resistance. When there is a jack in Out1, Out2 has a stray piece of wire attached to it, not bothering anybody.

At this point the pedal worked with the Timefactor, but not so well with my Zoom G2, why? Well despite using a stereo cable, the pedals are still only seeing a mono signal (resistance between tip and sleeve). Timefactor doesn't care, but Zoom does.

So I tried the insert trick mentioned above again, and it worked! At 25K no less, but I ain't swapping no cables for different pedals, besides the Timefactor doesn't care either way (good old Eventide). So all I did was to use the same switching trick, this time sending In1 and In2 to the ring lug on Out2 (when nothing is connected to the inputs).

Looking at the white wires above, they only do their job if nothing is connected to In1, In2, and Out1 ie. expression mode. Otherwise you can use it as a stereo or volume pedal as normal. So the expression mod is essentially hooking up the dual gang pot in its entirety (all three lugs of the pot) to the Out2 to give a 25k TRS based expression pedal

Lessons learned

• I didn't need to replace all the jacks, but seeing as I eventually needed 3 switched ones, it made sense.
• The pedal actually reads around 26k, which means the last 1cm or so of the treadle does nothing. Not sure how to remedy this, besides it feels ok to play. Can't really adjust the pinion/pot because it does go down to 0 at the heel end. 

Pedals that I think sound amazing

I thought I'd start this blog with a couple of videos. I watch a lot of pedal demos, so when I do post some here, believe me when I say I think there's something particularly special about them (even though I can't afford them)

 

Until I heard this pedal I had no idea I had a penchant for American voiced amp based overdrive. The saturated low end and crispy high end really does it for me.

 

A bit heavy for my tastes, but it has tone. Both these pedals are presumably JFET based, so maybe that's a road I need to head down instead of constantly checking out opamp based OD's/Distortions etc. These are both very good channels to subscribe to by the way.