Edwards Light Beam volume pedal repair / refurbishment

[Matt Kennan]

Hello all,

I'm brand new to SGF and joined after being directed here after a number of internet searches on several topics. I've seen a lot of great information here and respect the great comradery of the members! My reason for this post is to try to get some information on Edwards Light Beam volume pedals. I've read a lot of the posts here and elsewhere on the Edwards pedals and have some specific questions and maybe some useful information on how they work, why they degrade, and what I'm planning to do to perhaps resurrect or improve mine.

First, I'm not a professional musician but play guitar and a few other instruments. I have two Edwards Light Beam volume pedals (the volume-only version) in my collection. I bought the first after reading about Joe Walsh's gift of a '59 Gretsch 6120, a tweed bandmaster 3-10 amp and an Edwards Light Beam pedal to Pete Townshed of The Who, and how it became the little-known rig that shaped his tone on Who's Next, Quadrophenia, and countless other recordings. Since my first concert ever was The Who in the fall of '69 in Hartford, CT when I was 12 years old, they've always been one of my favorite bands.

My first Edwards (we'll call it #1) is in good shape and appears to be all original. Being a gear geek who fixes most of my own equipment (instruments, SS and tube amps, etc.) I immediately took it apart to see how it worked made a full schematic of the circuits inside. I agree with many of the posts I've seen about how they alter tone and wanted to find out why. Compared to my other potentiometer volume pedals, they also diminish the overall volume to a noticeable level, one of the topics I'll talk about below. My second pedal "#2" earned it's name by chronology, but turned out to be appropriate due to the condition it was in when I bought it (I didn't pay much for it). I had a cracked plate that the light shines through, a very poor job had been done replacing the power cord, including completely gooping over the splices with silicon caulk, the pilot light flickers, and most important it only got up to about half the volume of the other one.

First, a little about how they work, since I've seen a number of posts and videos on this and while mostly correct, there is some misinformation out there:

There are two completely distinct circuits inside. The first consists of the 120V power cord, which drives the 120V pilot light and a 120V to 24V transformer. The transformer in turn drives a 5W, 56ohm resistor in series with a type 57 off-the-shelf incandescent automotive bulb. This circuit is a simple voltage divider circuit designed to cut the 24V down to the 12V rating of the bulb, and has nothing to do with the signal path from the instrument to the amp. If the resistor has the same resistance of the bulb, they will should each be seeing 12 volts. Here's a tip if you want to check this: When I checked the resistance of the bulb out if the unit, it was only 5.6 ohms - 1/10th that of the resistor. This baffled me since they should be the same, until I thought about what happens when you put current across a bulb: it quickly heats up to the point where it gives off light and heat. This would logically increase the resistance of the bulb. When I did a little internet research I found that incandescent bulbs typically increase their resistance 10 to 20 times from cold to operating temperature. Checking the voltage across the bulb when it was on showed it to be 11.88 volts and the voltage across the resistor was about 13V, which means that that bulb was now at about 56ohms. That is the entire first circuit. Other than one side of the 24V output being hooked to the case and the ground side of the bulb, there is no direct connection between this circuit and the guitar signal circuit. Note that THERE IS NO CONNECTION BETWEEN THE 120v SIDE AND THE GUITAR SIGNAL CIRCUIT.

The signal circuit consists of the input and output jacks, a photo resistor and on my #1, a ceramic disk capacitor connecting the tip connections of the jacks. The photoresistor (light dependent resistor, or LDR) is connected from the tip of one jack to the ground (sheath) of the jack. LDRs change resistance when light shines on them, going from a higher resistance in the dark to a lower resistance when light shines on them. It’s a gradual change as the light level changes, and the design of a particular model determines what the highest and lowest values are. The inside of the Edwards pedal is painted black and the bulb is encased in a milled block of aluminum except for a small hole facing the LDR. Between the bulb and LDR there’s a sheet of brown translucent plastic with a groove milled into it that gets wider and deeper from one end to the other, which is attached the treadle. Going from fully toe down (loudest) to fully heel-down (off) allows more light through, lowering the resistance and therefore sending increasingly more signal to bleed to ground so there’s less and less to feed the amp. Pretty simple.

It's easy to measure the resistance with a volt-ohm meter. Plug in a guitar and hook the meter to the tip and sheath of the other end of the cable. Turn on the pedal and measure the resistance at fully down and fully up. On my #1, heel down was 140 ohms and toe down was 16.5K (16,500 ohms). On my #2, the low was 130 and but the high was only 4.73k (4,730 ohms). The lower resistance on #2 at full volume was bleeding much more or the signal to ground, hence the lower volume.

It took some time, but I located some LDRs that were listed to have dark resistance of 1.3 megohms (1.300,000 ohms and a light on resistance of 170 ohms. I bought 4 of them and when I tested them, they tested from under 100 ohms under my desk lamp light to over 1 megohms in the dark. It’s hard to tell what the fully lit level is inside the pedal and it never gets fully dark due to the open areas between the top and bottom of the case, but I’m hoping to see an improvement when I get around to replacing the LDR. I have no way to tell what the resistances were when they were new, but I'm hoping the drastically higher resistance at toe-down will result in much more signal getting through to the amp improving the tone and volume.

Some other notes:

People have mentioned getting shocks from Edwards pedals. Because the 120V is isolated from the case (unless there been damage or improper modification), I’m guessing shocks (as from touch a microphone while playing) were from polarity differences in the pieces of equipment and not caused by the 120V source at the pedal. The voltage difference would be present through the ground in the signal cables from the instrument to the amp regardless of whether the Edwards pedal was in the chain or not. Remember that guitar strings and mic casings are connected to ground, so if the polarity of the guitar and mic are reversed and you touch both, that 120V is being bridged by you. I’m old enough to have been zapped a time or two from amps in the 70s without grounded plugs, which not only ground the chassis but are also standardized as to which of the two flat prongs carried the line voltage and which is the neutral.

If anyone’s having issues with dead or flickering pilot lights that you’ve traced to the bulb itself, I found replacements on ebay. They are Leecraft/Snaplite #32, 1/3 watt 125V. I bought 4 of them on eBay for $10 + shipping.

I have a couple of questions for members who may have an Edwards pedal:

1. My #1 has a .05 microfarad 50V ceramic disk capacitor between the tips of the two jacks. My #2 does not, but it’s obvious some rewiring was done (and very poorly as well). From some online posts and Reverb ads, I’ve seen them on some but on others there’s just a straight wire connecting them. For those of you who have them (especially if you bought it new), can you tell me if it’s there or not and what markings it may have on it?
2. If I’ve sparked your curiosity about the resistances, could you please share the results of the readings at full toe and heel positions? If you get curious and measure the resistance of the instrument you're using, that would be interesting as well.
3. Any general thoughts on general performance of them would be appreciated (if it works better with this or that equipment, etc.). I’d especially like to hear if you’ve made repairs or modifications to your Edwards pedal, and what the results were.

After I get to replace the LDR in #2 I’ll certainly update the forum as to how it worked out. I may also try to fabricate some different control plates to try different volume curves if I get time. Replacing the 120V circuit with a 9 volt LED for standard battery or pedalboard power could be used might be another thing I might try someday. That would eliminate any concerns about the high voltage, possiibly remove any 60Hz hum, and make it much easier to use.

I’d appreciate anyone’s input on anything I’ve written and welcome corrections or other opinions.

On a different note, I also picked up an interesting first “pedal steel” which had only four pedals and no levers. I figured it would be a good way to try getting the sound without trying to learn all the levers. I spent a lot of time trying to find out about it since it had a Gibson badge, without luck until I saw a picture of a Harlin MultiKord. Mine is exactly the same except it bears a Gibson nameplate. I’ve never seen another with a Gibson badge and internet searches haven’t turned up anything related to Gibson having sold them. I’ll post more about it in another section of this site sometime.

[Dave Hopping]

Edwards pedals have been fairly common here in Colorado. My first one came from Don himself long ago and over the years I've lucked into 3 more volume-only and one volume-and-tone. The volume-only all work OK, the V-T not at all. I don't know enough electronics to do work on any of them, but I'm going to talk with my amp guy and see if he wants to deal with the volume-and-tone..

[Matt Kennan]

Thanks for the reply Dave. Any chance you can tell me if your volume-only pedals have the capacitor connecting the jacks? Also, any chance you can measure the resistances of your pedals? I'd be really interested in knowing what I'm trying to achieve in repairing mine. I could also meet up to measure them if you'd be willing, and we could trouble shoot your volume-tone pedal. I live in east Denver and am retired so my schedule is flexible.

[Dave Grafe]

I have one but haven't used it for years. It still works but I am not a fan of the 120vac scheme.

[Erv Niehaus]

I had a Goodrich pedal at one time that was 120 volts.
I sent it to Goodrich for some work to be done on it and they wouldn't send it back.
They were getting complaints that sometimes on a wet stage steelers were getting shocks.
Instead, they gave me credit on a new pedal.

[Pete McAvity]

Heya, Matt. I have one (I believe). It’s a photoreceptor vol pedal that looks like an Edwards but does not have a water slide or emblem identifying it as such. I got it from a local guitarist who received it in a package trade. Mine has a Speedy West Custom badge, meaning either it came new with a Marlen Speedy West model steel, or someone got hold of one of those emblems & slapped it on. I didn’t even know it was a light driven pedal till I got it- I was after a cool looking old pedal & I’m a Speedy fan.

It’s in a pile of equipment as I just moved houses. I’ll try to get to it in coming weeks to tell you what’s in mine. Remind me if I flake. High probability that I’ll flake.

[Jim Palenscar]

Thanks Matt for posting this. I had a stereo version of Goodrich's light beam(pre-LDR) that did suck the tone but never really had any problems w it. I have a mono one at the shop that has not been messed with but it would be cool to modify the tone sucking part of it so looking forward to seeing what you come up with .

[Matt Kennan]

Thanks for the replies. A quick correction: In measuring the resistance, I wrote "plug in a guitar", which should have been "plug in a guitar cord." I'm guessing that was understood. You then measure the resistance between the tip and sheath.

[Matt Kennan]

Dave Grafe and Erv, I agree the 120V is not a great design, especially as it's done in the Edwards pedal - without a fuse. My opinion about why people might have been getting shocked was because a polarity difference between one's amp and a mic or other piece of equipment can get you shocked regardless of whether there's an Edwards or Goodrich pedal in the signal chain or not, so I was speculating that it was less likely the pedal was a factor. If the 120V came into contact with the case, for instance due to a worn power cord where it passes through the case, it could certainly be the cause of the shock.