Taper in Goodrich light-beam vol ped.

Paul Brainard · From: Portland OR

I recently picked up a Goodrich model 6402 light-beam volume pedal, which has been a vast improvement over my previous pot model (better tone, full volume range, no scratching!) There is something I am curious about, and I wonder if anyone can shed some light on it for me (no pun intended. . .)Poking around inside it, I found that the amount of light that reaches the photo cell is controlled by a squarish plastic plate which is blacked out except for a sort of v-shaped pattern. This apparantly lets more light through as the pedal movement slides it up & down the face of the photo cell. This makes enough sense to me, but I have a few questions:

1. Are there other plates available from Goodrich or elsewhere with different tapers?

2. There is what seems to be Scotch tape on both faces of the plate, and it is slightly peeling off on one side - should I be concerned about this & replace it? Will it affect the performance of the pedal? What purpose does it serve, perhaps just to protect the black coating from wearing off?

Thanks to anyone that can answer my questions!

Bill Crook · From: Goodlettsville, TN , Spending my kid's inheritance

1) No, The photo-cell V/C isn't like a phsyicaly like a pot V/C. The V shaped pattern of the plate suggest a lenier control ( which INMHO) is the better control.
Changing the shape of the opining of the plate will only turn on the sound quicker or slower. It will NOT do anything for the taper of the control.

2)I wouldn't be too concered about the peeling, unless it starts to cause mechanical problems with the pedal. It shouldn't cause a problem with the operation of the pedal.

The make-up of the plate seems to be a 3 part thing,samwiched together,

1) Scotch-tape like plastic,
2) Black material,
3) Scotch-tape like plastic.

Other than the larger physical size of the photo-cell pedals, I think they do a far better job than the "POT" type controls, depending on how they are wired up, they do not have a tendency to changge the tone as the volumne is decreased.

Just my opinion....

Earnest Bovine · From: Los Angeles CA USA

I always thought that turning the sound on quicker or slower was the same thing as changing the taper.

Lee Baucum · From: McAllen, Texas (Extreme South) The Final Frontier

EB, I was going to say the same thing.

I have an old Goodrich Model 400 photocell pedal that I bought new back in the mid 1970's. It has the same arrangement as you guys described. This pedal has two light bulbs. I put a dot of superglue under the corners of the "scotch tape". I believe the tone of my Mullen is much crisper and more defined using this pedal vs using my Goodrich Model 120 pot pedal. I use a Hilton Digital Sustain unit between the guitar and volume pedal. Believe it or not, the Model 400 still has the orginal light bulbs in it. I bought a couple of new bulbs about 2 years after I bought the pedal, just in case they ever burned out. The new bulbs are still waiting in my pac-a-seat.

Lee, from South Texas

Paul Brainard · From: Portland OR

Yes, changing the volume taper is what I'm after. . .I'd like to make it a little more gradual (seems like most of the swell is in the last 20% of the pedal's range.) I guess I could probably fashion something with a piece of plexiglass and a sharpie. . .

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Here is some information that might help you build a filter you like. I have never seen a Goodrich light beam pedal. From descriptions, the light control sounds similar to other pedals of that era. The light control sounds like that used by the Edwards Pedal. Don Edwards had a music store in Denver, Colorado, and was ahead of his time when it came to a lot of things. He made some interesting pedals. I took one apart and studied everything about it. The filter was made from some brown acrylic plastic. A "V" shape was cut using a "rat" tail file. The deeper the cut with the rat tail file,the more light could come through.
Earnest Bovine said exactly what I was thinking! A "V" shape can be expressed as a mathematical equation on a X-Y axis. That is how linear, audio, logrithmic and other tapers are described in "all" books. The use of Cds cells, or photo-resitors is a quality way of controlling audio sound. Hope this helps!

[This message was edited by Keith Hilton on 16 January 2001 at 09:39 PM.]

Bill Crook · From: Goodlettsville, TN , Spending my kid's inheritance

Terry Downs · From: Wylie, TX US

I have never experimented with a light pedal. Can anyone tell me how they are wired. It seems that it would be difficult to make a passive unit considering that the CdS photocell is a two terminal bilaterial network. If you wire it in a shunt configuration, you reduce the impedance the pickup sees thus reducing high frequencies. If you wire it in a series configuration you get high frequency loss by the cable capacitance. You also raise the suseptability to noise by making the loop impedance very high. I realize that a fixed resistor is probably used as the substitute part of the T pad. Does the Goodrich model use drivers and buffers to overcome this? Or does the photocell control the gain of an amplifier in a feedback loop?

------------------
Terry Downs
http://nightshift.net
terry@nightshift.net

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Terry, the things you mentioned caused loss of high end in many light beam pedals. The Edwards pedal had that problem. I experimented with Cds cells several years ago. The way I got around the problems you mention is as follows: Install a high input impedance OP amp as the first stage. This keeps the pickups from seeing low impedance and having the highs milked off. The OP amp also gives you a stronger output as you can alter your feedback resistor, in the feeback loop, if you use the non inverting method. Then, what I liked is using two Cds cells in the form of a voltage divider. This caused me to build two filters instead of one. Cds cells do great with audio signals. Then there is the "big" problem of stray light getting in. Outside ,or in bright light, many of these pedals would not go all the way off. I solved that by making the operation only work under really intense light. That took some special LEDs. I still have this pedal I built. The electronics are installed in a old Edwards pedal body. It works great. I went further with my experiments and found a much better, and totally different concept. My infrared pedal works like the bar code scanner at the grocery store check out. I hope this information helps. You can build a good pedal using Cds cells.

Paul Brainard · From: Portland OR

This brings up something else I noticed about the filter plate in my Goodrich - in what I guess is the CDS or photo cell, there are actually two little holes, and the filter plate has two separate V-patterns of clear plastic to let light through, but they are inverted with respect to each other. As the pedal is depressed (poor pedal!), one of them opens up and the other one closes down. So maybe this is a voltage-divider like you described above?

Lee Baucum · From: McAllen, Texas (Extreme South) The Final Frontier

Keith - My guitar sounds incredible with my old Goodrich photocell pedal. No loss of highs whatsoever.

Of course, I use a Keith Hilton Digital Sustain unit between the pickup and the pedal!

Lee, from South Texas

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Yes, Paul, it sounds just like what I decribed. If there are two "V"s, inverted to each other,used as a filter, then there is a voltage divider arrangement being used. In this case there will be 2 Cds cells. This is a "much" better arrangement than just shunting signal to ground with one Cds cell. Even with a pre-amp in the front, seemed to me there was a loss of tone when signal was simply shunted to ground with only one Cds cell. Like I said, I have never looked inside a Goodrich pedal. All of the light beam pedals I have ever looked at only used one Cds cell. I still bet the filter is a dark piece of acrylic plastic made by using a rat tail file. The inverted "V"s are made by simply turning the plastic around, and filing from the other direction. To alter the taper, you would have to use a thinner ,or thicker, rat tail file. Acrylic plastic comes with paper on each side of sheets. Many times when they made these filters they did not remove the paper. That may be what you see pealing off. Another common thing they did to these light filters was put dark tape on them to block any light, except in the "V" shape. This also may be what you see pealing off. If you will look close at the "V" shape, you will see that the wide part of the "V" has the deepest cut. This allows the most light through the plastic. The V's are arranged where the once Cds cell lets signal in. Signal then has a choice of going to the line out or to ground, as the other Cds cell determines the amount of signal going to ground. With the V's reversed, as signal is cut off to the line in, the line to ground has less resistance. As signal is increased to the line in, the line to ground has more resistance. By the way, a Cds cell is nothing more than a light sensitive resistor.
It's resistance is changed and determined by the amount of light hitting the cell.

[This message was edited by Keith Hilton on 18 January 2001 at 05:12 PM.]

Forum Index > Electronics		Next oldest topic :: Next newest topic

All times are GMT - 8 Hours	Jump to: