Morley volume pedal: how do you replace the bulb with a LED?

[Carlos]

Hi everybody!

I've got a nice chromium Morley volume pedal. It's mains powered. I would like to power it with 9V DC that I use for the other effects.
That means I will have to replace the bulb with one or two LEDs.

Has anyone done this conversion?
Which LED color works best with the Morley LDR?

Regards

Carlos

[Carlos]

Anyone?

[Peter Snowberg]

Which model?

http://www.morleypedals.com/downloads.html

The Vactrol CdS units use a yellow LED and the only info I could find quickly says they respond best to frequencies between 300uM and 400uM.

You just need a source of light in there that is able to get "bright enough". A high brightness yellow LED and a proper resistor for the supply voltage should be all you need. I have a Morley Power-Wah which uses a fairly high DC voltage. The bulb in that thing is wired in series with a small resistor.

[Carlos]

Hi Peter!

Thanks a lot for your answer. Here are the schematics for my pedal:

http://www.morley-pedals.de/tpl/default/media/manuals/04_manuals/03_schaltplaene_alt/vol.pdf

The bulb is a 387 bulb running at 28 volts!

regards

Carlos

[Peter Snowberg]

Great. 

With the schematic now we can see that the power is only there to light the bulb and there are no buffers to worry about. 

You can just add a DC jack with a high brightness LED and resistor and you're good to go. You could even put the LED next to the lamp and have the option of using either power source, but 9VDC sure is more convenient and it gets rid of a possible hum generator. Nice move!

Grab your meter, set it to ohms, plug the pedal in and measure the resistance between the signal connections of the in and out jacks. You need enough light from the LED to match that value.

For 9VDC, try a 1K resistor to start and see what LDR resistance that results in. You can drop the 1K to about 330 ohms at the minimum value. If you need more light than that to match the original resistance then you'll need to use a couple of LEDs.

Be aware that the light pattern from the LED and bulb are quite different. You may want to rough up the LEDs with some fine grit sandpaper to average things out a little.

[Carlos]

Thanks Peter!

I still wonder what color the LED should have. Or: for which wavelength/color is the Morles LDR most sensitive?

Regards

Carlos

[Peter Snowberg]

I Googled for "cadmium spectral response curve" and got this:



...from here: http://repairfaq.ece.drexel.edu/sam/CORD/leot/module4/module4.html

From that graph, it looks like anything from IR to green will work fine. Bright red LEDS are abundant and cheap. Give red a try and see if you can get a lower resistance than the original bulb gave. Some detectors need a specially matched light source but this looks like a case of "close enough for rock and roll".

[Mark Hammer]

When a buddy's Morley wah lost the use of its bulb, I contacted the company on his behalf to inquire about how to buy a replacement.  The fellow I spoke to said "No problem" and at his insistence shipped me two, no charge.  Whether he still works there, and whether that i company policy and whether they still have any to be so generous with, I don't know.   But if it happened once, it can happen twice.  Try them.

[Carlos]

Thanks Peter and Mark,

you see I've got some spare 387 bulbs, but the point is they can fail and there is an AC cord running to my stage board. So I'd like to power the volume pedal with 9V DC I use for my effects.

Carlos

[Peter Snowberg]

High brightness white LEDs are another one to try.