Tube Compressor

[twiggybush]

Well, I have not messed with this circuit in many many years so I forget the specifics. It's super simple. The light bulb is the tricky part. Basically the louder the amp, the brighter the bulb, which opens the LDR, shunting the input to ground. To get a taste, put a LDR across the sleeve of a guitar cable. Use your hand to cover it from the light then back off. The trick with the bulb is to find a sweet spot. Suppose your amps on 5 and you've used a potentiometer connected to the bulb to find a sweet spot. If you turn your amp up or down, you've lost the sweet spot. I've not messed with this in years, but if I was, I'd look for a flashlight bulb and put a resistor on it. Perhaps a 1 meg pot. Just a guess, but a 10K resistor feeding a 10K pot gives you line level off a speaker. Find a flashlight bulb. Another source of parts is a 120V night light. It has a neon bulb and the LDR. I'm not sure if the speaker can kick a neon, maybe. Another obstacle is creating the LDR/bulb package sealed from out side light. For kicks I suppose black electrical tape will get ya goin. In the old days I used 35mm plastic film cans. For as simple as this circuit is, it is no less legitimate than any commercial LDR compressor using the same principle. To answer your Fender opto coupler question, a bulb and LDR package is an opto coupler. Commercial versions exist for easy pcb integration. The bulb is in paralell to the speaker. The voltage it taps is so small it will not affect sound. Taken to the limits a pot could be mounted on the amp. It works best for clean compression. Once an amp is driven to distortion it is compressed and has less of a voltage swing so the circuit becomes useless. So that's the long and short of it as I recall. It's a fun and super easy experiment. It probably never became popular because people wanted a compressed, distorted guitar sound which made compressor pedals more practical. Compared to an La2a, it does have iron and tubes. After this experiment consider a volume pedal. How about a stereo tremolo with a tiny motor driving a cardboard strobe between 2 LDRs. Have fun!

[Austin]

Well, I have not messed with this circuit in many many years so I forget the specifics. It's super simple. The light bulb is the tricky part. Basically the louder the amp, the brighter the bulb, which opens the LDR, shunting the input to ground. To get a taste, put a LDR across the sleeve of a guitar cable. Use your hand to cover it from the light then back off. The trick with the bulb is to find a sweet spot. Suppose your amps on 5 and you've used a potentiometer connected to the bulb to find a sweet spot. If you turn your amp up or down, you've lost the sweet spot. I've not messed with this in years, but if I was, I'd look for a flashlight bulb and put a resistor on it. Perhaps a 1 meg pot. Just a guess, but a 10K resistor feeding a 10K pot gives you line level off a speaker. Find a flashlight bulb. Another source of parts is a 120V night light. It has a neon bulb and the LDR. I'm not sure if the speaker can kick a neon, maybe. Another obstacle is creating the LDR/bulb package sealed from out side light. For kicks I suppose black electrical tape will get ya goin. In the old days I used 35mm plastic film cans. For as simple as this circuit is, it is no less legitimate than any commercial LDR compressor using the same principle. To answer your Fender opto coupler question, a bulb and LDR package is an opto coupler. Commercial versions exist for easy pcb integration. The bulb is in paralell to the speaker. The voltage it taps is so small it will not affect sound. Taken to the limits a pot could be mounted on the amp. It works best for clean compression. Once an amp is driven to distortion it is compressed and has less of a voltage swing so the circuit becomes useless. So that's the long and short of it as I recall. It's a fun and super easy experiment. It probably never became popular because people wanted a compressed, distorted guitar sound which made compressor pedals more practical. Compared to an La2a, it does have iron and tubes. After this experiment consider a volume pedal. How about a stereo tremolo with a tiny motor driving a cardboard strobe between 2 LDRs. Have fun!

So would the neon bulb sound different than the flashlight bulb? Would an LED work?

[exclamationmark]

People use LED's all the time in commercial opto-compressors, well at least the photocoupled sort you find for channel switching amps. They'd definitely all give you different results, mainly because they require 3 different ways of driving them! (and their luminous intensity vs voltage/current curves being all different, as well as having different time constants). Ignoring this though, I'd think the LDR is the limiting factor in an optical compressor, as it has a sloooooow response time (though people seem to love the sound of the things). I've used things like photodiodes to get really fast attack/release times with LED's.

[twiggybush]

Austin, a Led is dc. The speaker is ac. I've never tried a neon. I just mentioned it as there is one inside a night light. I have to laugh because back in the day LDR's were critisized for slow response. Today they are as popular as ever. Actually the slow response can be advantageous to letting the initial attack pass. Lowell was looking for a clean tube sound. This is such an old school approach I thought it needed to be mentioned. Photodiodes require dc and support parts. Might as well buy a pedal if going that route. For anyone wanting to learn more on this subject, I'd suggest looking up Craig Andertons 'Electronics Projects fo Musicians'. It's from the 70's and has a easily built compressor surrounding 741 type chips.

[Enzo]

Neon needs a lot of voltage to fire, your speaker would have to have that much across it for neon to even try to work. Neon is not linear, it is dark until the threshold is met, then it comes on. From that point, increasing the voltage through the tube can increase brightness a little, but nothing at all like controlling an incandescent or LED.

LEDs run on DC, and typically do not like much reverse voltage, but simply adding a regular diode in series will block reverse, and then the LED can respond to the AC of the speaker. Incandescents care not about polarity or AC/DC.

[Mickey]

Simple Compression Circuit

Well, I have not messed with this circuit in many many years so I forget the specifics. It's super simple. The light bulb is the tricky part. Basically the louder the amp, the brighter the bulb, which opens the LDR, shunting the input to ground. To get a taste, put a LDR across the sleeve of a guitar cable. Use your hand to cover it from the light then back off. The trick with the bulb is to find a sweet spot. Suppose your amps on 5 and you've used a potentiometer connected to the bulb to find a sweet spot. If you turn your amp up or down, you've lost the sweet spot. I've not messed with this in years, but if I was, I'd look for a flashlight bulb and put a resistor on it. Perhaps a 1 meg pot. Just a guess, but a 10K resistor feeding a 10K pot gives you line level off a speaker. Find a flashlight bulb. Another source of parts is a 120V night light. It has a neon bulb and the LDR. I'm not sure if the speaker can kick a neon, maybe. Another obstacle is creating the LDR/bulb package sealed from out side light. For kicks I suppose black electrical tape will get ya goin. In the old days I used 35mm plastic film cans. For as simple as this circuit is, it is no less legitimate than any commercial LDR compressor using the same principle. To answer your Fender opto coupler question, a bulb and LDR package is an opto coupler. Commercial versions exist for easy pcb integration. The bulb is in paralell to the speaker. The voltage it taps is so small it will not affect sound. Taken to the limits a pot could be mounted on the amp. It works best for clean compression. Once an amp is driven to distortion it is compressed and has less of a voltage swing so the circuit becomes useless. So that's the long and short of it as I recall. It's a fun and super easy experiment. It probably never became popular because people wanted a compressed, distorted guitar sound which made compressor pedals more practical. Compared to an La2a, it does have iron and tubes. After this experiment consider a volume pedal. How about a stereo tremolo with a tiny motor driving a cardboard strobe between 2 LDRs. Have fun!

You're testing my memory but in the 60s I used to fit a 6.3v dial light connected in series with a 50 ohm 5 watt wire wound potentiometer to the voice coil winding of numerous Fender Bandmaster amplifiers (all the rage back then). This light was placed next to a Philips LDR (tightly wrapped together with insulation tape) and connected across the input grid of either the first or second amplifying valve/tube.
I recall that the compression was rather severe if the potentiometer was turned way up but back then most guitarists thought this was a very cool low cost modification. The songs that were often played by bands at that time and that made use of this compression was invariably by the UK group The Kinks (You Really Got Me) but I also remember one guitarist who used this effect to play tunes recorded by another UK group The Shadows a mainly instrumental group.
It's been a while since you posted this thread but it just came to my notice. Cheers, Mickey

[d95err]

How about a stereo tremolo with a tiny motor driving a cardboard strobe between 2 LDRs.

Not stereo, but at least it is programmable:
https://www.youtube.com/watch?v=ojtroKSb3IA

Although, electro-mechanical is even cooler:
https://www.youtube.com/watch?v=lNJzm7dwf9w