#1
I read that you can put an audio signal through a light bulb for some neat compression/sustain effects. Countless google searches yielded nothing, any thoughts on where to look? I also heard LEDs can be used for compression (IE, not in the same way they can be used as diodes for clipping). Links anyone?
#2
i've heard u can use a light bulb for an attenuator inside a Valve Junior. but i'm not too sure.
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#3
It's really easy. You put the lightbulb just afer and in searies with you rectifier. This can only be done on tube amps and it'll give you compression and sag similar to a rectifier tube. Wire wound resisters will work better though because a wire wound resistor has greater inductance which will make the compression sound more like the natural compression from a tube. If you use a resistor the value should be anywhere from 100 to about 200ohms. When you get much higher than 200 ohms it creates issues with the amps bias. I have a 470 ohm in my amp which meant I had to add a second bias pot so that when I flip the switch it rebiases the amp. This type of mod doesn't just add compression. It lowers the voltage running through the amp giving you a much browner tone so keep that in mind when you mess around with it.
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#4
now pardon my ignorance, but I have a few questions.

1.) The rectifier is just a transformer that gives you a lower DC voltage from the high AC mains line, right? If that's so, how would putting something right after it, before the audio signal, effect the signal?

2.) Ive searched all over, but what is the importance of biasing equipment?

3.) What's a "browner tone"? Just kind of darker? less highs?
#5
Quote by ECistheBest
i've heard u can use a light bulb for an attenuator inside a Valve Junior. but i'm not too sure.
True. This can add a noticeable compression, since the resistance of the light bulb changes dramatically from cold to incandescence.

It's a bit tricky, though, selecting the proper sized light bulb. In the case of a stock VJ combo, this gentleman chose to use a 12 volt, 7 watt bulb with the stock speaker and 8 ohm transformer: http://www.valvejunior.com/

I suspect using a 6 volt, 3 watt bulb in series with an 8 ohm speaker will give even more compression when run on the 16 ohm tap. The bulb will glow much brighter when you near maximum power output. But it might be a bit risky. If you dime the amp and exceed the voltage of the bulb for long and it burns out, tube amps do NOT like the absence of a load on the secondary. The voltage swings on the primary will be great enough that you might punch through the insulation and short the transformer. Or arcing can occur in the output tube.

Quote by flashbandit
now pardon my ignorance, but I have a few questions.

1.) The rectifier is just a transformer that gives you a lower DC voltage from the high AC mains line, right? If that's so, how would putting something right after it, before the audio signal, effect the signal?

2.) Ive searched all over, but what is the importance of biasing equipment?

3.) What's a "browner tone"? Just kind of darker? less highs?
1 - No, a rectifier is like a one-way valve allowing current to flow in only one direction. It's used to get direct current from and alternating current source.

A transformer is an impedance and voltage matching device. It allows you to get the desired voltage out from the voltage applied to the input.


2 - Biasing is the process of setting the voltage on the grid of a tube so that the idle current in the plate is at the proper level. Too little current will cause the output tube(s) to go into cutoff rather than saturation when the signal is large. It allows the tubes to last longer, but sounds "colder" and more "brittle". Too much current will shorten the tube life dramatically. Just there right amount of current will be the best compromise of tube life and a "warm, creamy tone" when the amp is pushed hard.

3 - "Brown" in this context is the starvation for voltage from the power supply when a large resistance is added in series. It stems from the expression "brown out", when municipal power companies can't supply enough voltage to meet consumer demand. It's not a blackout. The lights still come on, but they're dim.

Part of the "brown sound" is the sag that occurs when the voltage droops soon after the initial attack occurs when playing a note. The other part is the rich harmonic distortion as the output is clipped somewhat.
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#6
Quote by flashbandit
now pardon my ignorance, but I have a few questions.

1.) The rectifier is just a transformer that gives you a lower DC voltage from the high AC mains line, right? If that's so, how would putting something right after it, before the audio signal, effect the signal?


a rectifier turns ac into dc.

Think of the wires and resistors in your amp as roads and think of the electrical current in your amp as cars. The louder you play your guitar the more current you need or the more cars are driving on the road. Each time you strumm a note it's like rush hour on the freeway and the decay of the note is like non peak hrs on the freeway. Putting a resistor after the rectifier is like closing a couple lanes on the freeway. During rush hour if you close down have the freeway traffic slows down untill it's almost stopped. As the amount of traffic decreases traffic flow speeds up and things return to normal. When this happens in your amp, it means that when you strum a note too much power tries to go through the resistor and it slows everything down giving you less volume. As the note starts to decay the traffic jam clears up and your volume increases. This delay in power and volume is compression and when people are using rectifiers it's called sag. A solid state rectifier has almost no resistance which means no sag. A tube rectifier has an internal resistance so you get sag naturaly. Using the light bulb or resistor trick is just a good way to immitate the sag you will get from a tube but it's not quite the same because a resistor has a fixed resistance and a tube has variable resistance.

2.) Ive searched all over, but what is the importance of biasing equipment?


Keeping with the car analogys, biasing your amp is like filling your tires with air. If you fill them too full they pop, if they are not full enough they don't work very well. The bias is how much current your tubes draw when they are idle. If they draw too much current they overdrive too easy, burn out faster, or just stop making noise. It can blow up tubes fuses, transformers, and caps if your tubes are drawing too much current. If your tubes are not drawing enough current then they just don't perform very well just like a car doesn't perform well if the tires are not full enough.

There is no such thing as "the right amount" to bias your amp. Lower current means you have more headroom and tighter high end but it sounds colder, higher bias breaks up easier and sound warmer but you get less high end and less headroom.

3.) What's a "browner tone"? Just kind of darker? less highs?
Brown amps are typically thought of as the amps from the early 60's. They ran on lower voltages so they had a warmer tone with floppy bass and rolled off high end plus everything someoneyouknew said.
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Last edited by CorduroyEW at Nov 11, 2007,