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LED as current source/sink

4/20/1999 7:01 PM
Dave Stork
LED as current source/sink
It's funny that this topic has come up. A preamp I designed uses a low current type LED in place of a cathode resistor on the third stage. This acts as a current source (or sink, depending on which terminology you prefer) as well as holding the cathode at a relatively fixed voltage. I didn't detect a big change in timbral balance versus using a regular cathode resistor and bypass cap, but use of the LED gives an interesting compression effect that I like, evening out the volume difference between clean and distorted tones.  
Use of diode-type current sources in cathode circuits is not a new idea. These circuits are well known in hi-fi but are less understood in the guitar amp world since one often assumes when one sees a diode that it's being used as a clipper. Even our friends over at Mesa experimented with constant current diodes (the TCR508 in particular) for a period of time.
4/21/1999 10:30 PM
Steve A.

Even our friends over at Mesa experimented with constant current diodes (the TCR508 in particular)...  
    Can you fill us in on the TCR508's? I always wondered what that funny symbol was that looks like an Egyptian ankh...  
    In previous threads, we figured that the TCR530x was mainly used to properly bias the optional TR1009 FETRON in the Mark II. Is there any way that we could "cook up" a TCR5xxx from stock components available from a vendor such as Mouser? Or just use the low current LED as you suggest?  
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4/22/1999 7:59 AM

LEDs are not constant current devices, they are closer to a voltage source with some small resistance in series.  
Biasing with LED's is a form of constant voltage biasing.  
Constant current diodes are just JFETs with the gate tied to the source. The current is Idss for the JFET. They're sorted to get the values all the same, or close. Some CC diodes are JFETs with an adjustable source resistors, laser trimmed down from Idss.  
You can make your own CC diodes from JFETs, just picking one with an Idss larger than you need, and using a resistor gate to source to set the current down from that. Use a pot to dial in the current, then replace it with a fixed resistor.
4/22/1999 11:22 PM
Dave Stork
Re: LED as current source/sink (Long)
Can you fill us in on the TCR508's? I always wondered what that funny  
symbol was that looks like an Egyptian ankh...
The TCR508 is just a "constant current" diode, which is a type of JFET. The symbol you mention is just a generic symbol for a current source, much like a sine wave inside a circle represents a signal source or generator.  
Is there any way  
that we could "cook up" a TCR5xxx from stock components available from a  
vendor such as Mouser? Or just use the low current LED as you suggest?
First of all, I should point out that I was being a little fast and loose with terminology in the previous post. An LED actually approximates a voltage source. It will draw whatever current is necessary to maintain a constant voltage across its terminals. (For an LED, this is usually 1.5 to 2 volts). When you place a resistance in series with a voltage source to limit the current in the circuit to a certain value, the resistor is acting as a current source. In the case of putting an LED in series with a cathode, the resistance consists of the plate resistor and the plate resistance of the tube itself. The LED is in fact providing fixed bias by holding the cathode voltage at about 2 volts. The whole assembly (plate resistor, tube and LED) approximates a current source, albeit imperfectly, although a little closer to the ideal than when using a cathode resistor.  
By the way, you don't need to use the low-current type LEDs if you don't care whether or not it lights up :-) A regular LED will work just as well; it just won't light up very brightly on 1 or 2mA. This doesn't matter if you're not planning to use the LEDs as a visual effect :-) Since the LED is not a perfect voltage source--it's more like a voltage source with a small series resistance--the voltage across its terminals will drop slightly as you vary the current. You can smooth this out by putting a cap across the LED. I used a 25uF because I had one handy. With the cap, the bias stayed pretty solid no matter how hard I drove the tube.  
There are a few ways to build a simple constant current source. (Or, for those who prefer to think in terms of current flow from + to -, a current sink). The easiest is to take a JFET and tie its gate to source, then connect the drain to the tube cathode and the source to common (ground). Current draw will vary from JFET to JFET but is usually on the order of a milliamp or two for typical general-purpose JFETS like you might buy at Rat Shack. For an adjustable current source, hook the gate to common and put a pot in series between source and common.  
Another method is to use an NPN transistor, collector to cathode, emitter to common, and bias the base with a constant voltage source. This voltage source can be an LED or a couple of silicon diodes in series, cathodes pointing to common, fed by a large resistor from B+. If you do this, be sure to calculate the power that will be dissipated by the resistor and use a part of the appropriate rating.
4/24/1999 9:02 AM
Dave Stork

Incidentally, you can add a diode in series with the LED if 1.5 to 2V bias isn't enough. Or you could put a couple of LEDs in series to double the bias. You could also use rectifier diodes in place of LEDs, but you would have to use more of them to get the same forward voltage drop.
4/22/1999 8:38 AM
Geoff Van Brunt
Re: LED as current source/sink
The Marshall 2210/2205 used a silmilar setup to Traces. A 1N4007 across a 10k cathode resistor. I don't know why it didn't strike me to experiment with this before. I guess it's because I hated the sound my 2210. I guess I'll have to spend hours of swaping and listening, just like I did with resistors and caps... :)  
4/23/1999 6:40 PM

Let me know what you discover!!!!!!  
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