The folks at Bottlehead produce a constant current source/sink kit which they have dubbed the C4S (scroll down). It is inexpensive and relatively simple to use. However, while the booklet contains pretty much all of the information one needs to use the kit, this information is a bit disorganized, and at times amazingly obtuse -- this is particularly the case if this is one's first use of a CCS circuit. This is not intended to be a harsh criticism. The Bottlehead booklet spends a great deal of time discussing theory and explaining how the circuit works which is quite valuable. However, because the practical information is buried within the theory, it does make actually using the circuit a bit cumbersome. So, this page is intended to offer a simplified set of instructions to supplement the Bottlehead instructions.

Just a note: this page is not intended to substitute for the purchase of the Bottlehead kit, and as such, it won't show the actual schematic or any other propriatary information. The kit is inexpensive, and while the parts can be duplicated for about $10 at Digikey, it is worth purchasing for the background, the schematic, and some of the other information contained within. However, one can certainly build a CCS with information from the web, and if one is wanting to do that without the purchase of a kit, then the tradeoff is a little research. You should probably start with Gary Pimm's pages and go from there.

Some Background
In the simplest sense, vacuum tubes output current from their plate. To turn them into voltage amplifiers, one much convert that current into a voltage. The usual method for doing this is by adding a plate load resistor. Since Voltage equals Current times Resistance (V=IR), by adding this resistor (Fig. 1), the current is converted into a voltage. One can represent this conversion as a graph of current vs. voltage.

Fig. 1

It is usually drawn on a graph of a tube's loadlines (Fig. 2) as this is a tool for estimating a tube's performance (this is not strictly true here as stated -- instead, the input impedence of the device that the signal is going into -- usually an output transformer -- governs the slope of the load line. For a decent explanation, take a look at this page For more indepth background starting at a much simpler level, see the NEETS manual..)

Fig. 2

Instead of using a resistor, however, one can also use a what is known as a constant current source. As the name implies, a constant current source is essentially a circuit that maintains a constant current across it by letting the voltage vary. By loading the plate of a vacuum tube with a CCS, one can essentially convert the varying current from a tube to a voltage more efficiently -- you can also think of this as adding a near infinite resistance as the next stage for the tube.)

Fig. 3

That is, since V=IR, by holding both I and R constant (rather than just R with a resistive plate load), only the voltage can change. If one draws the loadline from a CCS loaded tube on a graph, the line is (ideally) horizontal (see Fig. 2). This matters primarily because tubes tend to be more linear here which means there is less distortion, and (in theory) better sound.

Picking Parts
The real trick to using the Bottlehead kit is in choosing what additional parts one needs. This comes down to choosing a couple of resistors, determining the amount of heat they will dissipate, the amount of voltage they will be subjected to, as well as determining whether one must place heatsinks on the transistors. The following table summarizes the decision making process.

Part ID	Value	Heat Dissipation
R1	Choosing R1 is done by simply reading the value off a chart included in the kit. You will need the kit in order to do this.	The kit indicates that a .25W resistor is sufficient here for currents up to 100mA.
R2	R2 sets the bias of the LEDs. To do this, simply divide the B+ voltage by the desired bias current to get the resistance. Traditionally, for most cases, this desired bias current is .002A. So, for example, if the B+ is 300V, you will need a 150K resistance (300V/.002A = 150000R). The only difficulty is that since the B+ will often be high and since most resistors break down at high voltages, you should probably string a couple of resistors in series to get the desired resistance and relieve the stress on the resistors, i.e., use 3 50K resistors rather than one 150K resistor.	Here, just multiply the B+ by the bias current (e.g. 300V*.002A = 0.6W) to get the amount of heat the resistor needs to dissipate. You should use a resistor that can dissipate about double this required rating. So, for the example, use a 2W resistor (or more to the point, use 3 equal resistors each of over .5 Watts.)
Q1	This part is supplied with the kit.	The challenge here is to determine whether or not to use a heatsink. The kit offers no real information which suggests that a sink is not necessary. The part is in the TO-18 form factor, however, so a heatsink can be used.
Q2	This part is supplied with the kit.	The combination of the current set by the CCS and the grid bias will determine the tube's plate voltage. The CCS needs to have a B+ that is at least 30-40V higher than the aggregate of these two voltages (this is to allow for adequate swing in the tube, and to not stress the CCS at it's extremes. This example is for a linestage, so if you are using a CCS for a power amp (or perhaps a phonostage with high gain) your voltage differential may be higher.) So, for instance, looking at Fig. 2, if one were to choose a current of 40mA and a grid bias of -40V, reading down on the graph this equates to a plate voltage of approximatly 95V. Thus, the B+ needs to be at least 30V to 40V higher than 95V + 40V, or approximately 175V. To determine whether you need a heatsink for Q2, multiply the amount that the B+ exceeds the plate plus grid bias (this is how much voltage Q2 must drop) by the current. Here, this is 40V * .04A = 1.6W. The guide suggests a heatsink for anything over over half a watt. Q2 is a SOT-32 transistor which should fit on many TO-218 or TO-220 sinks.

A Note on LEDs
The instruction manual goes to great lengths to instill fear in the end user that the LEDs used are some sort of magic HP parts with special curves. Indeed, there is a warning that any other LEDs will require an oscilloscope as there is a danger of oscillation. However, this post, from Doc B himself, indicates that not only will pretty much any Red LED do for the circuit, but that the folks at Bottlehead don't even know what the magical part number is.

The late breaking news from Doc B is that they do indeed now know what the magic LEDs are, and that they do seem to work better with regards to circuit stability. However, Doc B did not seem to think that there would be any effect on the sound.

A Note on LED Bias
According to Paul at Bottlehead, while the instructions concentrate on bias currents of .001 and .002A, for higher plate currents it might be better to use a bias current of 10% of the plate current. So, if you look in Figure 2 you'll notice that for this load line, I am using a plate current of 40mA (or .04A) (this curve is for the 6080/6AS7-G b/t/w/). Thus, here it is actually ideal to use an LED bias of .004A.

So, looking in the example where we found that a B+ of 175V is ideal, this equates to a resistor of 175V/.004A = 43.75K. While you can source a 43K7 resistor, the B+ is high enough that one would do best with a 23K7 and a 20K in series. The wattage rating for Q2's heatsink stays the same.