Good power supplies are very important for good sound. Tube amplifiers need heater power supplies. Especially when directly heated triodes (DHT's, like 2A3, 300B, 50, 45, VT25, KC1, KC3, LP2, P2, RE404, RS241, RS242, etc) are used, the quality of this power supply is very important. The heater of the tubes is directly in the signal, any harmonics from the power supply will degrade the sound quality. Non-DHT's (normal tubes like ECC83 and E88CC, etc) are much less influenced by this problem, but good quality power supplies still will improve the sound.

When DC voltage is used, power supplies mostly are regulated. Voltage regulators like the LM317, LM337, LT1086, and the 78-series have very limited bandwidth. Most already loose their good properties above 1kHz, in fact there is no regulation anymore at 100 kHz, just where the problems start. They should be used to get rid of hum alone. Hf is almost passed completely, both when they are used as voltage or as current sources. These passing Hf harmonics influence the sound a lot. They make sound harsh, uneasy and reduce image and make the background less dark.

A good solution for reducing Hf is the use of discrete FET current sources. A current source has very high dynamic impedance, so the Hf harmonics will drop over the power supply. A voltage source has very low dynamic impedance, resulting in all Hf components to drop over the tube heater wires. A schematic drawing of this can be found in this picture. Another advantage of discrete FET current sources is that bandwidth is really high.

Voltage dropped over the tube heater wires must be constant and exactly the value of the tube data sheet. When a current source is used, changing the tube or when the tubes gets older might change the heater resistance and thus the voltage over the heater wires. This reduces tube life. It is important to get the right current resulting in the right dc voltage.

The VCCS DIY kit takes care of all the above. For DC (very low frequencies) it adjusts the current so that exactly the right voltage is applied to the tube heater wires. For AC (including all Hf) frequencies, the VCCS acts like a pure current source, with a large bandwidth, and high reduction of AC harmonics. VCCS will improve the sound a lot compared to normal regulators.

One other point is that when applying voltage over cold heaters results in high inrush currents. When the heaters are cold, the resistance is much lower than when they are hot. Sometimes tubes even flash, but most of the time you do not see it, but it is there. The VCCS provides a slow start up, giving the tube time to get to temperature and their real resistance and not to draw high inrush currents, prolonging tube life.

Features:

• Simple and versatile, easy to build and apply mono VCCS boards for a wide range of voltages and currents.
• High damping of ac components (typically about -60 dB) over a very wide bandwidth range, resulting in more easy clean sound and better sound stage.
• Can be used for 0.5V to 12.6V heater voltage outputs, over a very wide range of input voltages (max input 16Vdc, or 12Vac in the AC version, if you have higher voltages, please ask for a special version at the same price).
• Available as stabilisation units for DC input and as complete supply units for AC input (including rectification and smoothing)
• Works at power supply voltages of 2-3.5V above the output heater voltage, depending on current (2V for 0.5A and lower, 3.5V over 1A). 4V drop or higher is recommended. For the VCCSac an estimate of the input voltage is Vac = Vdc output + 4V, so about 9Vac for 5V dc output.
• The normal versions can handle max 3A continously, if the current FET is cooled properly. FET is isolated and can be mounted directly on the amplifier chassis or a heat sink. For higher current, other types of FET's can be used. Typical the fet can handle upto 12W of dissipation, this is the drop over the transistor, times the current. If you are insure about your application, please ask me.
• No more manual adjustments of current to get the right voltage, this circuit does it all for you, continously!
• Slow startup, reducing inrush currents on tube heater wires.

Kit	Price kit	Price fully build
VCCS DC. Single VCCS for dc input, including PCB, FET and all other parts. Heat sink, smoothing capacitors and rectifier not included.	27.50	39.50
VCCS DC. High current upto 8A. Single VCCS for dc input, including PCB, FET and all other parts. Heat sink, smoothing capacitors and rectifier not included.	32.50	44.50
VCCS AC. Single VCCS for ac input, including rectifier and low esr smoothing capacitors, PCB, FET and all other parts. Heat sink not included.	55.00	75.00

Building and using VCCS:

Before you start, read this article about soldering and the disclaimers. If you have questions about the manual, about the kit, about your configuration, or anything else, please do not hesitate to ask us. We will do all we can to help you getting the best out or our DIY kits.

Description VCCS:

1. Mount the parts like you see in the picture. The colour codes of the resistors are noted in the partslist. Connect the plus of the capacitors the plus symbol on the PCB (printed circuit board)
2. Mount the transistors and the IC on the PCB. Take care the shape of the transistors (thick line is the tab side) and the notch on the IC match the shape and notch on the PCB
3. R1 (VCCS DC) and R1, R2 and R8 (VCCS AC) get hot and should be mounted floating off the PCB (see pictures)
4. The FET dissipates a a lot of power in most applications. It needs good cooling. Because we use an isolated type, it can be mounted directly onto the amplifiers chassis or a heat sink. Heatsink always is recommended, chassis cannot dissipate a lot of power, upto 3W it will work depeding on the material. The FET can be mounted at the top or below the PCB, just what is most convinient to you. Using heat conducting pasta is recommended
5. VCCS AC uses an external rectifier. This rectifier can be mounted on the chassis. Connect the ac voltages to the ac pins and the + and - of the rectifier to the + and - of the Vin on the VCCS PCB
6. The tube should be connected to the Vh+ and Vh- outputs of the VCCS PCB
7. On the VCCS AC PCB there also is a GND connection. This only should be used with non directly heated tubes, it balances the VCCS to the ground of the amplifier. For optimal performance the output of the VCCS should only be connected to the amplifier star ground by this pin (heater power supply must be floating fully for the rest). Of course you also can take external measures for balancing or connecting. If not sure, please ask
9. For testing the VCCS, it is recommended to connect a resistor with about equal value as the heater wire of the tube (Rtest = Vheater / Iheater from datasheet) to the output of the VCCS. Apply input voltage. The voltage on the resistor should be the heater voltage. If not adjust with P1, the output voltage should be easy to adjust. Check if the FET does not get too hot. If you are not sure the VCCS is working properly, please do not proceed to the next step. It could result in tube damage. Please ask us your questions if you are not sure, this what we are here for!
10. Now apply the VCCS at the tube pins. The tubes should heat up and the voltage should go up to exactly Vh. Take a close look at the heater wires and the voltage during start up. They should glow orange for normal tubes or white in case of Tungsten heater wires (you know the color). When the orange or white gets too bright, put the VCCS power off immediately. If the colour and voltages are ok, the circuit is working properly
11. Always feel the screw which mounts the fet to the heatsink with your finger after 1 minute and after about two hours of operation. If you can hold it for about 1 second before "burning" your finger it is ok (about 70 degrees). It is a good check if it does not get too hot
12. Fine-adjust the VCCS with P1 after about 4 hours of operation, when the amp is still hot (on). This way the VCCS will be exactly at the right values all the time
13. Tip: An extra 100nF capacitor can be used right at the heater tube pins, to filter out the last Hf noise and noise picked up underway. Sometimes this can causes stability problems (VCCS cannot be adjusted to the right value), it is recommended to try out in your own amplifier.
14. Please note: There have been cases (totally unpredictable) where the VCCS is osscillating (VCCS gets hard to adjust, normally it is very easy), or a 300Hz hum in noticable. It depends on the way a tube is biased, the capacitors round the cathode can cause instability with the normal VCCS setup. One way to solve it is to add a large electrolytic directly on the tube pins, typical 2200uF or more will do. This will solve problems in 99.9% of the cases. The other way is te remove all capacitors after the VCCS (except the biasing and decoupling capacitors of the tube of course), also remove C4 for VCCSdc and C6 for VCCSac from the PCB. If you still keep problems, please let us know.