The UL40-S2 Stereo Valve Audio-Amplifier DIY Kit


The UL40-S2 valve amplifier is born out of love for music, for music reproduction with all details and excellent spatial image. The amplifier is based on many years of experience and study of the designer. The newest techniques and knowledge are used in this design. Here we mention some details.

The toroidal output transformer is designed according the most modern techniques. Superior materials, insulations, kinds of copper magnet wire and core compositions are used (the ultimate in toroidal transformers with superior silver/gold wire is now also available at extra cost). All this results not only in an extreme wide frequency range, but also in excellent reproduction of details. The valves are carefully selected by computerized measuring equipment and after that measured by hand. The electrical circuit excels by simplicity, realizing minimal strain of the signal by means of well chosen components. There is no use at all of negative feedback from the output to the input. The concept is amplification straight forward. The printed circuit board has an extraordinary structure called “LEP = Logistic Earth Patterns”. This means that the layout of the ground tracks is such that the degradation of the audio signals is prevented. Even the mains ground is connected in a special way to a central point. The components like resistors and capacitors are selected with extreme care and they all contribute to an open and detailed sound image. The amplifier can be operated in several modes (Triode, Ultra-Linear and Pentode, also read the newest Super-Triode modification as published on this site) making it possible to experiment with famous valve configurations. In the power supply fast electrolytic capacitors are being used, leading to a clear and clean high frequency conduct of the amplifier. Even for the filaments high frequency decoupling is being used, making the PCB very poor in radiation.

The amplifier is available as a DIY kit. The construction manual is very detailed (36 pages) and many figures help to create an optimal product. The several components are soldered step by step in functional blocs on the PCB. See the figure below, where the blue components are already placed on the PCB and the yellow ones have to be soldered now.

  The next figure from the manual shows how to connect the output transformers.
  The picture shows the finished product. For this picture I used the power amplifier version (the UL40-S2P) of the integrated amplifier UL40-S2. The volume potentiometer and the input selector are not used in this case. The gray front of the amplifier is adapted as well, now only containing two switches and two LED’s. The picture also shows a very smart way how to modify the amplifier into the Super-Triode mode. Under every power valve the extra capacitor (10nF/1000V) and resistor (2M2) are combined in series and soldered directly between pins 3 and 5. This UL40-S2P preludes on the new preamplifier MCML05, also designed by Menno van der Veen. Amplimo is preparing this all valve preamp with line and MC/MM inputs to be released as a kit in 2006.

The next photo shows very rare power tubes. Normally EL34 valves are used, but also the KT88 or 6550 are very well suited. Changing power tubes is possible with minor changes in the electronics. In this case the famous original KT66 valves of General Electric Company are applied. These tubes are my personal collector items and not for sale. The input and phase splitter tube can easily be changed as well. Standard the 6N1P is used, but replacing this tube by ECC88 or E88CC-01 or equivalents goes without any problems. The design of the amplifier is very robust and can easily handle all kinds and brands of tubes; the minimal demand is pin compatibility. Recently we have brand new Vanderveen toroidal output transformer designs in stock. These are the gold doted silver magnet wire toroidal transformers. The impact of silver magnet wire in an OPT is tremendous, showing many more details in the soundstage. We have two versions: the VDV-6040SILpp with a silver wire secondary, while the VDV-6040SIL2pp has silver wire primary and secondary. Look for more details at

The output power of the UL40-S2 is two times 30 Watt in de UL-mode. In Triode mode two times 15 Watt is available, while in Super-Triode mode the full 30 Watt per channel is available again. The –3dB frequency range starts at 8 Hz up to 80 kHz without any negative feedback! This is indeed an extreme wide frequency range for a non feedback tube amplifier. The major cause is found in the superior toroidal output transformers. Also the short signal path with minimum components contributes to this clean and transparent behavior. The damping factor lays between 2 and 4, depending on the amplifiers mode. The UL40-S2 is intended for magneto dynamic speakers with an efficiency close to 90dB/W,m or better. The amplifier is not very well suited for electrostatic loudspeakers, because these mostly need a larger damping factor.

Amplimo bv is the main dealer of the UL40-S2 for Europe. At their site you will find all the information about prices and how to order. An experienced do it yourselfer needs about 8 hours to complete the kit. For less experienced builders the manual guides the construction step by step. In that case the building time will give you a construction pleasure of at least 12 hours. The amplifier can be ordered ready built, ask Amplimo or Vanderveen for extra price details. Both Amplimo and Vanderveen deliver service in their labs to ensure that the kit will function properly.

The UL40-S2 is not the final stage of your listening enjoyment. It is the starting point of a discovery tour through audio land. The amplifier invites you any time to new activities, whether it is listening to a new CD or to try new valves or to try another mode. Every time you will be amazed by the many possibilities and beauty of this amplifier.

Zwolle: 11-10-05
Ir. Menno van der Veen
Ir. bureau Vanderveen bv