pdf version - Menno van der Veen

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I always used the UL40-S2 in its Ultra-Linear mode, because I needed the large output power of 29 Watt for my loudspeakers. In the Triode mode the sound quality was better, but the output power went down to a mere 15 Watt, which was too little in my applications. What I really would like to have is a smart invention that would offer me 29 Watt Ultra-Linear output power with Triode mode quality. But we all know that inventions only come when their time is right. Meanwhile I was busy in my lab with a new research about a large general system, that contains all possible topologies and concepts of valve amplifiers. The system not only covers Single-Ended and Push-Pull amplifiers, but also Paralleled-Push-Pull plus balanced circuits like the Circlotron and Unity-Coupled plus many other smart vacuum tube amplifier inventions. This new approach clearly showed that the different valve amp topologies all use a certain amount of local negative feedback around the output transformer and the power tubes. Following I tested many topologies in a universal amplifier (almost equal to the UL40-S2). These tests clearly taught me that there are two winners: the pure triode circuit (SE or PP, both are excellent) and the Super-Triode circuit which I had invented and published in 1998. Comparing these two winners, it became clear that their sound quality was equal, but the output power of the Super-Triode was twice the power of the Triode circuit. Knowing this, I was close to the new invention which I will discuss. The only issue I had to solve was the implementation of the Super-Triode concept into the original UL40-S2. From Triode to Super-Triode: The major difference between Triode and Super-Triode circuitry is that the Triode only uses one control grid, while the Super-Triode uses a control grid and a screen grid connected to the Ultra- Linear taps on the primary of the output transformer. See the figures 1 and 2 for more information. input 1 3 Vht inverse input 5 Fig.1 : Basic Triode circuit. Copyright 2004 Van<strong>der</strong>veen
1 input 2 3 Vht 4 inverse input 5 Fig. 2 : Basic Super-Triode circuit. Copyright 2004 Van<strong>der</strong>veen The standard output transformer (VDV6040PP or PAT4002) inside the UL40-S2 does have Ultra- Linear taps (taps 2 and 4) on the primary. However, this output transformer does not have a separate cathode feedback winding. Would this mean that the Super-Triode circuit is not possible in the UL40-S2? By experience I know that the words “not possible” ask for closer attention, and therefore I focused on finding an alternative solution. Fortunately my “new system” showed me the way how to solve this problem, and a new invention was the result. I now will discuss this new modification that will change the UL40-S2 into a Super-Triode amplifier. The price of this modification is little, only a few Euros, while the net result gives a tremendous improvement in sound quality. Background of the modification: When we convert the UL40-S2 amplifier into a Triode amplifier, we connect the screen grid to the anode in each EL34 pentode. This is how a pentode is converted into a triode. This simple action delivers two results. Firstly, the lines in the Ia-Vak-Vgk characteristics get closer toward each others and move to the right side, resulting in a smaller maximum output power. Secondly, the lines in the characteristics get a steeper slope. This means that the effective plate resistance of each EL34 has become smaller, resulting in a larger and better damping of the loudspeaker (higher damping factor). How can a change in screen grid connection have such a clear influence? Because the screen grid functions like a weak control grid on which a certain amount of local feedback is applied. The screen grid receives as a local feedback signal the same alternating voltage as present on the anode. However, when we connect the screen gird to the Ultra-Linear tap (tap 2 or 4) on the output transformer, then only 1/3 of the alternating voltage at the anode is sent to this screen grid as a local feedback signal. This is because the UL-tap is placed on 1/3 of each primary halve. Compared to the Triode situation we now have less (1/3 part) local feedback. The lines in the Ia-Vak-Vgk characteristics do hardly move to the right side of these characteristics, which means that the output power stays large. Their slope is a little less steep as compared to triode, but much steeper as compared to pentode, resulting in a reasonable amount (although less as compared to triode) of damping of the loudspeaker. Compared to Ultra-Linear, the essence of the Super-Triode circuitry is that I apply extra local feedback at the cathode. I connect each cathode to a separate winding on the output transformer. This gives me the extra local feedback to make the power tube function as a Triode, while the output power stays large (the position of the lines in the Ia-Vak-Vgk characteristics is only determined by the local feedback on the screen grids and not by the local feedback on the cathode).
Page 1: Modify the UL40-S2 into a Super-Tri
Page 5 and 6: How to implement this modification?
Page 7 and 8: With this trim pot P5 I could compe
Page 9: Remark (Nov-28-2004): A customer to

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