pdf version - Menno van der Veen

3) Connect an oscillator (1kHz sinus) to the amplifier input. Load the amplifier output with a 4 Ohm
dummy load. Check the output with an oscilloscope. Slowly turn the input potentiometer (volume
control) P1 to higher setting up till the output voltages is at the edge of clipping. Now trim with P3 for
symmetrical clipping. (It is interesting, during this procedure, to listen to the soft sound generated
by the output transformer. It is noticeable that the second harmonics of 1kHz disappears at correct
setting of P3). An earlier given test method for trimming P3 with a 100Hz square wave now will give a
very stable result without any overshoot because the EL34 tubes behave exactly equal.
4) Repeat the above given procedure for both channels of the stereo amplifier. Connect dummy
loads of 4 Ohm to the outputs of the two channels and connect a two channel oscilloscope to the
amplifier outputs. Set the volume potentiometer P1 at 3.00 o’clock. Apply an equal input of 1kHz
sinus to both channels up till the output level at both outputs equals 8 Volt peak to peak. Trim P2
for equal output voltage. With this procedure the left-right balance has been corrected. (Why P1
at 3 o’clock? Because the Alps volume controls have some deviation between its two channels
above 3 o’clock. The mostly used volume setting is below 3 o’clock (I mostly listen at 12 o’clock) , so
we should make the left-right balance optimal in that range of P1).
Summary:
The Super-Triode modification gives the UL40-S2 amp maximum output power of 29 Watt per
channel, with the damping and clarity of a pure superb sounding Triode amplifier. The modification
destroys nothing in the amp. Only a few inexpensive components have to be added or changed in
value. Especially trim pot P5 offers new ways to balance the power tubes of each channel on three
independent parameters. After this balancing the distortion is truly minimal, resulting in an extreme
open and clean and powerful sound reproduction.
I wish you much success with this simple and very good sounding modification.
Zwolle; 12-october 2004; Ir. Menno van der Veen, Ir. Bureau Vanderveen
This modification and publication: Copyright 2004 Ir. Bureau Vanderveen;
Super-Triode: Copyright & Registered1998 Ir. Bureau Vanderveen