This project started around mid December '09 and finnally came to a conclusion by 7th of March 2010 -mainly because of lack of time-.
The power transformer is fixed to the wooden case from the inside. This way some mechanical decoupling is provided. I found this very effective helping avoid the transmission of the core vibrations towards the furniture and the consequent humming that happens when fixing it directly to the chassis. This mounting method also allows to have a gap between chassis and transformer allowing for some ventilation.

The Amplifier Circuit

When I started the mechanical work, the idea of what the circuit would be hovered around the EL84 being driven by an SRPP with active biasing for the EL84. The rest would be very much like that of the original amplifier using stopper diodes and including the OPTs secondary in the cathode path to provide some feedback to the output stage.
Unfortunately it turned out that with this OPTs, no matter which way around they were wired to realize the desired feedback circuit, the amplifier would always oscilate in some way or another. With the 'better' of the two wiring ways (only a low whistle was perceived from the oscillation) the sound was dull and also the sensitivity became much too low. No other feedback being applied, I blamed it on the OPTs L-C and maybe on the HF caractersistics of the used EL84 (a long life russian 6N14P military version which is EL84 compatible). A different feedback circuit approach was necessary leaving the OPTs outside the feedback path. Finally the circuit was realized without overall NFB. Local feedback in the output stage is achived by wiring the EL84 in ultralinear mode. The OPT has 3 primary tappings: 7K, 5K2 and 4K. Using 5K2 and 4K makes the UL circuit operate close to the triode mode. Efficiency decreases this way limiting the maximum attainable output power to somewhere around 2,5W. On 90dB efficient speakers 3 watts are plenty loud!

Update November 2010

Shortly after this project got ready the Aikido circuit came my way. I have listened for several months to the Aikido driven EL84 SE Amplifier and in spite of the good performance of this EL84 remake it falls short of reaching the performance of the Aikido driven one so that I would rather recomend any interested DIYer to try the Aikido version instead. Nonetheless here are the schematics of this project:


EL84 SE Amplifier
Power Supply
inserted by FC2 system