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
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
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
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: