Aikido EL84, and then they were three...
This is my latest EL84 diy project, an experimental setup to try the Aikido circuit
designed by John Broskie. Why did I never try the Aikido before?
Because one of my prejudices was to use as few valves as possible!
Why did I try it now? Because someone else triggered me to do so and I learned
the lesson.
This is a monophonic setup the left and right channels are added via
resistors before the input. These chassis are usually used to quickly
try out something before jumping into serious work. The last setup was for the
ECH81 / EL84 single ended amplifier, which later became the
Vintage EL84 SE Amplifier.
The speaker at my work bench is a Tangband W4-655SA mounted on a backloaded horn cabinet. This horn 'shouts' a bit so, unless driven at low levels, the ear is tired quickly. Not so with the Aikido driver!
The high frequency roll-off of the speaker sets on early, so it is not the brightest sound yet very detailed.
I had it running for a couple of hours. I could not move away from it, listening with amazement and changing tubes now and again from what I could find in my collection. I found a Tungsol 12AT7 and plugged it in (running at 6.3V but by mistake wrongly wired for 12,6V) in the first stage paired with an ECC85. This wrong heating voltage makes the bass sound like boosted, an interesting discovery.
Finnaly I hooked it up to my system and it has been in use for several days now, in mono! It keeps delighting me. I wonder what it would sound like in stereo maybe driving a 2A3... I am thinking about modifying one of my bigger SE amps from SRPP to Aikido. Mechanically it is a mayor undertaking so it will take time...
It took 6 weeks!
It makes great fun listening to this amp. As can be seen in the picture the OPT is tiny, inexpensive and not really intended for HIFI yet the sound from this setup is great.
Anyone considering to build a new EL84 SE amp should give this a try. In this setup ECC85 is used for both stages but several other types were tried with similar results: 6N1P, 6N2P (too much gain here), 6N3P with adapters, 6N6P (most likely under biased) and 6AQ8.
Please be carefull when wiring the secondary of the OPT!
In case you build according to the schematic shown below, depending on which side connects to the cathode of the EL84 via the electrolitic and which one to ground, the circuit is either an excellent amplifier or a powerfull oscillator. So first time it should be connected to a cheap speaker (any 8 Ohm 0.5W will do) for a test. If a strong whistle is heard swap the secondary connections else play some music and enjoy.My sincere gratitude to John Broskie (father of the Aikido) for sharing the circuit and for his superb webzine.
Related Aikido Links
Here is the original article .
Here the kit manual.
Also search at tubecad.com for 'Aikido', there is plenty of valuable material to be found.
Schematics
Note that the schematic just reflects the parts used. Some values like the 1uF/600V and the 30K pot. at the input must not necessarily have these values. A 50K input pot may be better and the 1uF may as well be rated at 400V. Also the 510 Ohms resistors may differ. Using 470 or 560 Ohms will work just as fine.
