Single Ended El84
Violetta Wagganer
However, I was not satisfied with the single volume/gain control potentiometer. In order to get some distortion, I had to crank the potentiometer up to a level that was too loud to play in the house.
The 12L8GT output tube used is pseudo exotic, but full of sweet tone. It is a dual pentode in a single bottle with an octal base. In this circuit the two sections work in parallel single ended. Gibson did something similar in a version of their Gibsonette, except they used 6V6GT.
Almost identical circuit to the EL84 amp, except output tube is a single ended 6V6GT. Very similar to a Fender Princeton (5F2). The biggest difference is the rectifier - the schematic is drawn with a solid state diode rectifier in place of the 5Y3 tube used in the day. Tone difference? Sure, everything sounded better back then...
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 learnedthe 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.
After a couple of hours the amp was ready to play, no hum but a VERY CLEAN and DYNAMIC sound.
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 amazementand 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 excellentamplifier 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 andthe 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.
A high quality 3.5W Single ended output transformer. Designed for the Mullard 3-3 amplifier but may be used in any single ended design with a valve having an output impedance of 5000 Ohms, such as an EL84.
This is the classic single-ended EL84 schematic from Mullard. The input stage uses a EF86 to drive a Mullard EL84. The power supply uses an EZ80 (6V4) tube rectifier. For more information about tube based power supplies, see the article Power Supply Design for Vacuum Tube Amplifiers.
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