I’ve recently delved into the world of paper crafting, dedicating nearly six months to perfecting my project. I’ve successfully completed my amplifier based on the Fisher X100A, a renowned El84 tube configuration from the American brand Fisher, which many of you might not have encountered before.
My goal in sharing this work is to guide fellow enthusiasts passionate about tube amplifiers and assist them in their ongoing studies, fostering an increased interest in tube amplifier technology in Turkey. As an amateur enthusiast, my aim is to contribute to the growing community of tube amp enthusiasts.
About a year and a half ago, I shared my experience with a single EL84 amp, and now, I’m excited to present a more potent and accomplished EL84 push-pull amplifier design.
LADY IN RED
The core of the circuit is derived from the Fisher X100A push-pull amplifier. It utilizes the readily available El84 output tube paired with 12ax7s for input voltage. The original X100A boasts a multitude of control and pre-amp components. However, it’s essential to keep the pre-amp and tone control separate. Integrating these features separately allows for future upgrades without needing a complete system overhaul.
I believe this circuit is perfect for home use, especially emphasizing the treble and midrange tones of the amplifier. It encapsulates all the hallmark features of tube amplifiers.
Regarding the original biasing method (cathode bias), I had to make significant modifications due to the limitations of my materials. If I lack a 3W potentiometer, I might omit this part. Additionally, I introduced low and high pass filters in the input voltage amplifier, although these specifics are less critical. The most crucial alteration lies in the negative bias setup.
Traditionally, El84 applications don’t involve negative bias; it’s more common in higher wattage amplifiers using tubes like el34 and 6550, ranging between 20-50W. However, in this circuit, a 5W potentiometer in the cathode bias line necessitates 10W strength resistors due to cathode resistance, causing current to flow from each tube. To eliminate these issues, I adjusted the negative bias to -20V (approximately equivalent to -15V of the El84 tube). This setup allows for easy adjustment of the bias current in the circuit.
I also made a slight modification in the feedback line by increasing the resistance, reducing the gain and minimizing noise. Additionally, I designed a new power supply circuit for the overall setup, presented in a simple and easily accessible diagram format.
The LM317 voltage regulator provides the negative bias voltage for the output tubes. This negative bias, a crucial aspect, needs to be noise-free and stable, unaffected by network fluctuations.
The amplifier design is straightforward, evident from the schematic, employing basic components like resistors and capacitors. These materials are readily available in the Turkish market, although finding specific sockets might be a bit challenging locally. However, sourcing these components from abroad should be relatively easy.
Another key aspect of the amplifier setup is the output transformers, which I personally wound. Through extensive testing, I am confident in the quality of these transformers. While similar high-quality transformers can be sourced internationally for comparable applications, I preferred to wind my own for this setup.
An essential aspect of the circuit design involves the casing. Typically, for lamps and other amplifiers, I opt for aluminum. Aluminum not only offers a pleasing aesthetic appeal but also effectively shields the magnetic field generated by the amplifier.
The lamp sockets will be affixed to the plate and directly attached to the interior ceiling of the casing.
Furthermore, I devised the input circuit board to be directly mounted onto the plate and securely fastened to the front panel. The amplifier’s output can be configured for either 4 ohms or 8 ohms. However, due to the limitations of my output terminals, this connection is manually adjusted. The quality of this potentiometer plays a pivotal role in determining the sound quality, serving as the primary factor through which the sound is attained.
The circuit assembly is completed, particularly focusing on the circuit’s divider section that incorporates 47kohm resistors with 1% tolerance, specifically chosen for their precision, alongside the 12ax7 lamp.
A crucial component is the feedback line, where the choice of cable is paramount. It’s imperative to use coaxial or shielded cable for this line. Circuits like these are highly susceptible to magnetic fields, making it essential to safeguard the feedback line from external influences.
Upon activating the transformer and fitting the necessary components, most of the amplifier assembly was completed.
The amplifier’s power supply circuit can be seamlessly mounted on the plate, although it requires a larger case due to the space it occupies. Initially, the LM317 bias regulator was externally installed. However, I later relocated this component to the plate for convenience. At that time, my amplifier was already enclosed since I had printed the plate before making this adjustment.
During my listening tests after completing the circuit, I noticed significant noise interference caused by my computer’s charging adapter. Whenever the adapter was plugged in, an irritating buzzing sound emanated from the speakers. The noise disappeared once the adapter was removed. This information could impact circuits similar to your amplifiers.
Following the power supply circuit installation, attention turned to connecting the card to the supply stage. One crucial aspect here is the winding of the supply cables, as depicted in the illustration.
This procedure should be particularly and specifically applied to AC lines. In my circuit, you can observe that the transformer’s 220V input is wound around the switch and the transformer after the main filter in order to minimize interference.
Another crucial detail involves the precise feeding of filaments for the lamps. To reduce interference, the filament feed cables must be twisted together and kept as far away from the plate as possible. If materials permit, it’s advisable to route the filament cables from the corners of the frame.
The incorporation of 100-ohm resistors, connected in parallel to the chassis in the filament feed, holds great importance in noise suppression.
Upon completing all these operations, the amplifier is ready. While I couldn’t detail every stage of this amplifier, if you have additional queries or if there’s another aspect related to this amplifier or general tube amplifiers, feel free to email me. I’ll do my best to assist you further.