The Birth of EL34-10W Single-Ended Class-A Tube Amplifier
The design of EL34-10W fully demonstrates the simplicity of a vacuum-tube circuit, the sound is extraordinary, showing a big difference from transistor or Class D designs. The antique components, giving a high bandwidth and linearity while adding an emphasis on its handmade craft skills, are again brought to life to create beautiful sounds—when being played by this small tabletop machine.
The best recommendation is to try it out yourself—you will get to know the fun part in building your own amplifier. You can try out new components, just like testing different parts on your car. You can experiment how different components contribute to the different characteristics of the sounds. Sometimes you’ll be amazed that every single component matters.
Let this article brings you to the journey of the birth of EL34-10W.
Accommodating to modern cozy living room for a Couple or fit to the size of your reading room, a 8-10W tube amplifier is good enough to pair with high efficient bookshelf loudspeakers like AP-1 or LS35-SP. Class A power amplifier is always the classic representative of supreme amplifier with super low distortion, and among this category, Class A with single-ended vacuum tube is no doubt the best of the best!
Then comes the choice of power vacuum tube. 300B definitely comes to the top list for Class A SET amplifier due to its good linearity, low noise and good stability, yet 300B can only deliver 7-8W with SET design, while pentode tube can be designed to have 10W output, which gives more dynamic range of different music lovers. And considering the fun part of tube replacement for DIY kits, Pentode with international octal base has many alternative vacuum tubes out there you can replace with, such as EL34, EL84, 6CL6, SY4307A. After numerous trials and tests, we finally chose EL34 to start with.
The Tube Circuit Design
Concise and exquisite circuit diagram:
The EL34 power stage uses “ultra linear” circuit design, which provides more power than triode and super low distortion than pentode operation, with relatively small power loss. The transformer from ENIGMAoustics Lab provides 43% ultra linear tap to travel through a 100 ohms resistor to grid 2 of EL34, and the signal input at grid 1 is also connected with a stopper resistor to avoid high-frequency oscillation.
To keep the power vacuum tube operated in a proper range to provide better tone, a proper bias needed to be taken care while tube is aging or new tube is replaced. EL34-10W leverages automatic cathode bias circuit to self-adjust cathode bias, so to prevent from bias adjustment issue. However the disadvantage is that some power will be absorbed by the cathode bias resistor and bypass capacitor (R9 and C2), and, as they are with cathode site, so to have critical influence on the sound performance, so high quality components are required.
Dual triodes are being connected to form a SRPP type (a.k.a. cascode amplifier) at the input—the benefit of this is its simplicity and clarity. The triode Q1B at the bottom is a common cathode type and its main function is to amplifier the signals. Meanwhile, the triode Q1A on the top is connected as a constant current source. Not only this unique formation provides the best plate loading system, the current coming out from the cathode also reduces the output impedance —obtaining a great ability to drive the power tube.
The addition of C4 provides the AC signal short loop for R3, and therefore increases AC voltage gain. The impedance of C4 will increase as the frequency is reduced. The frequency of -3dB is 1/(2*丌*R3*C4); as a result, the design in Fig 1 gives a low frequency cut-off approximately 0.4Hz. Moreover, with the support of negative feedback (after connecting to R11), the whole amplifier will have voltage gain of 25 (28dB), which means you will get an output of 12W full power when only giving an input of 0.4V.
Of course, if you want to experience having no negative feedback, C4 and R11 can be omitted.
The kit will be equipped with ECC803S, a long plate triode (similar to Mullard or Telefunken) with spiral filaments. Not like 12AX7, ECC803S can deliver more meticulous sounds, and provides large enough gainthe gain to the whole system either having negative feedback or non-negative feedback. The filament voltage is regulated by MC7812A (manufactured by On-Semi semiconductor which is the best sounding 7812 as I experienced) for clean and hum-less sound reproduction.
Two independent decoupling circuits are used for plates to ensure two channels (left and right) works individually as well as lowering down the cross-talk effect.
Both sides of a large electrolytic capacitor are connected to a good quality 0.1uF metal film capacitor. This is used to improve and further reduce the high frequency impedance in the power source.
The output of the first stage is coupled to the gate of EL34 by a high quality 0.22uF capacitor. This capacitor and the input grounding resistor(R8) will form a high-pass network. We have no intention to keep sub-lower frequency as low, so some sub-low frequency might be amplified and saturates output transformer while the sub-low frequency is so low that human ears cannot even hear.
Output transformer is critical:
Among tube amplifiers parts, output transformer is so critical that without a good output transformer, even a perfect circuit design is just a waste. Conceivably, a fine transformer may cost a big fortune.
Among many single-ended transformers, transformers made by Tango are incredible—low distortion, wide bandwidth, and high tolerance for DC, however Tango is no longer in production; therefore, hunting decent transformers worldwide has begun…
We know that many people love to use Hammond Transformers. The best-fit specifications should be that of 125GSE, and the price is also within a more reasonable range (about 120 USD). However, it would be better if it is able to tolerate at least 100mA DC and not being saturated. The DC specifications of 125GSE is fine, but the frequency response is not good enough after our test —the detail in low frequency is obviously lacking clarity, and the high frequency detail is not delicate enough as well.
Others like Plitron Transformer, and the well-known Sowter Transformer, are really a good fit. Sowter happens to have a single-ended output transformer for the EL34. It’s surely a good choice. The same time, ENIGMAcoustcis Labs starts to design output transformer by his own and the result is so satisfactory comparing to Sowter transformer.
To make a good output transformer, 6-8 winding machines are required for multi-layer winding process. Not only the iron-core, copper wires have to be top grade, the material used as the insulator and cushions in between the layers, and also the soaking varnish needed to be carefully considered and selected. Each minimal difference may pay off for quality in return. For example, the iron core can withstand the time and tide and still look just as new as it was freshly made even after 20 years if it is immersed in some good varnish. On the contrary, some poor varnish will not keep the iron core as new and become rusty soon. In addition, in order to provide different secondary output impedance and maintain the same frequency characteristics, different ways in winding around 4/8 ohm must be utilized.
This is why a good transformers are selling at an incredible price— it requires complicated processes and hours of hand-crafted labors to achieve state-of-art wide frequency range and good linearity. It definitely not something created under a mass production that only fits the purpose of saving the cost!
It is so said that nowadays, many resistors and capacitors selling on market can hardly be found having tinned copper wires. In order to cost down, most wires have been replaced with iron wires (or other metal alloys). In addition, bonding wires inside the IC (integrated circuit) are no longer made out of gold; they have long been replaced with aluminum wires for cost saving reason. The truth is, once the replacement is done for good, it is very hard to change it back. So, would it be ok for you to enjoy the modern version of NE5534 ? —the music journey into aluminum-iron wires! Maybe that is why antique components are getting more and more expensive.
Alright, let’s back to the business. The ENIGMAcoustics Labs transformer selected is a primary impedance of 3.5k as well as carries a tap of 43% for ultra linear connectivity. The secondary side provides 4/8—two impedance outputs, while allows 100mA DC to go through the primary for single-ended designs, and produces an output of 15W. Nonetheless, it has very good frequency response characteristics. Until this transformer is developed and tested, so we have high confidence to build a high-class single-ended tube amplifiers. and the overall frequency response as below is so encouraging.
5Hz~50kHz wide bandwidth