Introduction: Classic Vacuum Tube Amplifier
I decided to build a tube amplifier, working in pure A class, with the benefits of modern amplifiers like remote control, input selector or lamp hour meter. The amplifier's dimensions and colors were to match the Maranz Compact Disc Palyer CD-50 I owned. The cost of building the amplifier was not to exceed $ 500.
Did I manage to achieve the above goals? Familiarize yourself with the material and judge.
The purpose of presenting my amplifier is to inspire my solutions to people planning to build similar devices.
This description is not intended for inexperienced people and will not allow them to build a copy of my amplifier on their own. To build this amplifier, it is necessary to have knowledge and practice in the field of analogue and digital electronics, a general technical concept and awareness of the threats occurring in the project. There is voltage dangerous to life in the amplifier, EVEN AFTER DISCONNECTING THE POWER CORD. This voltage can cause your heart to stop or even lead to death.
Step 1: Analog Circuit
Class A amplifiers are characterized by pleasant sound, are liked by audiophiles but also have drawbacks. Their efficiency is low and they consume a lot of electricity. In the project I used a simple scheme as a base, available at http://skarabo.net/sid-21-se.htm, which I adapted to my requirements.
The main elements of the amplifier are electron tubes and transformers. In my design I used one 12AX7 (ECC83) double triode (L1) and two E84L power pentodes (L2). The supply transformer is TSL100/001 and the output transformers are TG5-46-666.
The filament voltage of the L1 lamp is stabilized by the LM317 stabilizer to avoid possible mains noise coming to the first amplifier stage. The filament voltage of L2 lamps is rectified by a Graetz bridge and smoothed with capacitors. Anode voltage is generated separately for each channel. The values of resistors and capacitors in power supplies (RC filters) are selected so that the supply voltage of the L2 lamp is 250V, and the L1 lamp is 220V. To discharge the capacitors in the power supplies after turning off the power, resistors connected in parallel to the terminals were used.
Step 2: Digital Circuit
The analog part is almost standard for every tube amplifier and is understandable for every tube builder. What makes the amplifier stand out from the others is the housing design and the digital part. I will briefly discuss the digital part in this section. The project was based on a solution provided by JarekC on one of the world's largest portals for electronics https://www.elektroda.pl/rtvforum/topic2920523.ht... JarekC did a great job, designed and built a tube amplifier driver that perfectly fits my needs. A driver set of elements together with PCB could be ordered from him. For those willing to make a PCB by themselves and program the microcontroller, I refer to the instruction "instrukcja_E.pdf" and the page with memory entries https://www.elektroda.pl/rtvforum/topic2920523.ht... The heart of the controller is the Atmel Atmega16 microcontroller. The BA6218 system controls the volume potentiometer motor. MBI5026 systems were used to control the displays.
In my amplifier, the driver is responsible for:
- volume potentiometer motor control
- switch on/off anode voltage (30 seconds for filament heating)
- input selector control (4 channels)
- operation using the remote control (RC5) and buttons on the amplifier's pannel
- monitoring amplifier status
- counting the operating time of electron tubes.
As a volume regulator i used ALPS motorized linear potentiometer 50k 50KBX2 for Hi End audio. PCBs for selector, relay, tact switches own production. I used the remnants from my other audio constructions or I used a universal PCB.
I adapted the controller to my needs using a terminal through the RS232 port. The controller software also allows you to program it using the buttons on the front panel of the amplifier.
Step 3: How Does It Look Physically?
All electronic components fit in the amplifier housing. Some of the components were mounted on the PCB, the rest was used for spatial assembly, which is not unusual in tube constructions.
I used shielded cables carrying the sound signal. I ran them as far as possible from AC voltage components. I placed a common ground point near the output of the anode power supplies.
Step 4: Housing
The whole amplifier weighs 14 kg. The main reason for this is the granite used for construction. This stone goes perfectly with the brass and red color of the electron tubes. Of course, it fit the CD-50 Maranz.
I commissioned the stonemason, who basically deals with the construction of tombstones, to create elements made of black granite. In the design of granite elements I put all mounting holes for lamp bases and of course ventilation holes (for cooling). Granite has been polished and its edges sanded. I connected granite elements with epoxy resin. I put a polished and varnished brass profile between these elements.
Structural elements such as bottom cover handles, front panel handles were glued onto good quality two-component epoxy glue.
The amplifier contacts the surface with stands made of soft rubber. Soft rubber washers have also been used to mount transformers to the housing. Custom made lower cover (profiled) made of aluminum with holes. Air flows freely to the amplifier through the holes in the cover to cool the amplifier's heating elements.
Transformer covers are commercially available cups made of stainless steel. Handles have been removed from the cups. The cups were painted with black powder paint.
Lamp base covers are metal elements made on a lathe according to the design. They were also painted with black powder method.
The front and back panels were made by an advertising agency from a composite board (aluminum, polyethylene core, aluminum). I designed the panels in Corel Draw in accordance with the agency's technological requirements.
The display cover is made of semi-transparent black Plexiglas.
Step 5: Plans for the Future
I intend to make the next amplifier in a similar way. I will use more powerful lamps (6C33C) also working in class A. Due to the weight I will probably be forced to make each channel in a separate housing.
Certainly I will make much more detailed a photo report of the project and put it on the portal.