One of the most rewarding Projects for beginners interested in Tube amplifiers is a Single Ended EL84 Stereo Amplifier. These Tubes are available everywhere at a reasonable price, and they sound excellent. The output Transformers are also available in high quality and off the shelf. Althogh the Output Power is only a few Watts, this is more than enough for listening at home.
But building a complete Amplifier from scratch is a demanding job:
A suitable power supply has to be built, The wiring has to be layed out properly to avoid noise and hum and so on..
For this Amp a PCB is available that eliminates most of the problems with conventional Design. With this PCB you can buld a compact EL84 Stereo Amplifier, and even the power supply is integrated on the PCB by means of two Mosfet stabilised supply voltages, one for each channel. This makes the AMP very quiet, no noise or hom can be detected.
All parts used can be obtained from the usual DIY sources, no special parts are used.
The whole AMP fits on a 100*160mm (EURO) size PCB !
Schematic (Click to enlarge)
The circuit is powered from an AC Voltage of about 220V, connected to X1. The Supply voltage is rectified and charges the Capacitor C1 to about 300 Volts. A stabilised reference Voltage of about 256 Volts is created by R3 and D1-D3. The reference Voltage is fed to the Gates of the two Mosfets, so the Source Voltage is about 250 Volts, ideally suited for the EL84 Tube. R3 charges C2 rather slowly, so the Plate Voltage for the Tubes rises smoothly, thus eliminating unwanted popping sound on power-on.
The Preamp Supply Voltage is further stabilised and decoupled by R21/C9 resp.. R22/C10.
The input Signal is routed via the Potentiometer to the Gate of the Preamp-Tube. You can use C13/14 approximately 100pF optionally in case of RF instability.
A Double Triode ECC88/6DJ8 or ECC82 aplifies the input Signal and feeds it via C7/C8 to the Main Tubes.
depending on your Input Stage you must set the Jumper Wires on the PCB for correct heating Voltage, as some Tubes (ECC82) must have their Filaments in Parallel.
Suggested Cathode Resistors for EL84/6BQ5:
|Pentode Circuit||Screen Grid at B+ Potential||Ua=Ug2=250V Output Transformer 5.2kOhm
Kathode Resistor 135Ohm
Idle Current 48mA
|Ultralinear||Screen Grid connected to UL tap of Output Transformer||Output Transformer with UL-Tap required|
|Triode||Screen Grid connected to Plate||Ua=250V Output Transformer 3.5kOhm Cathode Resistor 270Ohm
Idle Current 34mA
Selecting different modes of operation:
For Pseudo-Triode Mode the Screen grid is connected to the Plate, often through a 100-1000 Ohm resistor. The PCB allows for this Mode by means of soldering in jumper wires to the Pads left of the Tube Sockets.
The Cathode resistor for this Mode has to be 270 Ohms, the output Transformer is connected to X5/3 (plate Voltage) and X5/2 (Plate/Anode) resp. X6/3 and X6/2 for the second channel
X5/1 and X6/1 is left unconnected.
For Pentode mode the screen grid is connected to the fixed potential of 250 V (plate voltage in this case)
Place a Resistor R25/R26 of 100-1000 Ohm into the Pads left of the Tube Socket, so the screen grid is routed to X5/1 resp. X6/1. By placing a Wire Bridge from X5/1 to X5/3 resp X6/1 to X6/3, the screen grid is set to the desired potential of 250 V
The output transformer is connected to X5/2 and X5/3 resp. X6/2 and X6/3, the cathode Resistor is 135 Ohms
Der Kathodenwiderstand beträgt in diesem Fall 135 Ohm
The screen grid is connected to the UL Tap of the output transformer
Wiring similiar to Pentode Mode, Brigde Wires X5/1 to X5/3 (X6 similiar) are omitted, UL-Tap connected to X5/1 resp X6/1
The values given in the schematic / Component list are typical values for the Tubes used in this Project. The sound of a tube Amplifier is very dependent of its operating poing, so you should find out what sounds best for you. You may want to increase the idle Current of the preamp stage, or even decide to use completely different tubes, the Circuit and the PCB offer a great number of possibilities.
You may also want to experiment with Feedback, by placing Resistors R27 / R28, creating a global feedback point in the cathode path of the Preamp Tubes
Another modificacion could be the use of "audiophile' components, often sold for high prices, feel free to experiment.
The layout is designed to be very flexible, you can also use preamp tubes with 6.3 / 12.6 V Filament voltage by setting the correct jumper wires.
At the moment i use output transformers ATRA0288 supplied by Jan Wüsten, with excellent results, (no global feedback, straightforward SE Tride mode)
Building the AMP:
when using a PCB, the amp can be built very easily. No special parts are used.
The MOSFETs should by mounted with insuation washers, for safety reasons.
You will need a filament voltage of 6.3 V and a supply voltage of 220-230 Volts AC
Tube Circuits operate from voltages that are potentially life threatening!
Never operate this circuit direcly from main power, always use a suitable transformer!
When making changes to the circuit, ensure that no dangersous Voltage remains (LARGE Capacitors!)
Check with a suited Voltage Meter!
Always wait a few minutes, until the Capacitors have discharged to a safe level!
PCB's and Layout available on on request
Component List PDF here. (Thanks to Dietmar H.)!
Additional Information,Basics,DIY Kit, Links for Schematic