EL84 6BQ5 Universal PCB
When experimenting with tube amplifiers, you might be confused about the different modes of operation, like triode or pentode mode. To give a little guidance i developed this PCB. You can build small Amplifiers based on 6P14/6BQ5/EL84 easily, and the output Power of 2..6 Watts is more than enough for home use.
With this PCB you can try out the different Modes by means of Jumper Wires.
The Schematic is very simple:
This PCB is of course only one part of an Amplifier, you will need at least a Power Supply and output Transformers.
The PSU has to supply the filament Voltage and the Plate Voltage of about 250V
A suitable PSU can be found here..
Layout and compomnent place. High resolotion Layout can be found here here.
Filament voltage is connected to X2, Plate Voltage is connected to X3 and the output transformer is connected to X4
A Pentode like the EL84 can be operated in 3 different Modes:
Each of the 3 Modes has its unique sound characteristics, just try them and find out which one you like best!
An Overview how to connect the different modes
|Pentode Mode||Screen Grid connected to Plate Voltage
J1 in place, J2 not in Place output Transformer connected to X4/1 and X4/3
|Ua=Ug2=250V Output transformer 5.2kOhm Cathode Resistor 135Ohm Idle Current 48mA|
|Pentode Mode, sepeated screen Grid Voltage||J1 not in place,J2 not in place output transformer connected to X4/1 und X4/3, Ug2 to X4/2||Output transformer 5.2kOhm Cathode Resistor 135Ohm Idle Current 48mA|
|Ultralinear||Screen Grid connected to UL Tap of output Transformer
J1 not in place, J2 not in place
Output transformer connected to X4/1 and X4/3, UL TAP to X4/2
|UL Output transformer needed|
|Triode||Screen Grid connected to Anode
J1 not in place, J2 in Place
Output transformer connected to X4/1 and X4/3
Output transformer 3.5kOhm Cathode Resistor 270Ohm Idle Current 34mA
R1 Cathode Resistor, see table above
Selecting a Tube:
All EL84 Types can be used, such as 6P14 or 6BQ5. The circuit was tested with EL84.
The Cathode Resistors and the screen Grid Resistor should be rated for 1W, as the can get quite warm during operation.
The tube gets very hot during operation, so ensure proper venting!
An EL84 type tube needs several volts for full output power. Usage of a preamp is therefore recommended. A CD Player with headphone-out is a good start and usually enough to drive the Tube. the output needs to be DC-Free, so decoupling with a suitable capacitor might be neccessary.
Never connect a signal source with a DC Component to the circuit, this could lead to damage of the Tube and output transformer
Always ensure that a proper load is connected to the output transformer! Tube Amplifiers usually dont like being operated without load!
Here a picture of a complete AMP, consisting of PSU, power transformer, output Transformer and EL84 PCB
Transformers can be obtained from www.askjanfirst.com, for example.
Additional Information,Basics,DIY Kit, Links for Schematic