05-EL84 Amplifier III

Project 5 - an EL84 Amplifier build built as a demonstrator.

This amplifier was built for stock, as a demo unit to show anyone interested in commissioning an amp build what a 15 watt EL84 amp sounds like, given that the lower wattages tend to be the biggest offputting factor among people conditioned to high wattages from commercial gear.

Included in this amp are:
  • 2-triode Phono/RIAA stage
  • Tone Controls
  • Remote control volume, standby, and input selector
This amp was built on a small chassis - same size as the previous project: aluminium, sourced from AliExpress. The front panel is 8mm thick, brushed aluminium. It required pockets being milled on the CNC from the back to accommodate the controls mounted through it.

The lights are 3mm LEDs but I decided I don't like the bulging appearance they give when pushed through the front panel, so we laser-cut some 2mm clear acrylic into 3mm circles, so the lights on the front could be flat and flush. They press-fitted perfectly and the look was 100% what I was wanting.

The STBY LED is red, and the PWR led is dual-colour, it starts red at power-up and then when the HT switches on after 30 sec, it turns green, using a polarity-reverser circuit I designed based around a DPDT relay.

The power switch and input selector are a rotary encoder: push to toggle power, rotate to cycle through the inputs.




This is similar to the previous EL84 amp except that I found a supply of NOS Soviet triodes so I used the 6N2 and 6N1 in place of the 12AX7 and 12AU7. This necessitated changing the voltages and loads owing to the different charasterics of these tubes.

The output is exactly the same as the previous EL84 amp I built: EL84 in Class AB push-pull, ultralinear, fixed bias, with Hammond 1650E output transformers.

The tone control is the exactly same circuit used in the previous project.

The amplifier was built with the phono, tone, gain, and splitter stages on a single PCB at 160 X 100mm and holding 6 dual triodes.


The green boards are bought-in components: input selector, mains switch/standby, remote volume, and microcontroller.





Main board containing RIAA, Tone, Gain and phase splitter stages. The tube sockets are on the copper side, to allow mounting through the chassis. The sockets are flanged, and the boards mounted to the chassis with standoffs, so that removing the tubes does not risk ripping copper off the boards.


The power transformer is a Hammond 370FX - the 6.3volt secondary powers the heaters in the EL84 and tone control and gain/splitter stages. The heaters in the RIAA stage are powered from the 5 volt secondary, rectified with 4 schottky diodes (selected for their lower forward voltage drop) and smoothed with a 47000µF 10V capacitor. This gives 6.0VDC at the heater pins on the triodes.

The output stage uses the same fixed bias concept as the previous two EL84 amplifiers - the board running between the sockets provides the voltage trimmers and associated resistors, as well as the cathode shunt resistors. These are exposed to the top to allow fine-tuning by the user


The recessed bias adjust trimmers and exposed test points are discreetly positioned between the EL84 output valves 


At the back is just the usual configuration of inputs and outputs, with a separate GND terminal for the RIAA stage. The legend was laser-etched through the paint on the aluminium.



Rear panel

Some specs and details


Main Amplifier

Topology
Line-level amplifier, grounded cathode gain stage, DC-coupled cathodyne phase inverter, push-pull EL84 output in class AB using fixed-bias ultralinear topology, global negative feedback. 360V Plate Voltage.
Valve complement
Gain stage: 1 X 6N2 
Phase Inverter: 1 X 6N1
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1.5% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
500 KOhms
Input Sensitivity
1.0 v rms for rated power (2.8v p-p)
NFB
6dB
Frequency Response
10Hz – 24kHz ±1dB
6Hz – 32kHz ±3dB
-6dB at 50 kHz
Power consumption
230v 50Hz 190w nominal


RIAA Phono Preamp

Sensitivity
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Gain
+42dB at 1KHz
Freq Response
RIAA Compliant ±1dB
Topology
Cascaded Dual triode 6N2 implementing RIAA filter in NFB


Tone Control

Topology
unity-gain cathode-follower driven baxandall tone stack with NFB
Valve complement
2 X 6N2

Tone adjustment
±12dB at 100Hz and 10 kHz





Listening tests

The sound from this one is clean, detailed and very pleasing. The main amp stage is based on the well-regarded Fisher X100, with the appropriate modifications for the Russian triodes in use. 

The output stage is EL84 in fixed bias ultralinear, biased to 8.4W quiescent dissipation (70% of rated maximum)


Schematic

(click to enlarge, may need to save-as to read clearly)






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