“Hamilton” Amplifier

This is another EL84 amplifier that I built as a variation on the previous “Matariki” design. Home-built, as are all of my amps. It is simplified in that it has no tone controls, no phono stage, and no remote control. Being built in the same size chassis as the Matariki, this means there’s a lot of empty space inside.

The person it’s been built for has a Langcaster electric guitar with pickups and a little battery powered preamp built in, so the guitar input on the front is just a line-level input with both channels tied together. There are no distortion or non-linear effects added to the guitar input.

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Gallery click images to view

Insides and construction

Specifications

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. 330V Plate Voltage.
Valve complement Gain stage: 1 X 6N2
Phase Inverter: 1 X 6N1
Output: 4 X EL84 / 6P14P
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 100 KOhms
Input Sensitivity 1.2v p-p for rated power (0.4v RMS)
NFB 10dB
Frequency Response 6Hz – 22kHz ±3dB
Power consumption 230v 50Hz 90w nominal

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