Integrated amplifiers — BonaWatt Tamesis

The Tamesis is a valve integrated amplifier working in single-ended Class A, made by BonaWatt, the Polish maker whose products are handcrafted in Poland, and its name comes from the Latin name of the river Thames. It works in ultralinear mode with what BonaWatt calls Parabolic Power Bias. It was announced for the UK on 29 May 2025, with Feel Flows Audio as exclusive distributor, at GBP 8,795.

Key specs that matter

Two KT77 power valves drive the speakers, with two 6SL7GT, or 6N9S, valves in the preamplifier stages and a 5AR4 rectifier, also known as GZ34 or U77. Output is 2 x 11.5 W, or 2 x 8 W RMS within 1% at 1 kHz, into both 8 and 4 ohm. There are three RCA inputs with an input impedance of 20 kOhm and a sensitivity of 540 mVpp for 8 W RMS, a preamplifier output on a three-pin XLR socket wired unbalanced and a remote trigger output. Signal to noise ratio is 94 dB, THD+N is below 2.8% at 8 W RMS and 1 kHz and below 4.2% at 32 Hz, and frequency response is 6 Hz to 40 kHz at -3 dB at 8 W. Consumption is 130 W in operation, 220 W at most during start-up and below 1 W in standby. The amplifier measures 450 mm wide, 300 mm high and 350 mm deep and weighs 27 kg. Finishes are black, silver and gold.

Engineering and use context

Because the output stage is single-ended, one valve per channel handles the whole waveform, so it must run in Class A. The ultralinear connection connects each KT77 screen grid to an intermediate transformer winding, a compromise between triode and pentode operation. An automatic bias circuit sets the operating point of the output valves, and a preheating and cooling sequence controls how the valves are brought up to temperature and shut down, which spares them stress. A valve rectifier lets the high-voltage supply rise gradually. Volume is set by a logarithmic resistor ladder with 128 steps of 0.5 dB, a range of 64 dB, so the signal passes through fixed resistors. The output transformer taps let the amplifier deliver the same power into 4 and 8 ohm. The RC-2 remote controls standby, volume, input, mute and the display dimmer, runs on two AAA batteries and weighs 310 g.

Pairing

With its modest output, the Tamesis is intended for high-sensitivity loudspeakers in a room of about 20 m2. The preamplifier output can feed a powered subwoofer.

Alternatives

The BonaWatt Triton is the 250 W hybrid integrated amplifier with a valve preamplifier, a Class D output stage and modular slots for DAC, phono and Bluetooth.