About the big KT88 amplifier - tech details

Project 01.. the first project

This amplifier is an all-valve (tube) home-designed and built integrated amplifier utilising a Williamson topology.

Specifications / measurements

  • Topology: Williamson
  • Valve complement (per channel): 1 x 12AX7. 1 X 12AU7, 2 X KT88
  • B+ Voltage: 560V
  • Bias: 28W disspiation (50mA)
  • Measured power output: 80W into 4Ω at 1% THD (1kHz)
  • Freq. response: -3dB points at 13Hz and 46kHz
  • Weight: (approx): 27kg
  • Mains transformer: Custom-made
  • Output transformers: Hammond 1650R
Frequency Response measurements (yellow trace = input signal for reference, blue trace = amplifier output into 8R resistive load. Scale = dB on the millivolt, 1dB per division)

The amplifier is built using a custom-made mains transformer. The power supply components are all assembled onto a 150mm (6") square hand-made PCB which incorporates the main 560V power, 2 X 350V for the 12AU7s, and 2 X 280V for the 12AX7s, as well as the bias voltages for the output valves.

Custom home-built Power Supply board

The power supply consists of a full-wave rectifier from the transformer 400 – 0 – 400 secondary. Rectifiers are 1N5408 and these are doubled up (connected in series) as the Peak Repetitive Inverse Voltage rating is exceeded in a single diode configuration.

The Power Supply includes a 30sec delay on the HT, implemented by a 555 timer IC with associated componentry, and two relays which sit between the transformer secodaries and the rectifier diodes.

Main capacitors are connected in series, with balancing resistors, since the 560V B+ exceeds their 450V rating.

The amplifier uses a Williamson topology with the initial gain and DC-coupled concertina stages provided by a 12AX7. The driver stage consists of a 12AU7

Initial gain, inverter and driver stages, utilising 0.22µF Polyester Film coupling capacitors. Balancing potentiometers are present for the driver stage, as per the original Williamson design

The output stages are KT88 using ultralinear topology. Bias adjustments are made using a variable voltage-divider network with potentiometers wired such that a wiper failure will increase the negative voltage, thus decreasing the quiescent current through the valve. In other words, fail-safe.

KT88 bias in fail-safe mode

The full circuit diagram of the amplifier (minus HT Delay circuit)

Click to see larger / use browser zoom to view as necessary

The amplifier was built with some concessions to convenience: A separate small 9V transformer is included, this is powered independently of the large mains transformer. This powers the standby circuit, providing the capability of a remote control power switch. This is integrated into a modular motor-driven volume control and input selector unit purchased from AliExpress. This unit powers a solid-state relay which applies power to the large mains transformer.

The transformer for the remote sensor/control board also powers a small separate 5V power supply which is connected to a USB socket on the back of the amplifier. The purpose of this USB socket is to provide power for USB-powered source accessories. Much of this amplifier's source material will be delivered via a Chromecast Audio.

Custom home-built USB power supply board

The amplifier also includes a power-switched IEC socket to allow connection of other source components. In this case a separate valve phono preamp.

The front and top of the chassis is fabricated from 4mm aluminium with the bottom, sides and rear being 2mm. These are assembled onto a box-section frame with 3D-printed corners.

Chassis components were laser-cut to a custom design made for this amplifier.

This was the designers first amplifier build

More pictures at the photo gallery

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