A 50-W push-pull tube amp with KT88 power tubes. The design is based on the original «88-50» schematic published by the General Electric Company (GEC) in 1957. This push-pull tube amplifier uses a 12AX7 (ECC83), a 12AU7 (ECC82) and two KT88 tubes per channel. The output-power is about 50 – 60 Watts per channel. The schematic of this amp was first published in the book «An Approach to Audio Frequency Amplifier Design» by the General Electric Co. Ltd. of England in 1957, where it was called the “88-50″ (KT88 / 50 Watts). While I didn’t modify the audio part, I changed the power-supply design as follows:
- I replaced the tube-rectifier (5U4) with a solid-state one. A solid-state rectifier can deliver more power and is more rugged than a tube-rectifier.
- I still use the choke to filter hash and noise, but I added more capacitance to the power-supply. This results in better filtering and better peak current handling. Low impedance at high frequencies means that the power supply does not distort short transients.
- I use DC-voltage for the heaters on all tubes (including the KT88!) with a slow turn-on supply to prolong the tubes’ lives.
- I built the amp and the power-supply in two different blocks. This allows tweaking the power supply without taking the amp (and vice versa). An additional advantage is that the transformer’s magnetic stray field is far away from the amp, which reduces hum. The amps are smaller and nicer, too.
The case of the amp is a wooden frame with aluminium plates at the top and bottom. This is relatively easy to build but I still looks good. I used copper foil inside the case to shield the amp at the wooden parts.