88-50-B tube power amp

88_50_BA 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.

2 thoughts on “88-50-B tube power amp

  1. Kent wrote on 01. March 2017:

    First, thanks for posting tube related articles.

    I feel you should remove the word “Williamson” from this article; the design is nothing like a Williamson amplifier at all.

    I have been trying to educate a small number of beginners to tube amplification and the Williamson amplifier history. Someone in this group of beginners found this article and added it to the discussion. I pointed out that it is NOT a true Williamson circuit.

    May I suggest you re-title this “THE 88-50 POWER AMPLIFIER” which is really what it is. Here is a link to the original 1957 article in Wireless World-

    Please consider re-naming this article in hope to spread the historical information correctly.

    Thank you for your time. Kind Regards- Kent

  2. matthias wrote on 03. March 2017:

    Thanks for pointing this out. The “Williamson” terminology has been abused for many things, and it has become a bit unclear what it really is. But your point is well taken, I changed the post as suggested.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>