Unbalanced tube amplifier on a 300B tube

Details: Written by: Grzegorz Makarewicz; Published: 12 June 2025; Hits: 99

Photographs: Grzegorz Makarewicz, This email address is being protected from spambots. You need JavaScript enabled to view it.

At one time, fascinated by the 300B triode, a cult device in the audiophile community, I decided to build an amplifier based on this tube. As I decided, so I did. The amplifier has the form of two monoblocks. Below is a photo of one of them. I have to disappoint the purists - the monoblocks are not "visually" symmetrical, they are really identical. When you put them next to each other, they can arouse a certain distaste among aesthetes. I found out about it personally. My answer in such situations was always the same: we should listen to these monoblocks as they work together, not as they "look together". And it turns out that sometimes it helped.

The monoblock design ensures minimal hum, so there was no point in fighting for symmetry and a different tube arrangement and fighting again for the same electrical parameters.

I did not experiment and based the amplifier's electrical diagram on a proven configuration. This lack of "design passion" resulted from impatience and the desire to listen to this "miracle" as soon as possible.

Schematic diagram of one amplifier channel

As you can see, this is an asymmetrical system. I also made the anode power supply on an electron tube. It was supposed to be an audiophile tube amplifier, so there was no room for semiconductors (at least on the secondary side of the mains transformer). I made the power supply, speaker transformers and chokes to order. Where? Well. People who are familiar with the subject will look and guess right away. To make it easier - a Polish manufacturer.

I placed the monoblocks on solid metal bases. I even added wooden sides as a test. I planned to paint them, but it ended up being just plans. Maybe one day I will paint the sides, chassis, make covers for the transformers, etc. I have to admit that after turning on the amplifier and listening to it, I did not have enough energy to finish it. For me, it is simply BEAUTIFUL!!!

Okay, enough talking. I invite you to view the photo gallery.

The components are placed on a single printed circuit board made by the thermal transfer method (there is not a single jumper on it). The volume control potentiometer is unfortunately not audiophile. It's a good thing you can't see me blushing with embarrassment.

The connections of external elements to the printed circuit board are made using so-called "slips" attached to the wires. This makes disassembly of the amplifier very simple.