Tuesday, 31 July 2007

300B Dream

The DIY/Audiophile/Tube world seems to be filled with 300B designs. Many writers and builders place the 300B at, or near, the top of any 'most desireable' power output tube. Having never heard a 300B amp, good or bad, I have decided to follow the flock and build myself a 300B amp. There are a raft of designs around the Internet for the 300B, ranging from the original WE designs, RC designs (Angela, Audio Note Kit, Reinhardt, etc), exotic DC designs (Electra-Print), parafeed, transformer coupled, etc, etc. There are just so many!

Not being an audio designer, or electronics engineer, I have to rely largely on other people's designs, skill, knowledge and experience to put together something that will not; kill the builder, kill the end-user, burn or otherwise destroy property; and at the same time, sound decent and be reliable. It is therefore important that I not only use designs from people who really know what they are doing, but that I also understand the theory and design of the amp, so that I will not be simply soldering.

Not wanting to do the now conventional/standard RC approach, I decided that this amp should make use of technologies, designs, and topologies that are somewhat different. From reading the various information, it appears that the a 300B amp is best suited to its now 'classical' configuration, the Single-Ended (i.e., Class A) design. Further reading and research, suggests that the importance of the 'audio chain' begins backwards from the speaker. Taking this logical, the critical audio path, in regards to the power amp is as follows:

  1. Output Transformer
  2. Output Tube Stage
  3. Power Supply Stage
  4. Stage Coupling
  5. Driver Tube Stage
To thus 'rough-out' the basic design requirements. The output transformer should be as high quality as possible. Some of the commonly touted high-end transformer manufacturers include (in no particular order); Plitron, Tango, Lundahl, Sowter, Electra-Print and Tamura and Audio Note UK. Some of these transformers are in the hundreds of dollars, and some special versions, even more. More information will be provided as I research more deeply into this area.

In regards to the output tube topology, there seems to be four designs; Direct coupling between output tube-transformer-power supply), Ultrapath, Parallel Feed and a combo of Ultrapath/Parallel Feed. My main reference is an excellent article in a back issue of Vacuum Tube Valley. This is an important area that I will explore later.

The coupling between the two stages can take place in a number of ways; direct coupling; RC coupling, transformer coupled, Loftin-White, using active (semiconductor) devices, and others. Given that capacitors are the components that colour the sound the most, it would seem logical to use or not use only with full and proper understanding of its implications. An area that deserves far more attention.

Finally, the power supply and driver stage will be looked into much more detail later.

After all of that, a preliminary list of the designs that are being seriously considered is as follows; (this may grow or shrink)

  1. Electra-Print DRD 300B (A Loftin-White direct coupled 300B that also uses the Ultrapath technique for coupling the cathode to the output transformer).
  2. Andrea Ciuffoli's 300B PSE/SE (Transformer interstage between the 5842 driver and 300B).
  3. TubeLabSE (CSS 5842 Driver with a MOSFET stage between).

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