DHTs and cathode bias degeneration

A few years back I went down the path of exploring different DHT in pre-amp stages for sonic improvements on detail and dynamics. I explored all sorts of different DHTs as have posted in the blog and many more I never got around to write up about.

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Ba DHT Preamp Build

Just in the last days before the arrival of our second child, I had a spot of luck and managed to get some free time to work on my DHT preamp experiments. 

A pair of NOS Siemens “Ba” valves under initial testing 

Originally I was put off by the hum pickup susceptibility of the Ba German DHT. However, I decided it was time to listen to the design I worked and experimented in my workshop time ago. I remember listening to this valve and was very pleased with its sound. 

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DHT Audio Shootout @ London

Here’s the summary of the shootout day we had yesterday at my place with  Tony Rees and Andy Evans

Test Music

For testing the pre-amps Andy suggested the following 2 tracks:

  1.  Ravel Feria (Cleveland/Boulez)
  2. Janacek The Cunning Little Vixen Act 3 Scene 7 “A vixen’s running through the woods” Prague Theatre/Gregor


The Ravel is orchestrated for a large orchestra and percussion, and listening was particularly for the timbre of strings, brass and woodwind and the clarity of the percussion. The Janacek is for several different voices and chorus, and listening was for the tone and character of the voices and how well they were differentiated from each other. In both cases the overall clarity, life and crispness was considered.

Andy Evans

Although I wasn’t familiarised enough with these 2 tracks myself, I found them to be an excellent choice and a reflection of Andy’s musical experience and knowledge as musician. 

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4-65a SE Amplifier: testing filament regulators

The Rod Coleman Filament Regulators were already tested in the New Year. It seems a long time indeed. Here is a selection of pictures after fitting the regulators in their heat-sinks and doing a final test before assembly: