"I guess I don't need to tell you about the advantages of DHT tubes in preamps. They have an ease and immediacy that is better expressed than IDHT like 6SN7 etc. even though I have built IDHT that come very close. DHTs are hard to engineer with though. It's very hard to get them completely hum and noise free. Because the electron flow is coming from the filaments rather than the cathode being heated by a separate heater as in IDHT tubes, it takes extra engineering to get a DHT working optimally.
"As far as I know, Supratek was the first company to do a commercial DHT preamp. That was the 300B, many years before Manley brought out theirs. I also did PX4 and 45 preamps and one that could do 300B, PX4 or 45 interchangeably. Then I went to the 101D and used it for quite a few years despite reservations about its bass. I partly solved this by building the Cabernet Dual [positively reviewed by Srajan
back in 2006 - EK] which used the 101D and a 6H30 in two separate but integrated line stages. This was ideal for speakers which could be bi-amped to use the 101D on the mid/ treble and the 6H30 circuit on the bass.
"I started using Russian DHT about ten years ago. The engineering of Russian tubes is mostly excellent and some tubes are exemplary. Most of the early 1930s’ American DHT are hard to match perfectly but it's not uncommon to get a batch of 4P1L that all measure exactly the same. I really like the 71A and 801A/10/10Y of the early 30s US options in preamps but getting matched pairs is quite difficult now. Of all the Russian DHT I tried, I like the 4P1L most. It has a very good bass, sometimes the Achilles heel of DHT tubes, and has a very extended but sweet top end. The midrange is maybe not as euphonic as some but that after all is a coloration and usually becomes unconvincing over time.
"Supratek preamps are always high gain. I'm convinced that high gain equals higher dynamics and better resolution of microdynamics. This is usually achieved by putting a 6SN7 in front of the DHT as an input gain tube. In the Reference preamp, I elected to use a step-up transformer instead and make it switchable so as to enable user choice of utilizing extra gain or not.