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.
Tag: filament bias
300B SE Amp: 47 Driver
Going DHT end to end
As previously mentioned, I played around with the 46 driver. I love it sound and is a great valve. However, there were 2 reasons that pushed me to switch to the 47. Firstly, I wanted an extra tad of gain. Secondly, I have a nice stash of RCA 247 globe which measure extremely well. I’ve been reserving it for a 47 PP amp with local feedback (a la Pimm) and hopefully will get to in the future. Anyhow, the 47 in triode mode has a mu of about 8 which in combination with the SUT, gives me good gain to drive my 300B. After tweaking on the bench the stage for optimal swing and distortion performance, I ended up with the following circuit:
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.
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.
Continue reading “Ba DHT Preamp Build”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:
- Ravel Feria (Cleveland/Boulez)
- Janacek The Cunning Little Vixen Act 3 Scene 7 “A vixen’s running through the woods” Prague Theatre/Gregor
Andy Evans
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.
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.
Continue reading “DHT Audio Shootout @ London”Another DHT Preamp
Another DHT Preamp on the way with Rod Coleman regs. I will do a A-B comparison with some Tentlabs modules, it’s an interesting experiment to undertake. The preamp has the classic gyrator Rev07 and Source Follower boards…
300B Un-bypassed Rk Bias Line Stage
Many times I get emails from DIY Audio builders who embark on building a DHT preamp when they don’t need gain, but instead what they need is a simple line stage to drive their amplifiers and interconnect cables effectively. Then they come back asking: “can I reduce the gain of the 01a or 4P1L preamps?”
For those who don’t need the gain, here is an interesting idea which brings together several design decisions which makes the DHT sound to its best. The challenge with many of the best sounding DHTs of low-mu is that is very hard to implement with filament bias. I’ve done a driver with a 46 in filament bias which was a crazy idea. I could turn of the heating with the amp running! It was a nice experiment though. With exception of the 71a and some other few DHTs, if you’re looking for good anode current and low ra, you’re in trouble. The 300B, 45/46, 50 and some other variations can’t be used in filament bias.
Subject to your religious beliefs in audio, you may not want to add a capacitor in the cathode, like me. I won’t dive into this discussion which is pointless as is a personal decision. If you continue reading this is simply because you value the sound difference in the DHT without a capacitor bypass in the cathode. Keep reading then…
cx-112a DHT preamp
Introduction
This was one of my first DHT preamps. I found a quartet of NOS CX-112a Cunningham (globe) back in Buenos Aires many years ago and built one of my first DHT preamps. I loved it. I played with it before I moved into the 26 and then started the long exploratory journey with DHTs.
The CX-112a can be easily fit in an existing 01a preamp. Take a look at what Thomas Mayer recently blogged about this valve, worth reading it.
Well, you can get more current drive than 01a (nearly double) but no thoriated tungsten filaments. Anyhow, the gain is slightly higher but is very easy to adapt to my gyrator-based circuit, that I couldn’t resist to take the quartet out of my valve stash and make them sing again after so many years.
4P1L PSE Amp Finished!
I’ve been travelling a lot lately so haven’t had the chance to update on this project. A couple of weeks ago I finished the 4P1L PSE Amp:
The amp is outstanding, just like previous incarnations and tests I have conducted over the years. The level of detail and tone is unique. This is what I always loved from the 4P1L. I’m running it very hot (70mA per pair) and the output transformer is Amorphous Core 3K2 (more detail to be shared soon). It’s a simple stage with filament bias, so no cathode capacitor. The filaments are wired in series to reduce the heat dissipation. Despite this adds a bit more on the output impedance, the bass is powerful. I’m very surprised with the bass, but the level of treble is amazing. It goes over 40-50kHz, I will still need to undertake the classic measurements but so far is great!
It’s absolutely dead quiet. No traces of hum.
Some more pictures below:
And the full system below:
4P1L PSE Amp: assembling top plates
The additional machined top plates arrived yesterday. I assembled them this morning. Just need to assemble the chassis now before soldering!
HT and LT connectors
Rod Coleman’s boards
Filament resistors
Filament resistors
Speaker posts
Amorphous OPT and plate
Filament Bias: a practical example with 3A5 DHT
Introduction
Recently I was asked about whether I could write on my blog about how to design a filament bias stage. My immediate answer was:
- I don’t have much time these days am afraid to write extensive articles (and sometimes to even write-up at all)
- Thomas Mayer has written about it (see here). Of course, I completely forgot that Thomas never completed his intended series of posts around filament bias, so I decided to attempt explaining the practical aspects of its design in this blog.
Before you continue reading this post, I suggest you read first Thomas’ article above and get yourself acquainted with DHTs and triode amplification. I’m not going to cover any of that theory which I will give it for granted that the reader is experienced with valve circuits and in particular with the hybrid mu-follower amplification stage with gyrator load.
3A5 DHT example
Continue reading “Filament Bias: a practical example with 3A5 DHT”