For more than 10 years I’ve been experimenting, designing and enjoying DHT pre-amps. Without over-dwelling on this subject, to me DHT preamps bring the unique sound transparency and detail we all seek for. They can make night or day difference in any system. And is not about gain, hence I’m focusing on sharing this blog entry.
I’ve seen too many builders complaining about the gain of the DHT preamps. Well, in many cases you don’t need the extra gain, perhaps a few dBs but not clearly over 15dB which is what you get from a typical DHT of at least mu of 8 and with a hybrid mu-follower configuration which I praise on its sound. Of course, you can implement a step-down transformer (which isn’t easily affordable if you want the best quality) and also rules out easily most of the high anode resistance triodes.
Recently, I posted this preamp version which I’m currently using in my system and am delighted with.
There are few options out there to satisfy each one’s preferences and available valves. The purpose of this entry is to provide a summary of them to guide a few in their quest on the DHT sound.
CX-301a/01a
There are 2 options for doing this pre-amp. I covered the version based on SiC diode bias, however you can do it with a simple cathode resistor:
I will not repeat and say it only once. put the best quality resistor in R5 you can find. That’s my only advice as the impact on sound is significant.
26
The turn of the famous 26 DHT to provide a similar low gain stage. Here is the design:
The 1K2Ω cathode resistor will provide about 6mA when anode voltage is dialled between 140 and 150V. Alternatively you could add a 6-diode SiC array and adjust the bias point. I was asked recently a few times about the 26, and this is how I’d implement it with low gain (about 6dB)
2P29L
The 2P29L is a fantastic little valve. It’s cheap, accessible and provides a great sound and isn’t microphonic. I really love the 2P29L preamp I built and still have it with me. If you want a low-gain version, here it goes:
A 300Ω cathode resistor can be used and the above design will give you same levels of gain as before discussed.
4P1L
I forgot how many version of this preamp which I baptised “Siberian” over 10 years ago. The 4P1L really sings when you run it at 30mA and 150V:
A bit demanding in terms of HT current, but is a worth preamp to build. If you can tame the microphonic noise, it’s a fantastic valve in my view.
VT-25/10Y
Now, if you are a big fan (like me) of thoriated tungsten filaments and their sound, this valve is unique and sounds like probably no other valve (except 01a). Here is the low gain version:
Again, an array of 6 SiC diodes is possible in replacement of the 300Ω resistor.
CX-371a
If you want simplicity and don’t want to add a source follower, then you need to look at the low-mu valves. The CX-371a (or just 71a) is a perfect valve for this job:
I built one of this back in time and loved it. The above pre-amp is great and will deliver a gain of less than 10dB.
300B
The 300B can play great as pre-amp. Although I implemented in different ways in the past, here is how I prefer it now:
Simply with a 1KΩ cathode resistor, and biased at 30mA you get a very nice sound and extremely low distortion. The gain is just below 12dB.
46
I love the 46 preamp. The 46 (triode-wire) is as great as a 45. I could have included the 45 here as well:
With slightly higher gain at 14dB, this is a great stage.
Needless to say if you want less gain, you can add the attenuator and the source follower like previous designs in this blog post.
27
Wait a minute, 27? It’s not a DHT. Yes, of course. However, it’s a great IHT. I love the globe version with mesh anode. Have a few in my collection and would definitely put it back to use. The timbre and detail is unique.
This is as good as IHT preamp it can get. You can use many different triodes instead, your choice.
Hopefully you find this entry useful. I will add a few more since my list isn’t complete!