The low gain DHT pre-amp library

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:

01a low-gain preamp with cathode resistor (V6b)

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:

26 low-gain preamp

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:

 

2P29L low-gain preamp

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:

4P1L low-gain preamp

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:

VT-25 low-gain preamp

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:

CX-371a low-gain preamp

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:

300B preamp

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.

27 low-gain preamp

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!

Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

15 thoughts on “The low gain DHT pre-amp library”

  1. Hi Ale,

    All the above schematics employ cathode bias rather than filament bias.
    Does this reflect a new preference?

    Best regards,
    John

    1. Hi John
      I got asked that question a lot. Unfortunately the answer is depends. If you don’t have a mu-follower topology (like with the gyrator) then you are limited on what you can do. I found filament bias (as many others) to sound much better than the classic RC, mainly due to the cap’s influence.
      Now, filament bias con be impractical (45, 46, 300B, etc.) and in the hybrid mu-follower there is a clear advantage to not bypass the cathode resistor. I found using the SiC diode array to sound particularly good even not in filament bias. I couldn’t notice any difference. However, more than 6 diodes isn’t the best route to go. So there is a compromise. For higher bias, the simple good quality cathode resistor is the way to go. I have it in my 47 driver stage and would not change it. Same for a VT-25/801a stage, you are approaching the limit on what you can do with filament bias.

      Years ago I implemented a 46 with filament bias, yes it was nuts. A real heating system for my listening room. It was great but wouldn’t do it again.

      Hope this helps?
      Ale

  2. Hi, Ale,

    Thanks for the wonderful summary article. One question, have you compared 01A with very good line out transformer with your version with Gyrator? Are they about equal sonically?

    Thanks.

    Wayne

    1. Hi Wayne. Not easy to answer as it depends on many things. Every time I did a side by side comparison, my mu-follower implementation sounded better hence I ended up exploring this topology for so many years. Having said that, if you use an extremely good choke or OPT (which demands a big budget) then you get a very close sounding stage in my experience.

  3. Very interesting stuff here! Well documented. I stumbled upon your page cos i am toying with the idea to make a 45 preamp , driven by an EL3 tube. You got any schematics for something like that?

  4. Dear Ale,

    Great introduction to the DHT preamps! I have 2 foolish questions to ask:
    1. Does the gain of the preamp affect the size and dynamics of the soundstage?
    2. Does the quality of the chokes and other transformers (non output) affect the quality of the sound much? Or rather should I be investing money in the non output transformers/chokes?

    Thanks!

    Regards
    GH

    1. Hi
      Thanks for feedback. I’ve been struggling with the website as have had a virus infection on the hosting server. Not easy to deal with when you’re not an expert.
      to your questions:

      1. I have not perceived any difference in dynamic response. Personal judgement, however when levels and gain is right – overall listening experience becomes optimal. I’d say you can get to perceive better the dynamic responses / transitions this way.

      2. I always say that with the hybrid mu-follower (gyrator load) configuration you get a fantastic PS noise rejection, hence a big big isolation from the HT supply. A well designed supply with ample current capacity and fast enough to handle the load should be fine. Now, the filament supply is a different matter. I’d spend money if you can in good chokes. Look at the designs I published in the blog to give you an idea. The filament (cathode) is much more sensitive to noise and overall sound impression varies big time. Also if you can get hold of EI transformers with split-bobbins between Primary and Secondary that is very useful to keep the leakage capacitance low from the Mains and reduce interference.

      Good luck
      Ale

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