2Ж27Л / 2Z27L DHT Preamp

Back in 2012 I did some experiments with this Russian valve.  The 2Ж27Л / 2Z27L is a fantastic valve. I discovered that if I run it hot, it’s extremely linear and non-microphonic. Bingo! With its highish μ of about 16, it’s ideal as preamp stage or driver. 

Here is my implementation for an DHT preamp (as usual with the gyrator load):

This is very similar to the 2P29L which is described here. You only need to change a resistor on the Rod Coleman regulators and you’re done. It took me 15min to do the upgrade, I wish a mod in a preamp could be done every time like this. The loctal pinout is the same. The anode voltage is reduced to about 116-120V to push the valve to the maximum cathode current which is about 4mA. 

The result is great, check out the distortion profile:

2Ж27Л / 2Z27L THD @ 2Vrms output
2Ж27Л / 2Z27L THD @ 5Vrms output
2Ж27Л / 2Z27L THD @ 10Vrms output

Distortion is mainly H2 and below 0.03% for a 10Vrms output. This is very nice.

When you tap hard the valve you get some microphonic noise at about 2.5kHz. Surprisingly, when I built it on my Mule preamp, no noise is picked up! It’s extremely quite for a 24dB DHT stage! Nice discovery.

The frequency response is good, see below:

Flat from 5Hz to 170kHz (-3dB points). Expect worse response when fully loaded. The only disadvantage of this valve is the 4mA maximum current. If you need, you will have to add a FET follower at the output to drive more demanding capacitive loads. 

How does it sound?

I will have to get back to this a couple of weeks later when I manage to spend some proper time to listen to the stage. The initial impressions are (after listening Freddie Hubbard’s Open Sesame record for some time:

  1. Treble is very clear and sharp
  2. Tone is very detailed and clear – I like this valve
  3. Bass not as powerful as a VT-25/10Y, well what can you expect with only 4mA through the BSH111BK FET 🙂

What a great valve…

Will report back in some weeks.

One Reply to “2Ж27Л / 2Z27L DHT Preamp”

  1. Despite it measures really well and sounds great, I found it a bit tiring after listening to it for some time. There is something in the treble that I don’t like. Specially when listening to any brass arrangements. I did not measure any significant H3, so am puzzled. Well that is why listening tests provide the ultimate veredict!

    I still think it would be a nice valve for a final stage in a RIAA preamp though.

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