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:
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:
- Treble is very clear and sharp
- Tone is very detailed and clear – I like this valve
- 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.