I was looking at my valve stock when I found the 2SH27L/2Z27L Russian universal directly heated pentode and asked myself the question of whether this could be a good candidate for a DHT pre-amp. This little fellow was developed alongside the 4P1L after the second world war and was intended to be operated by batteries.
If you remove the aluminium screening case, you can discover a lovely valve inside.
Looking at the key characteristics we can see that:
- Heater voltage: 2.2V
- Heater current: 57mA
- Anode voltage: 120V (200V maximum)
- Anode current 1.9mA (2.5mA maximum)
- Transconductance: 1.9mA/V
The heater requirements are minimum (120mW) so this could be easily run from batteries. I was intrigued by this valve working in triode-mode. How will perform?
Let’s have a look at the characteristic curves first:
The anode resistance is high, about 12K. The gain is also high for a DHT, so 16 is attractive for a pre-amp depending on the specific use/need.
What about its linearity? Let’s look first at the distortion at low signal levels (1Vrms output):
The distortion is very low, which is great for the preamp, however looking at the triode performance at large signal levels we can find that the linearity is not that great: 0.2% @ 10Vrms output:
Obviously the optimal bias point for low level signal is not good for large signal as grid current kicks in and distortion goes up. Limitations on anode voltage and current may impact our capability of pushing the bias point further right in the curve to avoid grid current al large signal.
If we look to reduce the anode voltage supply we can see that distortion increases but a good compromise point is Ia=2mA, Vgk=-2V and Va=72V in which THD is 0.026% @ 1V rms.
A 70V HT supply is more attractive, however if we think about using this valve in a preamp with a cathode follower output driver like this one, then the HT supply requirements will be enforced by the latter.
Finally, if anyone is interested in playing with this valve and simulate the preamp in SPICE, here is a quick model which I haven’t tested yet:
