4П1Л/4P1L triode curves

4P1L triode curves

4P1L pinout

4П1Л (or 4P1L) is probably one of my favourite valves. It was an unknown device to me until was suggested by some friends in the forum. Many discarded it as being a howling beast in pre-amp stages :). I found that albeit it can be microphonic, this can be controlled to a certain extent, but in my opinion this is a great valve in most of the roles: pre-amp, driver or output stage. Preferably is such a linear valve that can easily match 2A3 and 300B characteristics (when arranged in parallel) at a fraction of their cost. You can get a view of this beauty in the datasheet here.

Looking at the specifications, the key points to highlight are:

  • Filament Voltage: 2.1/4.2 volts @ 650mA/325mA respectively
  • Recommended anode voltage: 200V with 150V on the screen
  • Maximum operating voltage 250V anode or screen.
  • Maximum cathode current: 50mA
  • Anode dissipation: 7.5W
  • Screen dissipation: 1.5W

I became aware of this valve when Anatoly (a.k.a. Wavebourn) recommended the 4П1Л  directly heated pentode  which was used in military transmitters. It is very popular now among Russian audiophiles. Apparently is the Russian equivalent of the WWII era German Wehrmacht RL2 / 4P6 RF oscillator / transmitting amplifier tube. It’s a brilliant valve when triode strapped, better than 2A3 / 300B in terms of linearity. See my post around THD here and will see why 4P1L is at the top of the chart with less than 0.03% THD @ +22.22dBu!

As Anatoly suggested, they are very nice for class A in triode, and give up to 2.5W per valve when driven with up to +12V on control grid. It is easy to parallel them, since they are consistent and very linear: paralleling linear valves you are loosing power on mismatch, i.e. the valve with higher transconductance will draw more, no distortion raise caused by mismatch. 2A3, for example, has a pair of paralleled triodes inside. You can parallel ten of 4P1L matching them (it’s easy), to get 100W dissipation and 25W output.

Many found a sweet spot around Va=235V, Ia=40 mA, Vg=-18V providing Pout= 2.5W on a 5K OT, triode connected. Capacitance between anode and first grid for 4P1L is 0.1 pF. Capacitance between screen grid and control grid is about 1 pF. It has a  10 pF Miller capacitance which is not high value, and for 20 KHz it is slight less than 1MΩ impedance. Any driver with 10 mA idle current will make it happy.

4P1L Siberian DHT Preamp

I tested this valve a lot as a DHT preamplifier with great results. Starving filaments and suspending the socket with cord can reduce significantly its microphony to very low levels. I could listen to it perfectly fine whilst my friend Tony still have some issues with a 30sp DHT stage bolt to the aluminium top cover 🙂

I loved the sound of the 4P1L pre-amp. I will build a 4P1L SE in the future, is on my list…

4P1L Siberian DHT pre-amp

Recently, a friend from the diyaudio forum asked me for the 4P1L curves which I posted previously. Here is a new trace of the curves under the following testing conditions:

  • G2 and G3 tied to anode
  • Filaments are in parallel, so If=650mA powered by DC supply.
4P1L-triode curves


You can create your model or use the curves to produce your load lines, etc.

Hope this helps

2 Replies to “4П1Л/4P1L triode curves”

  1. Hey, I’m about to build a single preamp stage for my guitar amp and I have one 4P1L tube. I thought of building your “Siberian DHT Preamp” as im new to tubes and this simple circuit is just what I’m looking for. But in my country it’s really hard to get a depletion-mode mosfet like DN2540. Could you advise what other device i could use or maybe it’s possible to even omit it by altering the schematic a bit? Many thanks in advance.

    1. Hey Mangis,
      Not that easy to find a depletion mode FET sometimes. I used BSP129 which is easy to find in the UK. Alternatively, you can look at the enhancement PMOS gyrator option. I posted a circuit here.

      If you look on eBay you may find DN2540s though.
      Hope this helps

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