The 2K2M is an unknown directly heated pentode from Russia with has variable mu. I have several pieces of these, and was wondering how useful they could be. This is part of my on-going saga of the “Mule“.
There is not much information around this valve. Here is the data sheet and here is in Russian:
It is also interesting to read this “confidential” report from the CIA about this valve.
The filaments have low requirements: 2V and 60mA. The maximum anode voltage is 120V and transconductance is about 1mA/V. Anode current is about 2mA. The internal resistance in pentode mode is about 1MΩ .
The first test I conducted was in pentode mode, albeit being vari-mu, you should expect a high distortion, H2 in particular:
The circuit is simple. A gyrator load with a load resistor (RA) as explained on this post. The screen regulator is actually a gyrator PCB modified with the addition of a current limiter transistor. The output follower is added given the high anode resistance and limited current drive capability. This was already covered in the previous post here.
So, a simple test was made with this valve in pentode:
The minor dip at 1.5kHz wasn’t noticeable in other sample valves. Either way, you can see a nice 56kHz bandwidth for 32dB gain. The gain is set by RA, 68kΩ in our case and the transconductance.
Let’s see the distortion profile:
As expected with a vari-mu valve, the H2 is high. We get 0.33% for 1V, which ramps up to 0.65% for 2Vrms. If you would like to use it as second stage in a RIAA amp, then probably it will add too much H2 distortion in my opinion.
Now, for a guitar FX stage, this may be an interesting valve in pentode mode.
Well, what about triode?
We always need to experiment and see if we can get better performance out of them. Let’s see.
Wired as triode, the gain is about 16-17. Very nice for a DHT:
A lovely 180kHz bandwidth for nearly 24.5dB. Not bad at all!
Well, let’s see the distortion in triode mode then:
Which is very low 0.0081% for 1Vrms output, so for 10V is still below 0.05% which is outstanding:
the harmonic profile is what we want. Hey, I think we have another candidate for the Mule. I should wire it up and listen to it, hopefully soon.
