6Ж49П-ДР/6J49P-DR High Gain Stage

Time ago I wrote about this sterling Russian valve. It’s extremely linear in triode mode, sounds superb and isn’t microphonic. My friend Paul LeClerq has used it as first stage of his guitar amplifier and is delighted. A real dormer one. I hope it doesn’t disappear when valve hoarders go out and grab every big lot of valve that exists. Anyhow, I have more than I need myself, so I’m not worried.

The triode driver

Well, the easiest implementation is the triode driver. In triode mode this valve has a μ of about 50. So you should expect a maximum gain of 33-36dB. Very useful in phono stages (2nd stage), or as a driver for high demanding output stage (i.e. transmitting valves). This lady can swing 150Vpp at only 0.13%! The penalty of course is the Miller capacitance.

Here are two snapshots of their frequency response using the gyrator load:

The above test is with a BF862 FET. Let me clarify that I have added a 1μF as C1 in the board as I was intending to do tests in pentode mode (as you will read later below).  If you use instead the new Rev07 PCB which can fit BSH111BK FETs instead, then you will get a bit more bandwidth as the transconductance of this FET is higher than the BF862 at same anode current:

 This is fantastic. However in triode mode it can’t be used in a phono stage. I haven’t measured the Miller capacitance of this valve, but I’d expect the resulting input capacitance to be higher than 200pF. 

The Pentode Driver

Ok, now you want to use a pentode driver in a RIAA stage. Here’s the testing rig I’ve used for this valve:

Actually C1 is 1μF as I mentioned before. I conducted a series of tests with different gain setups by simply changing RA1:

The 68KΩ resistor provides a gain of gm⋅RA1 or about 52.3dB for a degenerated gm of about 6.2mA/V. Without the SiC Mosfet follower output buffer you get a bandwidth up to 87kHz. Very good indeeed. If you want to extend this a bit further, you can add the follower to get up to 100kHz.

There is a dip about 2kHz which I can’t explain why. Perhaps is the valve itself, as you can notice some bump on the triode mode test. I should re-run the tests with other samples to diagnose where this is coming from. 

You can bump up the gain by increasing the value of Ra1.  Here is the response of a 100KΩ resistor:

You can get 54.5dB but HF pole is impacted slightly down to 68kHz without the buffer. 

Now, let’s see the performance of a 60dB stage. You will need to increase Ra1 to 270KΩ: 

You can see that at this gain level the response is limited further down to 38kHz. The dip is a bit more evident with about 1dB difference. Yet, this stage is fantastic at 60dB gain with still enough bandwidth to perform well in a RIAA stage. The input capacitance of this valve is less than 10pF (close to 8pF) in pentode mode. So it can be used with MC cartridges.

If you add the buffer you can increase the bandwidth to 45kHz.

The distortion in pentode mode is high. However, for lower signal levels is good enough. As a reference see the output of 2Vrms:

The disctinct H3 signature of a pentode is present. At lower levels like 500mV in a fist stage of the phono preamp we should expect THD to be around 0.15% maximum. I think it would do a good job as first stage.