After a long delay, I finally did what a friend asked me. Test the lovely 955/VT-121. This acorn-type RF valve has a 6.3V filament, 1.6W maximum anode dissipation and 250V maximum anode voltage.

As per Paul’s recommendations, I looked at minimising distortion around 6mA/250V.

Here is the distortion using a DN2540 Cascoded CCS load driving the output to 10Vrms:

And here is the distortion at 1 Vrms output level:

This little fellow is a very linear valve at low signal levels and still performs really well at 10Vrms output when THD is only 0.11%. A great candidate for a preamp so would be keen to read some comments around the sound of it…

Edit

Again, as per Paul’s request, I looked at  what the THD was for lower biasing points. The operating point was set for Ia=6mA and distortion measured for small signal, i.e. output voltage of 1Vrms as shown in the table on the left.

It’s very interesting to see that there is a minimum at -1.4V Below this grid current effect increases the distortion sharply as expected:

 

What is more interesting though, is the harmonic distribution change. H2 is reduced whilst H3 it is not. H2 and H3 levels are equal. Wonder if this is the residual harmonics from my oscillator 🙂