Measuring triode linearity

Today decided to do a quick distortion test of on a sample of a variety of different valves. All either triodes or triode-strapped pentodes/tetrodes. As per my previous tests, distortion was measured at +22.22dBu (10 Vrms) at the output of the valve in common-cathode mode. Valves were loaded with the CCS I use in my curve tracer. The operating points were quickly optimised at hand, so I’m sure there may be some better operating points for some of the valves below which may improve their overall THD. If you have any suggestions, please let me know!

Need to retake these measures as the soundcard interface got damaged and results are showing significant distortion

Interesting to see in the chart above, that 6e5p and 6C45p are the best ones. This is in line with their reputation as drivers as they are capable of swinging many volts and producing very low distortion. In terms of harmonics I noticed that 6e5P provides a richer H3 and H5 as being a triode-strapped valve, whereas the 6c45p provide a dominant H2.

Also good to see that my favourite 46, 4P1L and 6CB5A (all triode-strapped) are very linear with anode currents of 40mA (with the exception of 46 as I measured THD on a previous operating point used for transconductance measurement). I should retake the 46 and drive it harder, I’m sure it will perform better at higher current.

Surprised with the results of the 6N6P-I. Was expecting this one a bit better, but perhaps it’s the pulse version distortion, so may need to get hold of an 6N6P and compare the results.

Update:
It looks like I blew up Pete Millett’s interface after measuring THD in float mode and exceeding the 10Vrms limit in this mode. Therefore measures such as 26, CX301a and others are not accurate. When testing 26 with my Ferrograph test set it came out to be 0.05%…
Stay tune until I repair the unit!