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!
Ale Moglia, hello!
Read this article for a lamp modes 6C45P
http://musatoffcv.narod.ru/Docs/6S45P.htm
Hi Svetlana,
Thanks for sharing the info. Google translator is not perfect but managed to get most out of it.
Clearly 6C45P is best with choke, transformer or CCS/Gyrator load. Very interesting analysis.
Do you live in Russia? Would be interested in seeing if you can get hold of tubes for testing over there which are more difficult to find around here…
Thanks
Ale
If the above is true then what is the optimal value and operational point for the 6C45p?
I am trying to build a new phono pre and want the lowest distortion with most gain for each of the two stages. LCR in middle.
My Idea: 30H choke @595ohm :::: B+ of 200V::::::Ck:2488uF :::::::: Rk:64ohm
As Svetlana kindly pointed out the THD table reflects the Ia in ampere, not ma.
Updated distortion measurements in the following post:
http://www.bartola.co.uk/valves/2012/06/17/thd-benchmark/
Hi Ale,
Thank you for your excelent job.
I have work on a set of THD like yours but ECC99, 6N6P, 6N30 and 6N1P tubes.
TUBE Va Ia H2 H3
ECC99 100 25 -37 -27
6N6P 100 25 -32 -21
6N30P 100 25 -43 -15
6N1P 100 10 -32 -22
6N6P and 6N1P seems very close, 6N30P has an extrange distribution (high h3 and very low H2) and ECC99 is best of all to my test and ears.
If I become less lazy, i would like to make a better setup with a Variac, CCS, 1kHz sine and FFT
What output signal did you run the tests? The H3 level is a bit strange for a triode, how many samples?
Best test setup is with CCS or gyrator. Soundcard and FFT.
Cheers
Ale