26 THD analysis

ux226 globe valves

As I’m proud of my 26 DHT pre-amp and also looking to use this valve as the first stage of my 4-65a SE amplifier shortly, I looked at how linear this valve is.

Unfortunately I don’t have an extensive set of valves of same brands, so albeit I have about 40 valves if this type there is a big mix of different brands and many of them are used ones. Having said that, I think this may be the case of many of you out there, so I think that probably the results of this test may be relevant to you (if you are still reading this post).

So in summary, this is the sample set I tested:

  • 39 valves
  • 12 brands
  • ST (34 valves) and Globe (5 valves)
  • Type: 26, 226 and 326
  • State: used (80%) and NOS (20%)

All DUT were tested with the same test set and operating point:

  •  Operating point: Ia=5.5mA and Vg=-10V
  • Anode load: Cascoded DN2540 CCS set to Ia
  • Test signal: 1kHz, Adjusted to obtain Vo=10Vrms (+22.22dBu)
  • THD analysis: Audiotester via Pete Millett’s interface:
    •  32768 sample FFT (2.96Hz resolution)
    • 256 averaged windows
    • Van Hann window
    • THD for H2+H3+H4

So in summary the results showed that you should expect a 26 to have around 0.08% (Std  Dev = 0.00047). Here is an histogram showing a summary of the tests done:

26 THD histogram

Looking at brands, the following average THD ranking was produced:

26 THD by Manufacturer/Brand

Interesting to find National Union leading the chart. I found RCA and Sylvania to be my preferred ones in terms of sound. THD shown above are average of sample sets of 4 valves or more. Although I tested 12 brands in total, some of them were just a pair so they are not a representative subset.

Finally, here is a sample THD of a very nice 26 valve:

26 DUT

 

I wish I could have a bigger collection of 26 to improve the accuracy of this statistical analysis. Either way you can get a view of what you should expect from this great valve…