Category: Uncategorized

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…

 

CCS load for THD measurements

Here is a simple point to point soldered cascoded MOSFET CCS using the classic DN2540. A very simple design: carbon grid 1K resistors and two potentiometers I had at hand: 2K (coarse) + 100Ω (fine). I can set the operating point of the valve under test from 3mA to 100mA. The anode output is directly coupled to a BNC connector which is hooked to the Pete Millett’s interface. No capacitor used as the interface has a DC blocking capacitor.

 

I used an old aluminium box and build this takes less than 30min!

 

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46 THD analysis

GM tester modified today to add an option to disconnect the CCS bypass capacitor so can drive the valve with an external signal and measure THD from output in common-cathode mode. The input is calibrated to produce 10Vrms (22.22dBu) at the anode and then signal is fed into the PC through the Pete Millet’s interface:

GM/THD tester

Did some sample tests with a set of lovely globe CX301a achieving THDs from 0.27% to 0.35%.

When looking at a driver valve such as 46 (triode-strapped) got THD values of around 0.05-0.09% for good valves. When picked up the faulty one I had discovered yesterday with the curve tracer, the THD shown to be 0.20% and over 0.35% in the worst one.

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

 

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46 THD analysis

The PC generates a low distortion sine wave which is fed into the valve grid through the input capacitor. This is the same setup used for the transconductance test. The CCS in the anode is unbypassed to ensure the anode signal is not shunted to ground. The output is then taken out through the output PIO capacitor and fed back into the PC input adapter (Pete Millett’s sound card interface). Audiotester is used then to measure THD at 1kHz.

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