For those who are keen on the 26 preamp in filament bias and OT, here is my latest incarnation of this great pre-amp.
Tag: CCS
26 THD analysis
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:
Looking at brands, the following average THD ranking was produced:
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:
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!
Measuring Transconductance (Gm)
GM tester jig circuit
After a bit of work, got the transconductance jig working fine. Made an obvious omission which was not bypassing the CCS. The CCS present a very high impedance in AC to the circuit, therefore not developing the current variation on the measuring resistor. Bypassed by an electrolytic presents a path to ground.