New Lundahl LL2745 OT pair arrived today in the post. Very excited, Thomas Mayer kindly send me this pair for trial. These are specifically designed for the low current and high anode resistance DHT used in pre-amps (e.g. 26, 01a, CX112, etc.). Wiring and primary details are similar to the LL1689. Hope we can get a final datasheet in September 🙂
These are gapped at 8mA. So the primary inductance is about 200H in theory (160-180H in practice according to Thomas). The labels are incorrect. This is 2×2.8:4×1, providing the option of wiring it as 5.6:1 if both primaries windings are in series and all secondary windings are in parallel.
Will rebuild the 26 or 01a preamps and do some test.
I would like to build a quick breadboard with the 01a to do a comparison against the gyrator loaded preamp I’m using at the moment.
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…
