All about electronic valves and hi-fi
Finally, here it is.
Improved sound compared to LL1660 version. This OT (LL2745/8mA)is better suited for the 26 in this configuration. Sacrificed output impedance a tad to get more gain. Filaments starved at 760mA. Distortion is as low as 0.02% @ 3Vrms output.
First listening session with Mingus, exceeded my expectations. Rounder bass and more space. Like this sound!
Will do some proper measurements but for now will enjoy and listen to it…
Today made significant progress on the new version of the 26 pre-amp. Only bits outstanding are the CCS modules part of the HT supply on board. Tested glow valves, output transformers and filament circuits…
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Today made some progress in building the modular pre-amp using my latest design on the 26. I’m planning to make this preamp a modular one to enable quick changes and replace pair of valves and OT. This will enable to use 01a and 4P1L as well…
I used a floor plank I found around as the base. The front is a simple aluminium piece that holds the stepped attenuator. Both 4-pin sockets are standing with the rubber suspensors that DHTRob kindly sent me. The two resistor arrays for filament bias are placed between the valves and the OPTs.
There is plenty still to be done: connectors, filament regulators and HT bits. Once all drilling is done, I will then move onto wiring…
@DHTRob: I’m still thinking on what cartoon can draw for you. For the time being, here is one based on recent listening tests 🙂
Today I managed to finally test the Lundahl LL2745/8mA specially designed by Lundahl for Thomas Mayer for a 26/01a preamp / line stage.
The circuit tested is here.
Initially did some tests with fixed bias, normal DC heater supply from my workbench and HT from a passive capacitor multiplier also available in my workbench. Circuit breadboarded has long cables and we shouldn’t expect good 50/100Hz noise levels as have many transformers and things around 🙂
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As we shown earlier in other tests, filament starvation reduces THD slightly as expected. The OPT performs really well. Probably will look at starving a bit more filaments whilst doing a listening test
Will now proceed to rebuild the 26 DHT preamp with LL1660 with this circuit 🙂
Before breadboarding a pre-amp with these nice OT provided by Thomas Mayer, I decided to simulate some options in LT Spice to see what results I got.
First one is my preferred CX301a thoriated-tungsten DHT. I wired the LL2745 in 5.6:1 step-down configuration. This should provide a low output impedance which is what we want in this configuration where we need to drive the cables to the amplifier with sufficient capability:
Looks very promising. Of course gain will be much lower than a gyrator-based pre-amp, the 01a anode load is optimised providing very low distortion: 0.015% based on my SPICE model with curves taken from real CX301a. Gain is low at 3.1dB, but we don’t want loads of gain in this pre-amp. Sound is what we are after…
Now it’s time for the revered 26. I used Dmitry’s model based on the RCA manual curves. I’d like to simulate this again using a model based on starved filament curves with a real 26.
Originally planned to bias the 26 in a different operating point based on feedback from Andy Evans, however after playing a bit with the OP I found that a more linear point was around Ia=5mA and Va=114V @ Vgk=-6.85V
26 looks more interesting in principle as the output impedance with this model is lower than the CX301a thanks to having a lower Ra (7KΩ against 11KΩ) so LF response will be slightly better in a side by side comparison.
Will be trying these two with filament bias and Rod Coleman’s filament regulators. The HT will be provided through a Salas HV shunt regulator.
Stay tuned…
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:
All DUT were tested with the same test set and operating point:
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…
I’m still in the process of testing valves, here is how the ranking is coming up so far. This is a mix of driver and output valves. All tested at Vo=+22.22dBu:
Looking at the chart above a couple of interesting points to highlight:
Have so many other ones to test, but limited time….
Expect this chart to be updated in the future, so stay tuned 🙂
I have tested more than 10 different ST and metal 6N7. Some GT, other simply old ST G ones and metal as well. Both triodes in parallel as usually this is the configuration used as an amplifier driver. Found a good operating point from a distortion perspective around Ia=6mA, Vg=-5.6V. As you can see you should expect getting around 0.09% THD. With some good valves reaching as low as 0.04%, but will have to be hand-selected.
Great driver from a sound perspective, with low distortion close to a 26 and on average slightly better than the 6J5. Need to review famous 6SN7, but there are lots of measurements for this one out there.
