Just got a couple of NOS EIMAC which I will be using in my SE design
4-65a EIMAC NOS
So did some test on distortion, transconductance and driving them to +22.22dBu output to check the quality of these two ones.
I used similar test rig as before. At some point will be able to get a proper filament supply for this valve, but for the time being I will continue to use the hum pot and the big electrolytic cap across my old bench power supply which can gently provide the 3.5A for the hungry filaments!
I tested them at the limit of my CCS and bench HT supply which at the moment cannot provide more than 360V @ 100mA.
Transconductance is in the region of 3,800 – 4,000 μmhos.
So biasing the valve at -2.5V and over 90mA of anode current, the harmonic profile looks like this:
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.
CX301a DHT pre-amp
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:
CX301a preamp based on LL2745
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…
26 DHT pre-amp
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 preamp based on LL2745
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.
Quick test on a EIMAC JAN-8165/4-65a NOS. Here is the THD response at output level of +22.22dBu:
4-65a THD
Artefacts at 50Hz and above are all a result of a poor filament power supply 🙂
Nice to see this valve providing 0.14% THD at a low operating point: Ia=85mA, Va=246V. At least this is a nice test to show how linear this valve it is.
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…
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:
THD analysis of different valves
Looking at the chart above a couple of interesting points to highlight:
4P1L is the most linear valve I’ve found so far.
6e5P and 6C45 are expected to be on the top five anyway.
12P17L despite of having similar characteristics as 4P1L is not that linear
6N6P and 6N6P-I disappointed me. I thought the would be more linear..
46 and 47 in triode mode are superb drivers
Have so many other ones to test, but limited time….
Expect this chart to be updated in the future, so stay tuned 🙂
6CB5a, as many TV power pentodes, is a great valve for a single ended amplifier. Specially now that popular valves of similar anode dissipation are getting really expensive.
Thomas Mayer came up with a great design for this superb TV pentode that wired in triode mode is very linear and can sustain at least 25W of anode dissipation. If you want to check Thomas’ design, have a look at his website which is very interesting.
If you want to look at the specifications, you can download the datasheet here.
When testing this valve on the curve tracer it was evident the fantastic features and linearity at every point.
As a power valve, testing a 6CB5a for harmonic distortion showed THD=0.07% @ Vo=+22.22dBu (10Vrms) at the following operating point:
Ia=40mA
Vg=-50V
Va=253V
6CB5a RCA NOS THD
Transconductance in triode is high: 6.4 mA/V @ Ia=40mA, Vg=14.6V, Va=100V. In summary:
Gm = 6.4 mA/V
μ = 4.5
ra = 700Ω
Here is a set of triode-strapped curves that you may find very useful if you’re planning to use this valve:
6CB5a RCA triode curves
Also you may want to try the following spice model. It can be improved, but I think is an accurate representation of the curves shown above. Please try it and let me know what you think about it…
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.
Some 6N7 under test
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.
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!
So did some test on distortion, transconductance and driving them to +22.22dBu output to check the quality of these two ones.
