All about electronic valves and hi-fi
A simple PMOS gyrator for driver stage anode load
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This is a very simple PMOS based gyrator with only one transistor. You can use a Zetex ZVP0545a which is easy to obtain around here and is not a surface mount type (in case you don’t feel confident in working with SMD components).
Steps to complete the design on this gyrator are as follows:
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Quick test on a EIMAC JAN-8165/4-65a NOS. Here is the THD response at output level of +22.22dBu:
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:
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…
A warm and subtle tone, CX301a…
Superb sound!
When your mind is on some other place (not at work really)
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 🙂
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:
Transconductance in triode is high: 6.4 mA/V @ Ia=40mA, Vg=14.6V, Va=100V. In summary:
Here is a set of triode-strapped curves that you may find very useful if you’re planning to use this valve:
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.
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!
