Everyone loves this thoriated-tungsten DHT valve. I’ve only used it in a preamp and was hooked with its sound. Really warm and nice. Downside is, it’s very pricey these days and also is quite demanding from a filament perspective. You can check the characteristics here.
For those who like testing their designs with LT SPICE, I produced a model which matches really well the traced curves. Would like anyone to use this one, to drop me a note with any feedback 🙂
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!
From an audiophile’s perspective, this is not the right approach to determine the optimal operating point. However, minimum distortion is a good indication of a good starting point for further refinement with your ears.
I have used mostly the 26 DHT with filament bias in the following point:
Vg=-10V
Ia= 5.5mA
26 DHT THD as a function of Ia
We can see in the diagram above that distortion decreases with the increase of anode current (lower ra and higher gm) and between 6-7mA it’s at its minimum of 0.04-0.05% at full output swing/
As posted previously, is well known that starving the filaments is a good approach to reduce microphony of the valve and THD as well. At the expense of increasing Ra.
THD impact of filament starvation
From the picture above we can clearly see that a typical 26 running at 5.5mA and with filaments at the normal level (i.e. 1050mA) can achieve a reduction of distortion of about 0.02% by starving the filaments to 700mA (66%).
I still need to test how this level of starvation will sound on my preamp, but is quite promising…(at least in theory)
Having repaired the Pete Millett’s interface (hopefully) I tested two 26 DHT triodes I had at hand. One was an used Hytron ST valve and then the other test was an NOS White Whestinghouse ST valve, which is actually in pristine conditions.
Both valves were tested with the same operating point:
Vg=-10V
Ia=5.5mA
input signal adjusted to produce Vo=10Vrms (+22.22dBu)
26 Hytron ST valve THD @ +22.22dBu
26 NOS White Whestinghouse ST valve THD @ +22.22dBu
Well, I think I have re-vindicated the 26 DHT THD performance at a decent swing. Surprised to see the WW valve achieving 0.03% THD.
THD analysis for 12P17L in triode-mode (left-handed)
Looking at the THD for the 12P17L in triode mode (left-handed) using a CCS load and driving the input with the TEST SET oscillator to achieve the output at 10Vrms (+22.22dBu) to look at valve’s distortion in particular. This valve is more linear at lower currents (Ia=25mA) rather than an operating point to maximise anode power (e.g. Ia=50mA) as would be in an output stage:
THD @ Ia=50mA
Interesting to see that there is a point where there is a second harmonic cancellation and only H3 component is visible achieving very low THD (circa 1.3%):
Minimum distortion (H2 cancellation)
THD analysis
A compromise point to obtain maximum output power whilst minimising distortion was found to be:
Ia=35mA
Vg=-15.2V
Va=241V
THD=0.13%
This puts the valve under 8.4W anode dissipation. Looking at the specs you can see that anode dissipation is 7.5W and g2 dissipation is 2W.
12P17L THD at maximum output power
I’d rather operate this valve at a lower dissipation point.
