26 DHT THD (continued)

Looking for the optimal operating point

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 operating point — 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.

26 DHT operating point vs filament current — 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)

Some distortion tests on the 26 DHT

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 THD 1 — 26 Hytron ST valve THD @ +22.22dBu

26 THD 2 — 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.

12P17L THD analysis

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:

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%):

12P17L thd3 — Minimum distortion (H2 cancellation)

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 3 — 12P17L THD at maximum output power

I’d rather operate this valve at a lower dissipation point.

Curve tracer finalised

After a long process, here it is. The curve tracer is finalised. It includes the following features:

Valve curve tracer

Sockets: 4 pin, 5 pin, 7 pin, octal, loctal, 9 and compactron (10 and 12 pins)
Anode sweep: 0-330V
Anode current: 100mA (max)
Grid step generator: 8 steps, 0 to -80V and 0 to -5V steps
Grid output for calibration
Oscilloscope: X (x10 attenuation) and Y
Y amplifier:

x1/x10 differential amplifier
1Ω / 10Ω sense resistor
Polarity inversion optional
Image sync adjust (coarse and fine)

Valve transconductance tester & THD meter

Anode current CCS 0-100mA (0.1mA resolution)
Anode voltage 0-600V (1V resolution)
Grid bias: 0 to -80V (0.1V resolution)
Transconductance meter:

0-2,000 μmho scale (1 μmho resolution)
2,000 – 40,000 μmho scale (100 μmho resolution)
Input test signal: 100mVrms @1kHz

THD meter:

Soundcard I/O BNC connectors
CCS load or external load

Valve THD analysis

Measuring triode linearity

Today decided to do a quick distortion test of on a sample of a variety of different valves. All either triodes or triode-strapped pentodes/tetrodes. As per my previous tests, distortion was measured at +22.22dBu (10 Vrms) at the output of the valve in common-cathode mode. Valves were loaded with the CCS I use in my curve tracer. The operating points were quickly optimised at hand, so I’m sure there may be some better operating points for some of the valves below which may improve their overall THD. If you have any suggestions, please let me know!

valve thd analysis table — Tested valves for THD

Need to retake these measures as the soundcard interface got damaged and results are showing significant distortion

Interesting to see in the chart above, that 6e5p and 6C45p are the best ones. This is in line with their reputation as drivers as they are capable of swinging many volts and producing very low distortion. In terms of harmonics I noticed that 6e5P provides a richer H3 and H5 as being a triode-strapped valve, whereas the 6c45p provide a dominant H2.

Also good to see that my favourite 46, 4P1L and 6CB5A (all triode-strapped) are very linear with anode currents of 40mA (with the exception of 46 as I measured THD on a previous operating point used for transconductance measurement). I should retake the 46 and drive it harder, I’m sure it will perform better at higher current.

Surprised with the results of the 6N6P-I. Was expecting this one a bit better, but perhaps it’s the pulse version distortion, so may need to get hold of an 6N6P and compare the results.

Update:
It looks like I blew up Pete Millett’s interface after measuring THD in float mode and exceeding the 10Vrms limit in this mode. Therefore measures such as 26, CX301a and others are not accurate. When testing 26 with my Ferrograph test set it came out to be 0.05%…
Stay tune until I repair the unit!

46 THD analysis

GM tester modified today to add an option to disconnect the CCS bypass capacitor so can drive the valve with an external signal and measure THD from output in common-cathode mode. The input is calibrated to produce 10Vrms (22.22dBu) at the anode and then signal is fed into the PC through the Pete Millet’s interface:

Did some sample tests with a set of lovely globe CX301a achieving THDs from 0.27% to 0.35%.

When looking at a driver valve such as 46 (triode-strapped) got THD values of around 0.05-0.09% for good valves. When picked up the faulty one I had discovered yesterday with the curve tracer, the THD shown to be 0.20% and over 0.35% in the worst one.

Need to retake these measures as the soundcard interface got damaged and results are showing significant distortion

The PC generates a low distortion sine wave which is fed into the valve grid through the input capacitor. This is the same setup used for the transconductance test. The CCS in the anode is unbypassed to ensure the anode signal is not shunted to ground. The output is then taken out through the output PIO capacitor and fed back into the PC input adapter (Pete Millett’s sound card interface). Audiotester is used then to measure THD at 1kHz.