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.

12P17L curves and Spice model

A great russian pentode valve similar to 4P1L, but with indirectly heated cathode. You can check the valve specifications here.

I tested transconductance in left-handed triode mode: Gm=9.6mA/V @ Va=150V, Ia=50mA , Vg=-5.6V.

Here are the curves for this valve:

For the ones who want to test the SPICE model here is my take on it:

You can try the model and please let me know your results! You can always email me

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

CX301a DHT pre-amplifier

CX301a DHT preamp

Here is my latest incarnation of the DHT pre-amplifier:

Many claim that the 26 is the best sounding DHT valve for a pre-amp. I will agree to a certain extent, however I personally found the thoriated-tungsten filament sound a bit more rewarding to my ears. A more clear and defined treble in my opinion.

Since I plugged in my CX301a incarnation of my breadboarded preamp, I just left it there as I loved its sound. Certainly there are things to be improved to enhance the dampening of microphony, albeit I have to confess it hasn’t been a problem to me. Have heard some valves to howl, and this is not one of those. Clearly suspending the valve socket or adding the rubber dampers to the valve holding plate or socket will help massively.

Filament bias is a must in my DHT designs. Since discovered it, can’t avoid not removing most capacitors that I can from the signal path. In this case the filament resistor R9 will increase anode resistance by R9 times (μ+1). This will also impact the stage gain, but here all this is not a problem. You may find this is way too much gain in your system. Rod Coleman’s filament DC regulators are crucial to provide a hum-free stage. Attempting AC or other DC regulator is likely to bring frustration to your design. Believe me, I’ve been there before…

Now turning our attention to the anode load I will not open a debate here (or a can of worms!). You can make your choice of using a superior quality output transformer (and by superior means a lot of money!) or you can look at various options. A choke is a great idea, but special care needs to be taken to ensure choke is not picking up any hum from the remaining parts of the circuit – specially the supply transformers, etc. I have experimented for some time various types of CCS or gyrators as sandy loads for the valves with excellent results. If you are one of those that feels that sand is a sacrilege, then I suggest you stop reading this post now.

Gyrators are superb. They can simulate the AC response of an inductor of 300H (but without storing energy as a real inductor) or above very easily at 1/100 of its cost. You can easily adjust the valve operating point ensuring this is maintained despite the ageing impact of the valve or the eventual replacement of it. The anode voltage will be fixed by the gyrator, the current not. Cascoded MOSFET gyrators provide better supply ripple rejection and isolation. Using Q3 as a CCS instead of a high resistance potentiometer to set the anode voltage is better as it helps providing a better frequency response as impedance on this node is increased. A higher value of R10 will help reducing the size of the gyrator capacitor and the smaller the better it will sound in my experience.

M1 and M2 can be your depletion FET of choice. M1 should be a 250V rated one at least. Depending where you live, you will be inclined for using BSP129, LND150 or DN2540.

Previously I mentioned in some other posts that the mu-follower setup of the gyrator here provides a better output impedance and improves the performance of this valve significantly given its high anode resistance compared to other more suitable DHTs for this purpose such as 4P1L, 46 or 71a.

I’m not going to cover the HT supply here, but using a shunt regulator such as Salas, is one of the best choices here.

With Russian PIO capacitors you will get a fantastic result here, no need to start burning serious money on the capacitors until you are happy with the end to end build and you can then start looking at how to improve the sound of it by replacing some bits with better (or preferred) quality components

With an operating point of Ia=3mA you can get THD=0.08% at Vo=10Vpp. This will be subject of the quality of your CX301a. Some older globe 01a’s have a great sound, but they are not that linear. Hard to pick and chose your precious ladies here without testing them for linearity.

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!

01a preamplifier

This is a superb preamp which I originally breadboarded and sounded so nice that never moved it to a proper chassis. The 01a exceeded my expectations. It’s running with a MOSFET gyrator with mu-follower output. Operating point can’t be definitively be improved as filament bias resistor at hand dictated the bias point. Anyway, sound is clear, very warm and with great tone. Better than 26 in my opinion, but have to do side by side testing to make final call.

I think I posted circuit before, if not let me know if your are interested in it

Thoriated tungsten filament have something special indeed

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.

Finishing the curve tracer

Today I did a bit of extra work on the curve tracer with a view of finishing it. It has been a long and painful journey, but I’m reaching the end of it.

The transconductance tester is working perfect. I need to use the following ranges in my true RMS AC voltmeter:

0-2,000 μmho: 100mVrms scale
2,000μmho-50,0000μmho: 1Vrms scale

It’s probably the DC bias which affects the low scale. As an example when testing a 46 in triode mode (see datasheet for details), I tried the following operating point: Vg=-33V, Ia=22mA and the measure should be around 2.35 mVrms over 220mVdc. But in my bench voltmeter, above 17mA in the 46 doesn’t like it and cannot measure it, so need to change scale. I tested low transconductance valves in the lower AC scale such as CX301a, 26, 4P1L, 71a and then using the high AC scale, used 6e5P, 6C45, 6N6P amongst others.

The tracer now has a common-mode mains filter. This was required as at certain times during the day, specially in the evenings when the mains is really noise or my wife is using the microwave oven!, when tracing curves with the 1Ω sensing resistor and low anode currents (e.g. CX301a) then the noise level was sufficient to impact and distort the traced image. With the common-mode mains filter it works brilliantly.

Now need to place bottom plate and standing feet. Job done then and will move to some proper audio work!

Testing the circuit today, I measured 29 46 valves. Ended up discarding two which measured low and then when tested with the tracer found that curves weren’t good at all. Probably electrode misalignment as they weren’t just with low transconductance. Will upload some examples as it’s very interesting to see the difference