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

46 RCA (triode-strapped)

[singlepic id=3 w=100 h=75 ]

Cunningham CX301a

These RCA Cunningham CX-301a have short pins so had to use an external socket to trace the curves. They look really nice and the pair I got measure nearly 100%!

Measuring Transconductance (Gm)

GM tester jig circuit

After a bit of work, got the transconductance jig working fine. Made an obvious omission which was not bypassing the CCS. The CCS present a very high impedance in AC to the circuit, therefore not developing the current variation on the measuring resistor. Bypassed by an electrolytic presents a path to ground.

Listening to the 45 Single Ended (SE) Amplifier

A perfect and warm sound

Hard to describe in words what I felt when I played first time “Goodbye Pork Pie Hat” from Charles Mingus. Perhaps it was the due to the fact that I play saxophone, but inevitably for me this song is one of the best ones to test the response of any amplifier. Sax duets, solos, a great bass line and varied moods along the way can take the amplifier to their best of their abilities. Being a jazz fan, couldn’t help myself when listening this amplifier for the first time. I used to play an EL34 push-pull, and liked its sound and power – specially for rock. But when it comes to the first watt, this amplifier is breathless. Pure and a warm tone. Playing this song made me understand all the fuzz about single ended amps. But, far from opening the debate about that, I’m sure that a 45/46 in push-pull has its own merits and would probably will one of those amps I’d like to build in the future.

Having a FE167E full-range driver that have a relatively high efficiency (94dB/W), the 45 SE doesn’t sound low. It has its own personality. Brilliant for jazz, it may need a tad of power perhaps when playing some rock and classic stuff.

It took me some time to optimise the driver stage. Started with an SRPP, then a paralleled 6SL7 to an 6J5 loaded with a gyrator in mu-follower achieving less than 0.20% THD at 150 Vpp. A great driver in my opinion. Now need to replace the 6J5 for the lovely 7193.

It looks like a Frankenstein valve, but it’s a lovely thing to me. Many claim that it’s a superior valve compared to the 6SN7/6J5 or even CV1988.

Adding the 7193 will require opening my 45SE and not having music at home for some time so at the moment is not what I have in mind.

Perhaps when I get the 4-65A up and running? It will take some time for that…

DHT preamp evolution

My 01a DHT preamplifier has performed flawlessly over the last 4 months or so. I do enjoy its warm sound and clear tone. I do prefer it to my 26 OT preamp, despite everyone says the contrary. I personally feel that the thoriated tungsten filament gives some sonic unique mark to the sound here.

I want to do the following test and compare differences in sound:

1) Gyrator load
2) Antitriode load
3) Choke
4) OT

Have my LL1660s which can take out temporarily from the 26 preamp, and also got a couple of chokes to use as well, oil caps, etc?

What do you think?

Updated 6e5P SPICE model

Having fixed the bias offset problem in my tester (actually my oscilloscope). I took again the curves this evening to get a better model….

If you want to read how did I manage to get here, please read this post