Yesterday finished to upgrade my THD test box by adding the screen bias section I needed to test the 24a tetrode. Here are the details
Category: valve tester
4P1L PSE tests
Testing parallel DHTs
After listening to a great incarnation of the 4P1L PSE in filament bias output stage from Andy Evans, I decided to have a look at the impact of unmatched pairs of triodes from a distortion point of view. Main reason was that when listening to Andy’s amplifier I noticed a bit of an uncomfortable treble with some strings. Perhaps the increase of odd harmonics, but wanted at least to see what was all about.
4P1L are very easy to match. you can easily get a pair with equal mu. Just randomly I picked from my collection a pair of valves with a difference of 0.5 in mu.:
THD is about 0.03% mainly driven by H2. It happened that one 4P1L from the pair had 0.02% where the other had nearly 0.04% distortion. The difference between H3 and H2 is about 8dB.
Then looked at a more closely matched pair (0.03 mu difference). The distortion wasn’t surprisingly different:
Again, nearly 0.03% and difference between H2 and H3 is down to 7.5dB.
Looking at the individual performance of the 4P1L, now biased at 30mA and similar anode voltage, we can see that despite having a lower THD, the difference between harmonics is just 5dB. This is the THD of the other 4P1L from the pair:
Well, how rthis compares to a 2a3/6C4C? The latter valves are two triodes physically connected in parallel inside the same envelope. So, no matching can be done:
The previous was a low distortion 6C4C I have. Distortion is higher than 4P1L PSE, but not that much. H3 – H2 difference is about 12dB.
My early thoughts:
- 4P1L are very easy to match
- 4P1L PSE performs really well. Distortion of the pair is lower than a 6C4C performing at same level.
- H3 component is higher in PSE and this could be the reason why is more noticeable when listening to strings – as I proved in practice.
THD, Gm and μ tester
As part of my repairing of the curve tester, I had to do some changes to the transconductance (Gm) meter section. Currently I’m leveraging most of the curve tester to also measure Gm, μ and distortion (THD). Albeit the latter is rarely used as I prefer an external equivalent CCS circuit that is not inside the tester as the output signal comes out cleaner. The curve tester provides all sockets, HT power adaptor, meters and bias supply.
This is my latest circuit:
The additional protection to fuse (F1) is the diode D2 which can protect the LCD panel meter A1 in case of an unexpected anode short. P1 and R4 were chosen to allow a precise setting of the anode current at low levels and some protection to the CCS when P1 is set to zero. M1 is bolted to chassis and is carrying all the effort when providing current at lower anode voltages. M2 on the other extent can be a TO-92 type. R1 was added to allow a bleeding path to C2 when not measuring transconductance. The bias section is a simple adaptation of Merlin Blencowe’s “Power Supplies for Tube Amplifiers”, which I suggest you take a look at as Merlin covers very well the most common valve bias circuits
With this circuit I can measure very accurately transconductance at any desired point. I highly recommend you Alan Douglas’ “Tube Testers and Classic Electronic Test Gear”, which has a lot of details around how classic valve testers work, challenges around Gm measurement and obviously some good ideas and suggestions for calibrating and measuring Gm correctly.
Obviously adding an amplifier to the Gm tester section could improve the accuracy of low transconductance valves. But that would be for another time!
955 / VT-121
After a long delay, I finally did what a friend asked me. Test the lovely 955/VT-121. This acorn-type RF valve has a 6.3V filament, 1.6W maximum anode dissipation and 250V maximum anode voltage.
As per Paul’s recommendations, I looked at minimising distortion around 6mA/250V.
Here is the distortion using a DN2540 Cascoded CCS load driving the output to 10Vrms:
And here is the distortion at 1 Vrms output level:
This little fellow is a very linear valve at low signal levels and still performs really well at 10Vrms output when THD is only 0.11%. A great candidate for a preamp so would be keen to read some comments around the sound of it…
Edit
Again, as per Paul’s request, I looked at what the THD was for lower biasing points. The operating point was set for Ia=6mA and distortion measured for small signal, i.e. output voltage of 1Vrms as shown in the table on the left.
It’s very interesting to see that there is a minimum at -1.4V Below this grid current effect increases the distortion sharply as expected:
What is more interesting though, is the harmonic distribution change. H2 is reduced whilst H3 it is not. H2 and H3 levels are equal. Wonder if this is the residual harmonics from my oscillator 🙂
A super-linear output valve and fireworks :)
Testing the 814 in triode mode
Worried about the G2 maximum voltage limit as per datasheet in class B, I decided to test the 814 carefully. So carefully that had a bit of smoke and fireworks (but not inside the valve)
Paul Leclercq suggested adding the grid resistors in G2 and G3 and do some baby steps whilst testing this lady.
Here is the test rig used:
The valve is astonishingly linear. At 600V anode voltage and 50mA quiescent current the valve only produced THD=0.01% at 15Vrms output!
Internal pannel meter wasn’t disconnected and apparently didn’t survive the 600V exposure so blew up and a domino effect provided some interesting fireworks inside my bench supply (series MOSFET regulator) 🙂
Evidently something went wrong with an unexpected short to ground, couldn’t be bothered with doing some proper root cause analysis so shut everything down and brewed myself a cup of tea….
4-65a EIMAC
4-65a EIMAC NOS DHT
Just got a couple of NOS EIMAC which I will be using in my SE design
So did some test on distortion, transconductance and driving them to +22.22dBu output to check the quality of these two ones.
I used similar test rig as before. At some point will be able to get a proper filament supply for this valve, but for the time being I will continue to use the hum pot and the big electrolytic cap across my old bench power supply which can gently provide the 3.5A for the hungry filaments!
I tested them at the limit of my CCS and bench HT supply which at the moment cannot provide more than 360V @ 100mA.
Transconductance is in the region of 3,800 – 4,000 μmhos.
So biasing the valve at -2.5V and over 90mA of anode current, the harmonic profile looks like this:
Both EIMAC measured about 0.12% THD.
VT-25 / 10-Y triode curves
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 🙂
Here is the VT-25 spice model. Let me know how it works for you!
4-65a glowing in the dark
These were taken at my lab in the attic during the summer evening twilight.
4П1Л/4P1L THD sweet spot
Playing around today with the 4P1L chap found a very good operating point where distortion is minimised at maximum anode power disipation:
- Va=250V
- Vg = -21V
- Pa=9W (7.5W anode + 1.5W screen)
- Ia=35mA
- THD measured at Vo=+22.22dBu (10Vrms)
Interesting to see this valve swinging beautifully at just 0.027% THD….
4-65a THD
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.
