Tag: uTracer

eTracer Build and Review

Tracing valves: an obsession

Since my early days of valves and DIY audio, I developed an obsession around testing and tracing valves. This led me to design and build my analogue curve tracer which I used for many years successfully until I build my uTracer, which was a great innovation in curve tracing. I do have many valve testers (some which I made myself) so why building another one?

Well, Chris Chang from Essues Technologies developed a fantastic new digital curve tracer for valves, the eTracer.  There are a few things which will grab anyone’s attention on this curve tracer. Firstly, the power supplies can accommodate a large range of valves which the uTracer can’t. HT can go as high as 750V @ 300mA and the grid supply down to -170V! This is exactly what you need to test your transmitting valves or even a 300B.  Secondly, the tracing speed is surprisingly fast. This is a nice feature, specially when you want to trace pentodes at various screen voltages to develop a Spice model  for example. 

Build process

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Valve Leakage Test

Years ago when I built my analogue curve tracer I added a small, yet very effective valve leakage test circuit. Due to my laziness, I failed to test a  transmitting tetrode which I bought on-line and despite being NOS it damaged my uTracer. I followed the repair and re-calibration process and got the tester back again running, however, I regretted not having used this simple one step test I normally did before.  Lesson learned, now I do use it back again!

Here is the circuit in case you don’t have a proper tester and you want to build something similar yourself:

leakage tester publishedYou can test for leakage current using a simple amplifier made out of a NPN transistor and an indicator. In this case I used a Russian Neon (80V/0.5mA) and the existing supply on my tester (+/-80V). You can replace all this with a simple LED and the supply you have at hand. The circuit is designed to turn on the bulb when 5 μA leakage current is provided on the base of Q1 thanks to grounding the valve element next to the one under test. So for example if we want to measure cathode to grid leakage, we simply ground the cathode and we connect the tester to the grid. Same process is repeated with the other valve elements.

When I asked for some help in the DIYAudio forum, someoone gently recommended this text. Unfortunately I don’t read German, but what I got out of this adivce was:

  1.  valves with poor vacuum (i.e. failed the test described on the procedure in 18A)
    1. preamp valves with less than 4 μA are usable
    2. output valves whit less than 10 μA are usable
  2. Valves that are good and show little Gas on the gas test:
    1. should have less than 0,6-1μA for preamp valves
    2. And should have less than 1.5-2μA for output valves

So the 5μA threshold was good enough in my view. It does work well and the beauty is that when neon light is very dim is an indication that it may be a workable valve despite the tiny leakage in particular with output valves.

Hope this helps

Ale

 

HY1269 curves

HY1269 I’m a big fan of the thoriated-tungsten filaments and I value them not just because they do look very cool, but fundamentally due to how they sound in a single-ended stage. I hope to build a nice push-pull amp with these type of valves soon.

I found a pair of HY1269 valves recently. This valve is not well-known amongst the used ones out there but they do have some interesting characteristics as a directly heated tetrode that would be interesting to consider it for a plate-to-grid (Schade) feedback configuration.  With its 30W of anode dissipation capability, it’s a good candidate for an output stage. However, it’s quite likely that you will have to drive it in A2 to get the most out of this valve. Like designs using 811a and similar transmitting valves, the HY1269 can be operated in class A2 even with no signal on the grid.

As per my “Robustiano” design using the 6P21S, it would be nice to see extracting 6W or more out of this  transmitting valve. I’m sure that the thoriated-tungsten touch will provide a lovely sound if properly implemented.

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6P21S tetrode curves and model

6P21STime ago I generated the tetrode curves for this great directly heated tetrode using my analog curve tracer. I originally used this tetrode in triode-mode. Although it’s a good candidate for a SET amplifier with its 21W in triode-mode, I always wanted to find out how it will perform with Schade-type anode to grid feedback. Building an accurate beam-type tetrode model was key. Luckily now, Derk Reefman has developed an accurate model for these type of valves.

I also worked with Ronald Dekker and insisted him to build in the “Schade” feedback capability in the uTracer using software rather than hardware:

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814 SE A2 Amplifier

Goodbye 4-65a SE, at least for now

IMG_1401After enjoying the 4-65a SE amplifier for many months, I couldn’t resist myself from upgrading the output stage to the 814s.  I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very encouraging. The 814 seemed to perform much better than the 4-65a in delivering 10W of class A2 sound at half the distortion levels. This to me, was only worth trying.

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4P1L model improved

 Improving the DHT model in A2

After playing for some time with the uTracer, I found that the tracer wasn’t measuring accurately A2 curves. Ronald clearly advised me (as explained on his site) that uTracer wasn’t designed for this purpose although there was a great trick to use the screen driver to generate the A2 curves and also measure grid current. Measuring grid current is key in A2 mode so a better grid current model can be derived to better simulate the non-linear and low impedance behavior of the grid in positive bias.

My incarnation of the uTracer is not neat. I’m using my existing analogue curve tracer. As shown in the pictures below, my current tracer have a plethora of sockets and just adding right jumper cables for anode, cathode, screen and grid connectors will still give me the flexibility I had with my analogue tracer

Additional 3,300uF capacitor across C13 to help A2 traces
My version of the uTracer
uTracer using my analogue tracer sockets 🙂

The process of generating the A2 curves (and SPICE model) starts by plotting the normal curves. The uTracer is great for this. Then you have to overlay the A2 curves in Excel (or whatever tool you prefer) to combine both set of curves.

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4P1L: Tracing A2 curves (first test)

Just finished the digital tracer project uTracer V3. Did some further tests this morning, now with my favourite DHT: 4P1L.

Started with a well known bias point for triode-strapped operation:

4P1L Va=200V Continue reading “4P1L: Tracing A2 curves (first test)”

uTracer up and running

After a stupid mistake which blew the PIC, I finished today my version of the uTracer. A quick trace of an CX-226 RCA Radiotron can be seen below. This is an incredible digital tester. I need to test it properly and do some comparisons against my analogue tracer, but as everyone would expect,the digital tracer has superior capabilities in terms of data manipulation and representation.

Highly recommended. Ronald provides a sterling service and specially when I damaged the PIC and send me a replacement at no cost!

More to come shortly…cx226 RCA test