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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Diode cathode bias

Playing with the semiconductor curve tracer I did a quick test of potential candidates for diode cathode bias:

The popular HLMP-6000 is a superb LED with its low impedance. The SiC diodes have proved to be a great match with an impedance lower than 2Ω. Bias voltages will be around 0.7-0.8V for low currents. The classic Schottky SB540 has a very low impedance, but its forward voltage is so low that is not practical for diode cathode bias. What surprised me was to see the 1N4007 to be a good match. The impedance is higher than the LEDs or SiCs, but good enough. The green LED on the opposite extent has a significant dynamic resistance over 10Ω.

Interesting to see that a minimum of 2mA should be run through with small signals to ensure the diode operates in the linear region. The higher the better. An arrangement with an extra source of diode current (e.g. LND150 or DN2540 CCS shunting current to the diode) can be used when dealing with lower cathode currents due to the valve being used.

Further tests are required….

Transistor Curve Tracer

Finally I’ve had the chance to test Locky’s Intelligent Curve Tracer. This is a great piece of testing gear if you’re looking to match transistors or derive some curves for solid state amps. Great for FET matching in RIAA stages as well.

Here are some quick tests I performed after installing the software:

SX-201a Sylvania

I have a pair of Sylvania’s SX-201a which sound superb. My preferred ones for 01apreamps. I’m looking at revamping my 01a line-stage shortly. Here is a trace of the anode curves for one of them:

The uTracer is a fantastic tool clearly. Well done Ronald!

Valve Tracing & Testing Service

If you are looking to test your precious valves and you don’t have the equipment, I can now offer this service for you which includes:

Valve health check
Valve bias point measurement (Ia, Is @ Va, Vs)
Transfer curves
Characteristic curves
Microphony (optional)
Distortion – THD (optional)
SPICE model for triodes (optional)

Some examples of a lovely EL34 Mullard tested on request:

My fees are very competitive and this service is worth if you are looking to sell your valves or match them.

Terms and conditions:

Price varies on level of testing and amount of valves
Postage is not included and depend on your location. I’m in London, UK so you can easily estimate the costs beforehand.

Update – August 2021

My apologies but due to pandemic and lack of free time, I’m not offering this service anymore.

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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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…