PC based Tube Curve Trace Generator

[silverfox]

Build your own or buy a PC based Tube Curve Generator.
Another Forum I use to attend posted a link to a PC based tube curve trace generator and it was suggested there may be interest here in creating a Topic with a link to the project.

Everything needed to construct one on your own including the programing or purchase from the designer.

The uTracer, a miniature Tube Curve Tracer / Tester.

Silverfox.

[mhuss]

I built one of these, the PC-side software is a little quirky, but it works great. The only downside is limited HV (~400v max).

The design is very clever, using a common 19.2v leftover laptop power supply and on-board switching converters for the high voltage.

Nothing like real curves from the actual tube you're using to give you an accurate idea about what is going on.

--mark

[R.G.]

I actually had an old Tektronix tube based curve traced that would trace tubes and other things. It was fun, but I eventually gave it away.

Tracing curves is a means to an end, looking for characteristics like gain and matching. Or it was to me.

I decided that if I were to build such an instrument - I didn't, as my life took one of those periodic unplanned byroads - that I'd skip tracing curves on tubes and measure the stuff that I really wanted out of it - gain, bias point, noise and microphonics.

I envisioned part of the PC based curve tracer, in that the B+ was program driven, as was some kind of biasing setup, and that it would measure things like plate voltage, plate current, Vgk, and so on. But the heart of the matter to me was to drive it with a test signal automagically and then capture the output. Feeding a 12AX7 section a sine, and recording the plate signal lets you compute gain direction in a model circuit, and tells you directly what to expect. Have the PC mute the input, then capture the noise, computationally subtracting out the thermal noise of the plate resistor, etc. ; then leave the grid held at ground by a typical 1M resistor and have that nifty surplus cell-phone vibrator motor shake the socket mounting, and see how much signal comes out of the plate for only vibrations in.

That ought to tell you pretty quickly what are good tubes, and if your PC recorded grid, cathode and plate voltages as well as AC gain, it would match them pretty well too, in circumstances similar to actual operation.

But that's just me.

I figured I'd modularize the biasing parts so the cathode resistor and so on were pluggable to match the tube under test type.

[Mick Bailey]

Just skimmed through the link and it looks a nice project. It looks like the tube isn't being operated under the actual conditions it would encounter in an amp. The B+ has very limited sustained current capability (very similar to Nixie PSU mini amps) and the tube won't be operating with actual screen and plate dissipation.

How different would the curve look if the tube was operating 'hot'?

[Merlinb]

How different would the curve look if the tube was operating 'hot'?

Not different at all. Valves are designed to keep the same characteristics across the temperature range (they'd be kinda rubbish if they didn't!)

[Mick Bailey]

Perhaps across an operating temperature range, but the anode/screens would be operating at well below their operating point on the curve tracer. I was thinking that secondary emission may be different at (say) room temperature to what it would be at 250 degrees.

[Merlinb]

I was thinking that secondary emission may be different at (say) room temperature to what it would be at 250 degrees.

It will, but it is reduced to negligible levels by the design of the tube. The fact that the uTracer curves match the data sheet curves is surely reassurance enough?

[Mick Bailey]

They're pretty close, and the designer explains some of the discrepancies in the resulting curves. It's a really impressive amount of work that's gone into this and the generosity in publishing the information has to be commended. The reason I asked the question about operating temperature is not to be critical, or appear to condemn the work, just that I don't know the answer.

It then leads me to think that the anode in an audio amp could be pulsed at SMPS frequency to get greater efficiencies.

[Merlinb]

The one things the u-tracer does get very wrong is curves for directly heated triodes, as the pulsed heater voltage does not give the correct voltage gradient along the heater.

[Enzo]

Fortunately for us, we don;t use many of those in guitar amps.