Tube Curve Tracer Ver 1.1

A curve tracer is a machine which will plot the Voltage-Current characteristic curves for a device such as a transistor or diode or something else. In this case we are talking about the plate V-I characteristics of tubes. By this, I mean a curve showing what current will flow into the plate of a tube at any moment depending on what voltage is present between the plate and the cathode. This current is also dependent on the voltages present on each of the grids in the tube. A photo of a family of plate curves is shown above for the 6AU6 pentode. There are several curves shown in this photo. Each one is for a different voltage on the 1st (control) grid. About 16 months ago I built a curve tracer for small-signal tubes to facilitate my understanding of the behaviour of some 6AU6s I was using in another project – a stereo amplifier. It was a fun build and I wrote an instructable for it.

Here's a link to it.
http://www.instructables.com/id/Tube-Curve-Tracer/

It also had its limitations. In this instructable I show what I did to expand its capabilities to allow power tubes to be plotted and fix a couple of small problems.

Step 1 About the Previous TCT

The existing tube curve tracer is shown in the first picture above. It could generate the plate voltage-current characteristic curve for a family of 16 grid voltage levels. The grid bias generator could generate either ½ Volt or 1 Volt steps only. Thus the maximum negative grid bias available was -15 volts as the grid was stepped from 0V to -15V. This allows only small signal tubes to be traced but not anything that needed greater bias. Also the vertical output, measuring the plate current, was at most 10 mA per 2Volts out. The vertical amp would run into the power rails at +15V so that limited the maximum plate current displayable to 75 mA. I needed to display much larger currents for power tubes.

Two main sections had to change. The stepping bias generator and the plate current amplifier. I include a PDF of the new circuit in its entirety below.

I also decided to change the pot used to balance the vertical output to the scope, R5. The pot I had was very noisy and was becoming a pain in the tush. After changing it out for the next lowest value pot I could scrounge up (50 Ohms across a 10 Ohm resistor) I show just what effect it has on output by connecting the unit up for regular operation. The 2nd photo shows a horizontal line with no plate current being drawn. I adjust the new balance pot from end to end with the storage scope painting all points on the screen. It shows that with no plate current a non-zero vertical signal can be generated from the plate voltage waveform which is a half sine wave. This would tilt the whole family of curves up or down. The vertical gain of the scope is magnified 10x to show this.

First I talk about another small problem which I had - the screen grid Supply.

Comments

[kelseymh]

A minor point. You can make the URL in your introduction an active hyperlink by using the appropriate icon in the I'bles editor (the globe with a chain). Highlight the text you want to be a link, click the icon, and a popup box will appear. Put the URL into the text input, and hit OK.

Other than that...Your original project was extraordinary, and is now Featured, as is this one. We need more Real Engineering projects here, rather than yet another LED cube :-/

[BasinStreetDesign]

Thank for the hyperlink tip. I had no idea how to do that otherwise.

I intend to produce more 'ibles' (interesting just how some words are contracted in the popular vernacular). I was initially worried that giving too much technical explanation would turn off most readers. I hope that is not the case. But I am willing to produce more "Real Engineering" stuff if there is any call for it.

What does it mean to be "Featured"? Who would see it who might not otherwise?
Just wondering.
8-)

[kelseymh]

Technical detail will attract some readers and repel others. If you're trying to reach the script kiddies and K'nex gun builders, well, you're out of luck. If you're aiming for a more sophisticated audience, I think you are doing well.

As for whether or not to produce more I'bles, Kiteman's Law is probably the place to go :-)

"Featured" (or more recently, "Editors' Picks") means that one of the members of the "Feature Team" has identified your Instructable as being among the best (there are some non-subjective criteria used). The idea is that Featured I'bles are displayed on top-level pages that random Internet searches might pick up, and they tend also to be passed around the DIY blogs.

There are three featuring levels -- "top level" means that your Instructable will be displayed on the home page for a while (until enough new ones push it off); "category" level is displayed on the main page of one of the half-dozen big categories, such as Technology or Play; and "channel" level is displayed on the most narrow classification (like "CNC" within Technology or "K'nex" within Play).

[kelseymh]

I did. Interesting that it didn't stick. And it let me do another...

[lemonie]

Give it a rating then?

L

[temper]

Hear hear!