This is for all those tube amp enthusiasts and hackers out there. I wanted to build a tube stereo amp that I could be proud of. However in the course of wiring it up I found that some 6AU6s just refused to bias where they should.

I have a 1966 copy of the RCA Receiving Tube Manual and having designed electronics of all sorts for about 30 years, I understand that the published data on a device need be taken with a wee grain of salt sometimes. But the tube data published in these books is definitely NO guarantee of behaviour in a real circuit for any one specimen.

I like the little plate curve family charts, as in the picture above, in the book and THAT is what I wanted to see for the tubes I had.  Using a tube tester, even a well-calibrated, high quality one will only give you one data point on one plate curve amongst that family.  And you don't even know which curve it is.  It's not very illuminating.  Buying a curve tracer on the market can be expensive and rare (You may find an old TEK 570 on EBAY once a year for $3000 or up) and finding one locally is out.

So I decided to build one.

P.S. I have completed some enhancements to this TCT here:

Step 1: The Circuit Design

I needed a circuit that would be relatively simple but would provide a high plate and screen grid voltages as well as a stepping control grid voltage with steps of ½ V, 1V each, etc. For the plate drive I used a half sine wave straight off a high voltage transformer winding since I realized that the plate current would follow the same characteristic path going up the wave as coming down. The wave form need not be precise, calibrated or any particular shape as long as it rose and fell in a non-abrupt fashion. It did not even have to be consistently the same shape each time it rose or fell.  The shape of the resulting curve is determined solely by the characteristics of the tube under test.  This eliminated any need for a precision high-voltage ramp generator but I still needed to acquire the transformer for this...

I wanted to have several tube sockets for the various existing base types but eventually settled on four: 7 and 9 pin miniature plus octal sockets. I also included a 4 pin socket to allow testing old rectifier tubes.

The stepped bias generator is a cheesy 4-bit R-2R ladder type digital-to-analog converter driven by a counter advanced by the 60 Hz wave from another winding on the transformer.

The filament voltage came from a transformer ripped out of an old ReadRite tube checker from the 1940's which provided many filament voltages from 1.1 V to 110 V AND a switch to select them.

Finding a switching method to accommodate all of the various and sundry tube base pin-outs proved to be futile at best so I avoided the whole issue and used patch cords with each numbered pin and each drive signal brought out to 5-way banana connectors. This gave me ultimate connection flexibility and prevented me from going mental trying to figure out a good switching method.

Finally, the biggest concern was measuring the plate current. I didn't measure the cathode current since it is the sum of ALL element currents including the screen grid. The place where the plate current is measured (at the plate) was elevated to about 400V at the top of the wave. So after dividing the plate voltage down to 0-6V with a resistor divider so OP-AMP ICs could work with it, a large gain, very-well-balanced differential amplifier was needed. The LMC6082 dual precision OP-AMP did this very well and to boot its signal range includes ground so it could be wired up as single-supply.

Both plate current and plate voltage readings were then output on BNC connectors to an oscilloscope operating in A-B mode so the final chart of these two quantities could be plotted against each other.

Some people have written asking for a clear copy of the schematic since the one that shows up was pretty fuzzy.  I have removed it and replaced it with a PDF version.  The green line encloses all of the circuit on the small hand-wired circuit board.  A couple of parts of the circuit are expanded upon in step 7.

There were a couple of surprises in the build and I will talk about those later.


I think a built-in meter would look great, but it would have to be of the analog, needle variety. This device looks to classy for one of those seven segment LED displays. Perhaps with some incandescent backlighting like al old VU meter. real steampunk-like
Great Idea... Looks Good... Please is there a way to get a 'readable' schematic?
Hi Greg, Yes, sorry about the readability of the schematic. I will send one to the email address you have published on your website. Tim
I'm not sure what to say about this...<br /> <br /> On one hand, it's a great build. Really well done. So good, in fact, that I'll rate you 5.0.<br /> <br /> On the other, there's absolutely no reason to retrace the characteristic curves of tubes, unless you're building something weird like a &quot;starved cathode&quot; project where the datasheets wouldn't apply. Tracing tube that are failing? I think it's time to buy some NOS tubes, my friend.<br /> <br /> But my tube projects aren't of the <em>audiophile</em> variety, so this doesn't make much sense to me. ;-)<br /> <br /> I guess this would be useful for some of the newer tubes that aren't really what they claim to be...<br />
Thank you for the rating.&nbsp; I am new here and it took me a bit to find out how the rating system works.&nbsp; But don't ever tell an engineer that there is no point finding out more about how components behave.&nbsp; It's always good to know what they will do, even in&nbsp; non-standard ways.<br /> Besides I have about 80-100 6DJ8s, 12AX7s, 6AU6's etc salvaged from some old Tek scopes from the '60s ( they are a treasure trove of tube project parts) and its much more convenient for me to know what I have rather than run out to spend a bunch of money on &quot;new&quot; old stock tubes.&nbsp; I t appears to me that very little of stuff hocked as &quot;NOS&quot; is really never used.<br /> I believe the ones I have have a lot of life in them yet even if they are about 35 years old.<br /> Anyway, the guidelines I had from the Secretary of the Treasury in my house were not to spend any money on the unit.&nbsp; So I didn't.&nbsp; It was made for $0 outlay.<br /> It was a fun build, in any case.<br /> Thanks again for the comments and the rating.&nbsp; I didn't think it would do that well.<br />
<em>But don't ever tell an engineer that there is no point finding out more about how components behave.</em> <br /> <br /> Point taken. <br /> <br /> I took the liberty of adding this to the &quot;Vacuum Tube&quot; group... Good luck with your tube builds!
Brilliant! Well done! I hope that you will publish your Amp as well.
Thanks for the kind words.&nbsp; The amp will be published sooner or later.&nbsp; It has been a tale that has grown in the telling for quite a while now.&nbsp; LOL<br />

About This Instructable


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Bio: 64 yo electronic design engineer, effectively retired. Historically sometimes employed, sometimes self-employed. Have always had a home lab. Just can't let it go.
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