Picture of Tube Curve Tracer


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:

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
ghanks5 years ago
Great Idea... Looks Good... Please is there a way to get a 'readable' schematic?
BasinStreetDesign (author)  ghanks5 years ago
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
gmoon5 years ago
I'm not sure what to say about this...

On one hand, it's a great build. Really well done. So good, in fact, that I'll rate you 5.0.

On the other, there's absolutely no reason to retrace the characteristic curves of tubes, unless you're building something weird like a "starved cathode" project where the datasheets wouldn't apply. Tracing tube that are failing? I think it's time to buy some NOS tubes, my friend.

But my tube projects aren't of the audiophile variety, so this doesn't make much sense to me. ;-)

I guess this would be useful for some of the newer tubes that aren't really what they claim to be...
BasinStreetDesign (author)  gmoon5 years ago
Thank you for the rating.  I am new here and it took me a bit to find out how the rating system works.  But don't ever tell an engineer that there is no point finding out more about how components behave.  It's always good to know what they will do, even in  non-standard ways.
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 "new" old stock tubes.  I t appears to me that very little of stuff hocked as "NOS" is really never used.
I believe the ones I have have a lot of life in them yet even if they are about 35 years old.
Anyway, the guidelines I had from the Secretary of the Treasury in my house were not to spend any money on the unit.  So I didn't.  It was made for $0 outlay.
It was a fun build, in any case.
Thanks again for the comments and the rating.  I didn't think it would do that well.
But don't ever tell an engineer that there is no point finding out more about how components behave.

Point taken.

I took the liberty of adding this to the "Vacuum Tube" group... Good luck with your tube builds!
Slowpoke5 years ago
Brilliant! Well done! I hope that you will publish your Amp as well.
BasinStreetDesign (author)  Slowpoke5 years ago
Thanks for the kind words.  The amp will be published sooner or later.  It has been a tale that has grown in the telling for quite a while now.  LOL