Tube Testing & Curve Tracing

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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

TriodeLuvr wrote:
Sat 08/14/21 3:03 pm
Thanks Josh, I'll keep this in mind. Yes, the uTracer was what I was thinking about. Your comments are enough for me to steer clear.

Jack
Good call. It was an expensive nightmare. In the end, their design was poorly executed. The PCB quality was absolute trash, and their firmware was mediocre. On top of that, the supplied parts in their kit were all really low end. Their firmware, with the same connected hardware, couldn't give a proper trace of the same EL34 I posted tests of above. I'll post more of the story on the tracer website when I get around to building it, including comparisons of the same test sweeps.

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

We've been spending more time working on the GUI. I'm really happy how it's going. The plot tabs can now be resized or full screen. Resizing vertically is great to make the sweeps more visible:
EL34-1_plot_1.png

If anybody is wondering about why multiple tabs, because it's cool, that's why. ;)

You can have separate tabs running during the test for different measurements. So that way if you have it auto-saving plots, it can save, for example, separate plots of Ia and Is, or other measurements at the same time. Yes, you can do 2 measurements on 1 plot, but that can be confusing. This way the information is nice and clear.

This screen current chart was run and auto-saved at the same time as the above:
EL34-1_plot_2.png
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Re: Tube Testing & Curve Tracing

Post by lakelandmusic »

True, I run a small studio as well as restore guitars and amplifiers, and have a small online store in which vintage and NOS tubes are a part of my sales. 90% of online tube dealers sell them blindly, with mostly just Gm or pass\fail tests. In a never ending hunt for good tubes, I've bought thousands of them at auction over the last 15 years with varying test results from many different basic testers, and while most have been excellent, many of them tested so completely opposite of how they were presented, it just makes no sense to rely on those results.

I built a very nice curve tracer kit, and wouldn't think of selling or using a pair or quad not matched by relevant parameters, as well as a few other test circuits that track plate current variations while simulating voltage spikes, gas test, etc.

Great post, I just joined the forum, so hello as well!
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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

lakelandmusic wrote:
Fri 08/27/21 4:37 pm
Great post, I just joined the forum, so hello as well!
Thanks, and welcome to the site! I'm glad to hear there's people out there doing it the right way.

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by southboundsuarez »

hurray Brilliant work! Absolutely brilliant.
As an avid user of "NOS" valves, I would love having something like this to play with whatever latest discovery of treasures that I may bring back from vintage hunting safari.
My current method involves testing transconductance on Hickok (which leaves alot to be desired) I then take them and match for idle current on a single ended amplifier test jig. The final and most definitive test is actually installation in the intended amplifier, where basically all previous derived data is pretty much moot anyways, and I audition for my perception of sound, tone and essentially playing response.
The most critical parameter at this stage is the tubes ability to remain free of microphonics, fizz and noise.
Not too scientific and very reliant upon personal perception.
Curious if there is a parameter within your data that might predict the suceptibility to a tubes inherent character to be or become microphonic?
Thank you for sharing your posts and keep up the great 👍 work!
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Re: Tube Testing & Curve Tracing

Post by southboundsuarez »

TriodeLuvr wrote:
Thu 08/12/21 12:48 am
Josh, I'm designing several amplifiers around vintage output transformers that I removed from older commercial gear. The transformers are approximately 3.6K ohms or 3.8K ohms anode-to-anode. The original amplifiers used a tube type that is no longer available, and only the EL34 now matches this impedance well. It's problematic though, because whereas the original tubes were beam power, the EL34 is a true pentode. The screen current requirements of the EL34 are much more challenging, and because these are hi-fi amps, I can't just install a 1K or 1.5K screen resistor and call it a day.

One type of data that's not readily available in any of the EL34 sheets (including the extensive documentation provided by Mullard) is a chart showing anode current as a function of screen voltage. I'd like to see a graph of what happens to anode current with a fixed anode voltage of, say, +400V, a fixed grid voltage of perhaps -26V or -28V, and a screen that's swept from +250V to +400V.

This is something I can do point by point when my current project is off the bench, but that won't have the resolution of a curve tracer. Is this something you're able to do?

Jack
Without getting too off track from OP, I was just curious what orig. tube you were substituting? 8417perhaps?
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Re: Tube Testing & Curve Tracing

Post by TriodeLuvr »

southboundsuarez wrote:
Sat 08/28/21 5:37 pm
Without getting too off track from OP, I was just curious what orig. tube you were substituting? 8417perhaps?
It's the 7355 tube. There have been a number of articles online suggesting the 7591 or various 6L6 variants as subs, but they don't work as well with these low Z transformers.

Interesting what you said about the Hickok; that's the same method I use, except I have a push-pull amp set up for power testing. I bought a large quantity of 6P3S-E a few years back, and I needed to test them four at a time.

We should start a new thread in one of the other forums if you want to discuss any of this. I'd be glad to tell you more about what I'm doing.

Jack
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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

If anybody is interested in building one of these tube tester / curve tracers, I have the final parts sets ready. The set comes with the main PCB, an external PSU connection PCB, socket board PCB, and a programmed firmware chip for $135 including shipping within the continental US. Outside of there needs to pay actual shipping. There are two options for the socket boards, one with through-hole devices, and the other one is mostly SMT (only one is included).

Here's a test on my beta unit with the new socket board:
EL84 Beta Unit_plot_1.png

Here's the PCB set (showing two options for the socket boards - only one is included):
FUtracer-PCBs.jpg



Here's my final tester (left), and the beta unit (right):
20211204_164353.jpg

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

I put the support website up. It's still a work in progress, but the meat and taters are there. Check it out: https://futracer.com

Thanks,
Josh

PS. I wrote it fairly quickly, and didn't do much proofreading, so please let me know if I did something dumb. ;)
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