Tube Testing & Curve Tracing

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Tube Testing & Curve Tracing

Post by JMPGuitars »

There are obviously varying and polarized opinions on the subject of tube matching.

My opinion is that:

1. Most people that sell "matched" tubes are basically lying. They'll typically match a single variable like gm (or some B.S. magic number a tube tester arbitrarily decides, but is still typically a single variable). In all of the cases "matched" means within a given percentage of variance, but not everybody agrees on what that percentage is. As we know, tubes have more than 1 variable to consider.

2. If you want actually matched tubes, you would need to use a curve tracer and match the tubes' performance against the voltages that you will see in the circuit, and/or across the entire curve.

Now how many people do you know that actually does number 2? 😇

I'm tempted to grab a set of tubes I purchased as matched and see how they actually perform on my curve tracer. Then compare them to other tubes not matched with the same magic numbers.

Thanks,
Josh

PS. For anybody that actually wants to accurately test / trace tubes, I'm working on a project PCB that people can build themselves. Eventually I'll probably also offer a prebuilt commercial version.
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Re:

Post by TriodeLuvr »

JMPGuitars wrote:
Sat 08/07/21 7:11 am


My opinion is that:

1. Most people that sell "matched" tubes are basically lying.
LOL, I think you're right, Josh! But I also think McShane burns them in and takes the time to at least match them for Gm and idle current. He's a good source for people who can't do it themselves.

I match 6L6 types by first measuring Gm with a tube tester. Then I plug them into a modified 30W/ch stereo amplifier, heat them for at least 1/2 hour and record the bias current with all grids at the same voltage. Finally, if I'm feeling especially brave, I compare the distortion spectrum tube by tube at a medium power level. The latter is really time consuming and requires disabling NFB, so I don't do it often. Someday I'll build a small test jig that I can power from my bench supply and add switches for EL34, 6L6, EL84, etc.

You know, even with a curve tracer, the uncertainty of divergence at various screen voltages has to be considered. Two pentodes that test close with the screens at, say, +300V, might not be so close with the screens at +400V. For instrument amps though, the most important criteria is idle current. If that's balanced, hum will be minimal and the output transformer will be happy. I know, I know... preaching to the choir. :)

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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

TriodeLuvr wrote:
Sat 08/07/21 8:25 pm
You know, even with a curve tracer, the uncertainty of divergence at various screen voltages has to be considered. Two pentodes that test close with the screens at, say, +300V, might not be so close with the screens at +400V. For instrument amps though, the most important criteria is idle current. If that's balanced, hum will be minimal and the output transformer will be happy. I know, I know... preaching to the choir. :)

Jack
That's the beauty of a good curve tracer. You can test at different screen voltages. My tester is similar to, but better than, a popular curve tracer (which was a huge PITA, and so was the creator if it). Here's an example of a recent test during beta testing the firmware for my device:
EL34.png

You can see specific measured data in the attached PDF file.

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

I think our firmware might actually be done. The results are pretty awesome. Here's some EL34 tests, I'll post the EL84 stuff soon too.
EL34-1_plot_1.png
EL34-2_plot_1.png
EL34-3_plot_1.png
EL34-4_plot_1.png
EL34-5_plot_1.png
EL34-6_plot_1.png
EL34-7_plot_1.png
EL34-8_plot_1.png
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Re: Matching el84's

Post by JMPGuitars »

Here's the EL84 tests. These were done I think with 15 test points instead of 30, I might do higher sweep count tests eventually too if somebody has a specific request.

Tube 1:
EL84 v2-1_plot_1.png
EL84 v2-2_plot_1.png
EL84 v2-3_plot_1.png
EL84 v2-4_plot_1.png
EL84 v2-5_plot_1.png

Tube 2:
EL84 v2-6_plot_1.png
EL84 v2-7_plot_1.png
EL84 v2-8_plot_1.png
EL84 v2-9_plot_1.png
EL84 v2-10_plot_1.png

This is supposedly a matched pair. The only number written on the matching stickers is "33" - whatever that's supposed to mean. I'd post the data files too, but there's no point really. You can see that the results are not the same, so for them to have the same magical matching number doesn't make a lot of sense to me. Is it a huge variance? No, like I said, they generally match them within a margin of error. But the magic number makes it sound like they should be the same.

Don't worry ZP if you read this, I'm not advocating for (or against) matched tubes. Only stating observations. ;)

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by TriodeLuvr »

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
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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

Jack, no problem at all.
EL34 v3-3_plot_1.png
I also ran the sweeps separately, but you've got all your data on here with a nice comparison in the PDF.

Let me know if you have any other tests you'd like me to run, or if you need more data points. I ran it with 20 points, but it can go higher.

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by TriodeLuvr »

Well, that's a real eye opener! The effect of screen voltage on anode current is much more severe than what happens with 6L6 and 6V6 types.

Josh, what is the difference between the solid lines and dotted?

Many thanks for running this!

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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

TriodeLuvr wrote:
Thu 08/12/21 4:57 pm
Well, that's a real eye opener! The effect of screen voltage on anode current is much more severe than what happens with 6L6 and 6V6 types.

Josh, what is the difference between the solid lines and dotted?

Many thanks for running this!

Jack
You're welcome!

The solid lines are Anode current, the dotted lines are Screen current.

But since you said that, now I want to run a similar test on a 6V6 and see what happens...but it will be a while before I can.

Because I love testing under stupid and/or extreme circumstances, I'm currently running a 300 point per sweep test, with 31 sweeps between 0 and -30 Vg, Anode voltage from 2 to 400, on an EL34. 39% complete after an hour. But it sure is pretty! lol

Thanks,
Josh
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Tube Testing & Curve Tracing

Post by JMPGuitars »

Here's my ridiculous / overkill test I did. 301 points from Va 2 to 400, with 31 sweeps of Vg from 0 to -30.

Ia:
EL34-1_plot_1.png

Is:
EL34-2_plot_1.png

I would attach the silly data file too, but it's 199 pages and over 6MB. 😂

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by TriodeLuvr »

What was the screen voltage during the plot of screen current? And what kind of hardware are you using for this work? This would be a terrific tool for designing new circuits, not just matching tubes.

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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

TriodeLuvr wrote:
Fri 08/13/21 8:10 pm
What was the screen voltage during the plot of screen current? And what kind of hardware are you using for this work? This would be a terrific tool for designing new circuits, not just matching tubes.

Jack
Screen voltage was 400VDC. Both are the same test, but I plotted the results from A/S separately so they'd actually be visible.

The hardware is a tube tracer I designed in response to a string of bad experiences I had with a somewhat popular tracer...and it's creator. Ech.

The firmware for this appears to be good to go, so I will eventually be offering this as a PCB + programmed chip set, with a link to a BOM for advanced users to build it themselves. The only issue right now is that some of the parts are backordered until early next year. I probably have enough parts to sell a couple kits with the harder to find parts, but I'm not sure if I'm going to hoard those parts or not right now. ;)

If this is something you (or anybody else) are interested in, you can email me at support at jmpguitars dot com to discuss it further. I don't have the website up for this, or any build docs done yet, so I dunno how fast anybody wants to rush on that. I miiiiiiight decide to auction the current unit I have after I build another one.

This is what my current build looks like:
_JMP2722.jpg

It's setup to test (from left to right):
12AX7 etc...
EF800 etc...
EF86
EL84
EL34/6L6/6V6 etc...

It can be setup for whatever tube types/sockets you want, but those are the ones I cared about, so that's what's on the tube pcb part.

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by TriodeLuvr »

That's a terrific machine, and those are the same tubes I would be interested in tracing. I seem to remember seeing a tracer that used jumper wires to configure the various sockets. That would work for me too.

Let me know if you make any of this available, and what you think the cost might be. I have so many amplifier projects in the wings right now, not sure I can take on a DIY project. Still, it wouldn't hurt to know what it might cost, and I'm especially interested in this one if you decide to sell it. Very cool!

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Re: Tube Testing & Curve Tracing

Post by JMPGuitars »

TriodeLuvr wrote:
Sat 08/14/21 8:23 am
That's a terrific machine, and those are the same tubes I would be interested in tracing. I seem to remember seeing a tracer that used jumper wires to configure the various sockets. That would work for me too.

Let me know if you make any of this available, and what you think the cost might be. I have so many amplifier projects in the wings right now, not sure I can take on a DIY project. Still, it wouldn't hurt to know what it might cost, and I'm especially interested in this one if you decide to sell it. Very cool!

Jack
Thanks!

Well, for the PCBs and programmed chip I'm probably going to charge about $125 + shipping. Something like that. A bit more if people need me to do the SMT stuff for them. The BOM on Mouser for quality parts is between $176 to $200 depending on whether or not you need the SMT stuff. Then you need a laptop PSU (I paid $14 for the one I'm using, and it works great).

All that said, I don't know if I would sell the one I have for less than $1000.


The windows GUI is free/open source and awesome. I'm currently working on the theming for the GUI, and it's looking pretty noice:
futracer-gui.jpg
All the graphs above were exported from that.


In case you haven't guessed (or read my opinion somewhere else), the tracer that pissed me off was the utracer. So naturally mine is called the "FUtracer" in response to my experience with the utracer and its not-very-nice creator. :lol: :lol:

Thanks,
Josh
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Re: Tube Testing & Curve Tracing

Post by TriodeLuvr »

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
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