I have a pile of 12a*7 tubes, mainly purchased 20yrs ago when I first dabbled with tube preamps. Many are NOS, but I also have quite a few that are definitely not brand new (usage TBC). I wanted some method of measuring the performance of each, primarily for my own interest, but also to help me gift/sell the tubes if I decide to do this in the future.
I purchased the Lampemètre Ultra-Compact kit from Alain at RadioElec. Alain was extremely helpful and answered a number of questions before I ordered the unit.
Lampemetre Ultra-Compact - Digital Vacuum Tube Tester MO0002 : RADIOELEC : Electronics Components and Modules, specially for Audiophiles and Radio-Ham
My plan is to test each section of each tube at defined operating points, then use Excel to calculate GM/Rp/μ.
Initially I chose the following operating points:
Va 145V, Vg 0.0V
Va 150V, Vg -2.0V
Va 150V, Vg -4.0V
Va 250V, Vg -8.5V
Va 250V, Vg -10.5V
I've realised that I'll need to add at least one more operating point with higher Va and same Vg, so I can calculate Rp & μ.
Before I start working through the tubes one by one, please can you let me know if I'm on the right track, or if I'd be better to change the operating points? I'm not really sure of what I'm doing here so any guidance would be massively appreciated!
I purchased the Lampemètre Ultra-Compact kit from Alain at RadioElec. Alain was extremely helpful and answered a number of questions before I ordered the unit.
Lampemetre Ultra-Compact - Digital Vacuum Tube Tester MO0002 : RADIOELEC : Electronics Components and Modules, specially for Audiophiles and Radio-Ham
My plan is to test each section of each tube at defined operating points, then use Excel to calculate GM/Rp/μ.
Initially I chose the following operating points:
Va 145V, Vg 0.0V
Va 150V, Vg -2.0V
Va 150V, Vg -4.0V
Va 250V, Vg -8.5V
Va 250V, Vg -10.5V
I've realised that I'll need to add at least one more operating point with higher Va and same Vg, so I can calculate Rp & μ.
Before I start working through the tubes one by one, please can you let me know if I'm on the right track, or if I'd be better to change the operating points? I'm not really sure of what I'm doing here so any guidance would be massively appreciated!
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Tube testers of old had standardized operating points for the popular tubes. (I don't know if you remember the old tube testers of the 1960s.) You looked up your tube, set a few dials, and pushed the button. I was doing it when I was 10 years old.
I don't know if there's guidelines like that with this product, but you could look up specs on the datasheet and set your parameters accordingly.Since you're in charge you could test it at different points and draw a transfer curve for each tube if you're so inclined.
That's a cool product, and you did a nice job of making a durable piece of test equipment. I cobble together test circuits as needed but I am inspired to build a few test gadgets. Thanks for sharing.
I don't know if there's guidelines like that with this product, but you could look up specs on the datasheet and set your parameters accordingly.Since you're in charge you could test it at different points and draw a transfer curve for each tube if you're so inclined.
That's a cool product, and you did a nice job of making a durable piece of test equipment. I cobble together test circuits as needed but I am inspired to build a few test gadgets. Thanks for sharing.
Thanks for the feedback. I looked at second hand testers but I was reluctant to spend several hundred £ on something that would probably need new capacitors, switches etc. and probably couldn’t be trusted unless recalibrated (more £).
My current preamp uses 12AU7/5965A/12AT7 tubes and has a B+ of 150V. I’m currently using Mullard E180CC tubes, as I love the sound and the NOS price is very low (no doubt due to high supply vs low demand). So for me it’s most valuable to understand performance at 150V and the bias level of my preamp (need to measure that...).
I wonder if there is a summary anywhere of the operating points for 12A*7 tubes for different classic testers (AVO etc)? It would be interesting to take measurements that should be equivalent to those devices, for comparison.
Alain shared a PDF manual by email after I ordered the tester (and included a printed one with the kit). It doesn’t prescribe operating points for different tubes - I’m fine with this, as his kit is designed to enable each person to test in the way they need to.
I’ll share some results for the first tubes I test, just for visibility. I’m open to any recommendations for the best testing methodology - I know I’m a bit out of my depth here!
My current preamp uses 12AU7/5965A/12AT7 tubes and has a B+ of 150V. I’m currently using Mullard E180CC tubes, as I love the sound and the NOS price is very low (no doubt due to high supply vs low demand). So for me it’s most valuable to understand performance at 150V and the bias level of my preamp (need to measure that...).
I wonder if there is a summary anywhere of the operating points for 12A*7 tubes for different classic testers (AVO etc)? It would be interesting to take measurements that should be equivalent to those devices, for comparison.
Alain shared a PDF manual by email after I ordered the tester (and included a printed one with the kit). It doesn’t prescribe operating points for different tubes - I’m fine with this, as his kit is designed to enable each person to test in the way they need to.
I’ll share some results for the first tubes I test, just for visibility. I’m open to any recommendations for the best testing methodology - I know I’m a bit out of my depth here!
Quick google search - http://www.philbrickarchive.org/12ax7_sylvania.pdf
Look at the transfer curves. See if your data is on the curve. I hope you have a schematic of the tester or user instructions. I'd have to look at it and figure out how it translates to datasheet curves. I'm hoping you can figure that out.
I'll try to help but I have my own science projects. I have a lot of work and if I can plow through and I'll actually have some money (something I haven't had for ten years until about a year ago, hard times). So good luck.
Look at the transfer curves. See if your data is on the curve. I hope you have a schematic of the tester or user instructions. I'd have to look at it and figure out how it translates to datasheet curves. I'm hoping you can figure that out.
I'll try to help but I have my own science projects. I have a lot of work and if I can plow through and I'll actually have some money (something I haven't had for ten years until about a year ago, hard times). So good luck.
Thanks Fast Eddie D, I wasn’t expecting you to dedicate more time to this - I just wanted to leave it open to others to give me a quick steer if they could. Sorry if I made it sound like I was leaning on you for more help - not my intention. Thanks again for your help - much appreciated!
I do like to help people. I hope I pointed you in the right direction.
Again, nice job. Learn to use it and you might find that you can use it to make an individual transfer curve for each tube. Think about something like that. Or you could measure mu at your specific operating point in your circuit. I think you could do that with your tester.
I haven't messed with tubes in decades. But some day.....
Again, nice job. Learn to use it and you might find that you can use it to make an individual transfer curve for each tube. Think about something like that. Or you could measure mu at your specific operating point in your circuit. I think you could do that with your tester.
I haven't messed with tubes in decades. But some day.....