Maybe help me learn a thing or two about tubes.
I purchased a set of 3 tubes that were advertised as a "Strong match".
The only shop in town with a tester told me that two of these were as good as possible. Above average for New Old Stock.
But the third was... not bad but not 'matched across sections' and definitely not a match with the other two.
Essentially the numbers were 1600/1600 for one, the same for the second, and the third was 1200/1400. I don't know what the units measured were.
When reading online I usually see numbers between 60 - 120 and I'm not sure what the units are. My shop is measuring something in the 1400 - 1600 range.
Here is a video of the tube he claims is 'good, not bad but not excellent.
https://photos.app.goo.gl/SA5wkFQSYGAeuVsC9
Does anyone have thoughts on the compatibility or matching of these three tubes?
Any schooling you do on the various tests and units being measured here?
Thanks.
I purchased a set of 3 tubes that were advertised as a "Strong match".
The only shop in town with a tester told me that two of these were as good as possible. Above average for New Old Stock.
But the third was... not bad but not 'matched across sections' and definitely not a match with the other two.
Essentially the numbers were 1600/1600 for one, the same for the second, and the third was 1200/1400. I don't know what the units measured were.
When reading online I usually see numbers between 60 - 120 and I'm not sure what the units are. My shop is measuring something in the 1400 - 1600 range.
Here is a video of the tube he claims is 'good, not bad but not excellent.
https://photos.app.goo.gl/SA5wkFQSYGAeuVsC9
Does anyone have thoughts on the compatibility or matching of these three tubes?
Any schooling you do on the various tests and units being measured here?
Thanks.
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Numbers between 60 and 120 would be for mu (dimensionless) if we are talking about 12AX7-oids.
That would be a lot of spread, as mu can match well even if gm doesn‘t. It really doesn’t get much above 100. There are derivatives of the 12AX7 (ie, 5751) which normally run around 60, but they are technically different types.
That would be a lot of spread, as mu can match well even if gm doesn‘t. It really doesn’t get much above 100. There are derivatives of the 12AX7 (ie, 5751) which normally run around 60, but they are technically different types.
That third tube is well out of the transconductance spec for a NOS 12AX7.
The seller should refund for that tube, if you still want the other two.
The seller should refund for that tube, if you still want the other two.
How did the seller test them ? maybe his tester was badly calibrated or maybe it was just a simple emission tester and all 3 tested the same, it's possible the seller may have honestly believed his own description.
A half decent tube tester will usually give you 2 bits of info that you can use to decide if the tube is still serviceable i.e. it has come to the end of it's useful life - anode or plate current and transconductance.
When tested with the correct voltages a NOS 12AX7 should have 1.2mA anode/plate current and a transconductance of 1.6mA/volts if you are in the UK - or - 1600 umhos if you are in the US, as the tube ages the anode/plate current and transconductance will gradually fall off until the tube eventually becomes so weak it fails to function, it is generally regarded that once these have fallen to approx 65% of NOS values the tube is no longer serviceable, however this I believe applies to tubes being put back into service and still have an expected life of a few years so don't be too quick to throw out tubes that fall into the unserviceable category, they may still work fine in your amp.
Where the 12AX7 is concerned 65% of 1600 umohs is 1040 so the suspect tube in question would appear to be serviceable but not particularly well matched for each triode in the envelope.
A half decent tube tester will usually give you 2 bits of info that you can use to decide if the tube is still serviceable i.e. it has come to the end of it's useful life - anode or plate current and transconductance.
When tested with the correct voltages a NOS 12AX7 should have 1.2mA anode/plate current and a transconductance of 1.6mA/volts if you are in the UK - or - 1600 umhos if you are in the US, as the tube ages the anode/plate current and transconductance will gradually fall off until the tube eventually becomes so weak it fails to function, it is generally regarded that once these have fallen to approx 65% of NOS values the tube is no longer serviceable, however this I believe applies to tubes being put back into service and still have an expected life of a few years so don't be too quick to throw out tubes that fall into the unserviceable category, they may still work fine in your amp.
Where the 12AX7 is concerned 65% of 1600 umohs is 1040 so the suspect tube in question would appear to be serviceable but not particularly well matched for each triode in the envelope.
@retailer . Thanks for the details. Unfortunately I can no longer access the listing or the photos of the test readings; only the listing title which includes the phrase "strong match".
The seller seems open to working with me. They reported that all three "had matched coding and the same manufacturer date" and that they all tested balanced. Anyway. Everyone seems to be on the same page w/r/t one tube being not as repeated. We will see if they can replace it.
Thanks again everyone for helping me calibrate my expectations and understanding of these tubes.
The seller seems open to working with me. They reported that all three "had matched coding and the same manufacturer date" and that they all tested balanced. Anyway. Everyone seems to be on the same page w/r/t one tube being not as repeated. We will see if they can replace it.
Thanks again everyone for helping me calibrate my expectations and understanding of these tubes.
Great question. Because I don't know if it's a big deal or not, I was erring on the side of caution.
They are being used in an Icon Audio PS1 phonostage: https://iconaudio.com/ps1-mkii-all-valve-phono-stage-ps1-mkii
I assume the two parallel tunes are left and right channels. I am unclear on how the third tube is being used. All three are listed as output.
Any thoughts?
They are being used in an Icon Audio PS1 phonostage: https://iconaudio.com/ps1-mkii-all-valve-phono-stage-ps1-mkii
I assume the two parallel tunes are left and right channels. I am unclear on how the third tube is being used. All three are listed as output.
Any thoughts?
if each tube is quite close for both halves, then maybe not an issue, and probably qualifies as being described as 'matched'.
Regarding earlier discussion about units and range. Here is another description I'm confused by;
Is it plate current or transconductance?
Is this just sales gibberish? Like, do they test at 110% or at 100%?
They can't be measuring umohs or mA here.
Is it plate current or transconductance?
Is this just sales gibberish? Like, do they test at 110% or at 100%?
Probably the third tube that is alone in the back is the output cathode follower for both channels . You could use there the "unmatched" tube because it doesn't matter that much .
Some testers like the B&K have a percentage scale to read transconductance - 0-120%. Some have an arbitrary scale with nominal value different for different tubes. Not that those which read transconductance number are better, or that two such will agree...
Those numbers are arbitrary, and are of no value other than comparison with other tubes.
Only mu and gm are meaningful. Here is a chart of typical parameters for the 12AX7.