Seeking information re: tube testing results.

barontan2418 · 1396

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Offline barontan2418

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on: March 02, 2017, 05:42:53 AM
Hi. First post so please treat me gently. I just purchased 2 Tungsol 5998 tubes which were advertised as showing "Low Emissions 50 to 60 %. not certain what that means in terms of what to expect. These were very cheap compared to normal 5998 prices so I was willing to take a chance. I already have a collection of power tubes 6080 and 6AS7, RCA, Sylvania Gold,  Mullard etc but the Tungsol 5998 low emissions or not seem to blow them all away. At present using a Tungsram E80CC as driver. Would love an opinion from someone in the know re: tube test results - showing low emissions and what effects they would expect in comparison to a higher rated tube. many thanks in anticipation.



Offline galyons

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Reply #1 on: March 02, 2017, 06:58:23 AM
An emissions tube tester only tests for shorts & cathode emission.  The test provides a relative good-bad reading by testing all the tube elements  connected together, but not  the cathode & heater.  As such, the emissions tester tests for cathode emission measuring the tube as a diode or rectifier, that is just two elements. The emission tester also checks for shorts. The emission tester simply tells you whether a tube is usable or not.  The tester gives you an indication how strongly  the cathode emits.

So the 5998's are "good"  because they not shorted and cathode emissions are 50-60%.  That emission rating is only accurate to the particular tube testers voltage, amperage and resistance on that tube.  Emissions can be roughly equated to remaining tube life. So put them in the Crack and listen.  In circuit is always the best test, once you know that the tube does not have any shorts.

More sophisticated tube testers measure  transconductance (Gm). All of the tube elements are tested. But, although a better overall test of tube health than emissions, it does not test the most important criteria in preamp/driver circuits.  In preamp and driver circuits, usually triodes, voltage gain (Mu) is important for matching tubes because it is a measure of the tube's gain.  Plate resistance, (Rp) determines the output impedance of the preamp/driver circuit. No readily available commercial tube testers measure Mu or Rp. So, again, the in circuit test is the best, once you know the tube is not shorted.

All of that being said, the 5998, in the Crack, is the power tube.  Give yours a try!!

Cheers,
Geary

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Offline Paul Joppa

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Reply #2 on: March 02, 2017, 08:44:28 AM
For what it's worth, the 5998 is rated for up to 140mA cathode current (emission). In Crack it runs only 30mA. So the 50-60% maximum emission is not an issue.

Paul Joppa


Offline barontan2418

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Reply #3 on: March 03, 2017, 06:03:21 AM
Really appreciate the informative and helpful replies, thanks galyons and Paul. As you say if it sounds good and it does, that's the real test.