tested tubes for sale, question

Why do you think when tubes are listed for sale, and they are tested... the seller says the tubes read 50 whatever with a minimum good reading of 25. don't I need more info, like the max reading to really get an idea how much life is left? or am i missing something?

scott

I'm certainly no expert, but this is what I've learned from this Board and other places.

There are two readings that are most relevent for both preamp and power tubes. One is how much life is left in the tube, which is described as mutual conductance or trans-conductance. Different model tube testers use different ranges. Reputable sellers will give you the make and model of the tube tester, the actual reading of the tube, the "mimimum good" (for used tubes), and the new tube reading for their machine for the type of tube being tested. Some tube sellers will state tubes are "matched" when the trans-conductance of two power tubes are very close. This is NOT how power tubes are matched.

The other readings are how much current the tube is drawing. These reading should be absolute irrespective of the tube tester as the tester should be reading actual current draw per triode for preamp tubes, and idle current draw at a specific plate voltage for power tubes. For preamp tubes, they should give you the number for each triode (as well as mimimum good and new readings), whereas for power tubes it is a single reading per tube, known as idle current draw. These readings are really how power tubes are matched, and is how Mesa sorts their tubes into colors (and Groove Tubes into their distortion ratings). For power tubes, a main shortcoming here is most tube testers read the idle current draw at 400 plate volts. The most relevant measurements for power tubes would be the idle current draw at the plate voltage YOUR amp draws. When I'm shopping for non-Mesa vintage power tubes, I'll ask for the idle current draw at a specific plate voltage. If they can't answer that, I walk away as I have no way of testing the tubes myself.

Truly knowledgeable tube sellers (Doug's, Eurotubes, etc) will know the plate voltage of your model amp and sell you tubes that meet the spec for your Boogie.

yes thanks dodger,

I was just wondering, 'cause I usually find mesa str 415's, so I don't worry too much about them being within range ma-wise. I just always ask the seller because most times they just list the min. good and the reading of the specific tube, but leave the max or what the new tube reading would be.

scott

spamalot said:

zodiac272 said:

I just always ask the seller because most times they just list the min. good and the reading of the specific tube, but leave the max or what the new tube reading would be.

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Probably because most don't know anything beyond what their tester tells them.
Or there tester has limitations (they don't all test for the same thing).
In general there are alot of clueless tube sellers on ebay using limited testing equip.

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this is true... sometimes i get an intelligent response from sellers, sometimes no response


scott

After reading thousands of tube listings I have a good idea what sellers mean by what kind of tester they are using in that listing. Generally a new tube reads a quarter to a third above the minimum value for a given tester. I have a very nice vintage Hickok tester. What I have found that is really interesting is how good sounding many old tubes with low test scores actually sound in circuit. To me the thing that is much more bothersome is a 12__7 that is excessively microphonic.

212Mavguy said:

After reading thousands of tube listings I have a good idea what sellers mean by what kind of tester they are using in that listing. Generally a new tube reads a quarter to a third above the minimum value for a given tester. I have a very nice vintage Hickok tester. What I have found that is really interesting is how good sounding many old tubes with low test scores actually sound in circuit. To me the thing that is much more bothersome is a 12__7 that is excessively microphonic.

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+1, or otherwise noisey (scratchy, swishy, etc..). The best sound I ever got from a preamp tube was just before it died! It was a bit noisey, but really put out some incredible tone.