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So over the past few months I have been collecting nos el84's to try in my TMB. I now have a nice variety of many different european and american tubes. Some sets are matched pairs, some are singles that I have paired up. I seem to prefer the sound when the pairs are not a perfect match. They seem to have more of a "3d" effect and sound livelier to my ears. Most of the tubes that I have tried run just fine in the amp with no redplating but I have come across a few that redplated instantly. Is it possible for a tube to test good on a dealers tester but its current draw is just to high to run safely in a cathode bias amp. I'm just not sure if the tubes are truly defective, or if their current draw is just to high to operate properly in my amp. I am currently running a 150ohm bias resistor. Has anyone else had this experience with a set of power tubes in their amp? I suppose a tester would let me know if they are good tubes but unfortunately that is not in my budget at the moment.

danman, just curious... any chance that the red plating tubes were ebay acquisitions?

Unfortunately yes! I have had very good luck so far with purchases but I know there is always a risk. Both tubes that ive had problems with did test good on the sellers tester, but I know that test numbers dont tell the whole story. I have gotten several tubes with great test scores but they were obviously very used tubes. These couple of tubes that are redplating appear to have had very little use. They have nice full flashing, no burn marks on the plates and very clear glass at the base. One is a Mullard and the other is an RCA. If they are truly good tubes I would like to keep them for the future. I could always raise the bias resistor value in my amp and see how they run but I was hoping to not have to go that far. Im hoping that their idle current draw is just to high for my amp setup, but may work fine in an adjustable bias setup. Just curious if any of you guys have run across powertubes that did this also in their amps. I suppose i'll have to break down a buy a decent tube tester so I can find out for myself in the future. Shame those things have gotten so expensive lately!

Are you using 150 ohm cathode resistor on the power tubes or 120? 150 is easier on tubes.
I've never seen a redplating tube in a 18watt I built at either value, FWIW.

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Stephen
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I haven't either Coco. Thats what made me curious as to whether anyone else here may have had that problem with certain tubes. I am running the 150ohm resistor so I figured that I should be pretty safe with most everything I try.
















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+1 You only need closely matched tubes for hifi.


These basic tube testers just test for emission levels. They just show what state the cathode is in and its ability to emit electrons. So a tube that emits lots of electrons measures as "good" or "excellent", but there's no indication whether the tubes' other characteristics, such as mutual conductance, are within spec. In this case you have got tubes with great cathodes, but which are way out of spec apart from that. Possibly the reason their cathodes are still good is that whoever first tried to use them back in the 1950s or 1960s found they were out of spec, and put them in their junk box, and then they later ended up on eBay.

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Great tones come in small glass jars!

I know its always a risk with the ebay sellers. I have bought quite a few pairs from ebay with no problems, so I feel pretty lucky that I have only gotten two bad ones so far. Like you said Zaphod, it is very likely that you will get bad tubes that someone should have tossed thirty years ago but never did. So what do you think...if they wont run properly with a 150ohm resistor are they more than likely unusable in anything?

You could try going higher on the cathode resistor, like 180 or 220 ohms. Also try reducing the screen grid voltage with those tubes. If they only redplate when the amp cranks, also changing 100 ohm screen resistors to 1k or 2.2k may do the trick. If they redplate on idle, you may need to make some changes to the power supply to reduce the screen grid voltage. Then you may get to a point where you need to consider whether there's really much point modding your amp any more for a sake of a couple of out-of-spec tubes.

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Great tones come in small glass jars!