Tube Testing Question

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Tube Testing Question

Postby FourT6and2 on Sat Apr 26, 2014 1:51 am

What does it mean when a tube tests higher than "100%"?

For example, I'm looking at some NOS tubes on eBay and the seller says:

Tubes measured:
Tube 1 - 94mA (134%)
Tube 2 - 98mA (140%)
Where 70mA is 100% for a new tube.

I'm guessing that 70mA/100% number is according to the tube's data sheet? And anything above that is higher than "spec"? I have no idea what machine the seller actually used to test or at what plate voltage the tubes (EL34s) were tested. But does a higher or lower number have an actual affect on how the tubes will perform?
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Re: Tube Testing Question

Postby OldSchoolDave on Sat Apr 26, 2014 3:16 am

FourT6and2 wrote:I'm guessing that 70mA/100% number is according to the tube's data sheet? And anything above that is higher than "spec"?


Exactly. Each tube tester (or, at least, each tube tester manufacturer) has its own measuring system.

FourT6and2 wrote:I have no idea what machine the seller actually used to test or at what plate voltage the tubes (EL34s) were tested.


Therein lies the rub. Without furnishing that information (and evidence that the tester was calibrated at the time of testing), such readings are questionable. Also note that testers do not put out the operating voltages seen in vintage amps.

FourT6and2 wrote:But does a higher or lower number have an actual affect on how the tubes will perform?


Typically, yes. But, that's not the only factor. Tubes should also be checked for leakage and shorts. And, microphonics (more of an issue in preamp tubes) can't be measured by a tester.

My tube tester has not been calibrated. I keep a set of new tubes as control units and adjust the tester's Sensitivity so they read at 100%. From there, I can tell if another tube is stronger or weaker. Relative differences are usable. The actual numbers can't be trusted on an uncalibrated tester.

Dave
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Re: Tube Testing Question

Postby FourT6and2 on Sat Apr 26, 2014 3:29 am

OldSchoolDave wrote:
FourT6and2 wrote:I'm guessing that 70mA/100% number is according to the tube's data sheet? And anything above that is higher than "spec"?


Exactly. Each tube tester (or, at least, each tube tester manufacturer) has its own measuring system.

FourT6and2 wrote:I have no idea what machine the seller actually used to test or at what plate voltage the tubes (EL34s) were tested.


Therein lies the rub. Without furnishing that information (and evidence that the tester was calibrated at the time of testing), such readings are questionable. Also note that testers do not put out the operating voltages seen in vintage amps.

FourT6and2 wrote:But does a higher or lower number have an actual affect on how the tubes will perform?


Typically, yes. But, that's not the only factor. Tubes should also be checked for leakage and shorts. And, microphonics (more of an issue in preamp tubes) can't be measured by a tester.

My tube tester has not been calibrated. I keep a set of new tubes as control units and adjust the tester's Sensitivity so they read at 100%. From there, I can tell if another tube is stronger or weaker. Relative differences are usable. The actual numbers can't be trusted on an uncalibrated tester.

Dave


Thanks! I guess I'm taking my chances either way then. Too bad I can't find the tubes I'm looking for from a reputable dealer here in the US. I've only been able to find them on eBay from Europe. Seller has good feedback though and the tubes—from the pictures—are legit. It's a gamble.
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Re: Tube Testing Question

Postby OldSchoolDave on Sat Apr 26, 2014 2:16 pm

FourT6and2 wrote: It's a gamble.


Always is :( .

In a perfect world, you could do a "test purchase" of a lower price tube from the seller before raising the risk.

Barring that, seeking opinions outside of the ebay feedback system can help to reassure a person.

What tube model can't you find in the States?

Dave
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Re: Tube Testing Question

Postby FourT6and2 on Sat Apr 26, 2014 4:12 pm

OldSchoolDave wrote:
FourT6and2 wrote: It's a gamble.


Always is :( .

In a perfect world, you could do a "test purchase" of a lower price tube from the seller before raising the risk.

Barring that, seeking opinions outside of the ebay feedback system can help to reassure a person.

What tube model can't you find in the States?

Dave


NOS RFT short-base EL34 and NOS Tesla EL34s from the '60s/'70s in good shape. I found what I needed on eBay, though. But had to get the RFTs from Poland and the Teslas from Ukraine. There are a few retailers here in the USA that have RFTs, but they're the ones from the '80s that are questionable and the Teslas are also from the '80s/'90s when quality went downhill. Or so I'm told.
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Re: Tube Testing Question

Postby OldSchoolDave on Sat Apr 26, 2014 6:42 pm

FourT6and2 wrote:
NOS RFT short-base EL34 and NOS Tesla EL34s from the '60s/'70s in good shape. I found what I needed on eBay, though. But had to get the RFTs from Poland and the Teslas from Ukraine. There are a few retailers here in the USA that have RFTs, but they're the ones from the '80s that are questionable and the Teslas are also from the '80s/'90s when quality went downhill. Or so I'm told.


PM Luc.Key here on the forum and see if he's got any.

He's in the Czech Republic and usually has a line on good Teslas and RFT's. I've purchased preamp and power tubes on several occasions and highly recommend him :) . No problem mailing to the U.S..

If he's out of stock, perhaps he knows the other sellers and can provide background info?

Dave
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Re: Tube Testing Question

Postby FourT6and2 on Sun Apr 27, 2014 2:56 am

OldSchoolDave wrote:
FourT6and2 wrote:
NOS RFT short-base EL34 and NOS Tesla EL34s from the '60s/'70s in good shape. I found what I needed on eBay, though. But had to get the RFTs from Poland and the Teslas from Ukraine. There are a few retailers here in the USA that have RFTs, but they're the ones from the '80s that are questionable and the Teslas are also from the '80s/'90s when quality went downhill. Or so I'm told.


PM Luc.Key here on the forum and see if he's got any.

He's in the Czech Republic and usually has a line on good Teslas and RFT's. I've purchased preamp and power tubes on several occasions and highly recommend him :) . No problem mailing to the U.S..

If he's out of stock, perhaps he knows the other sellers and can provide background info?

Dave


Wish I had known that earlier. I already bought them on eBay. But I'll shoot him a PM. :)
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