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B&K tube testers and false positives for shorts

post #1 of 17
9/25/09 at 1:55pm
Thread Starter 
I have a B&K 747B tube tester. I have had quite a few tubes that test positive for very minor shorts (the lamp doesn't turn on but the meter moves a bit into the area where the manual says says that I should assume that is is shorted). I have heard that some believe that these small shorts on the B&K's should be ignored because the B&K 747B is very sensitive, much more sensitive than Hickock's etc and that is may be picking up on something else besides a short. I don't really know anything about testers so I wouldn't be able to understand why one would be getting a false short. Many of these tubes I have bought from reputable dealers (Paul Lindemann, Brent Jesse, etc.) and they are tested on "real" testers with no shorts, etc before shipping. I just had my tube tester tested and calibrated so I know there is nothing wrong with the tester. Anyone else have this experience with B&K testers?

In particular, I just got a pair of Sylvania "Bad Boys" from a dealer for a good price on audiogon that tested good before they were sent (no shorts, etc.) but one of the sections shows a very minor "short" on the B&K 747B. The guy says he will refund the money but I am wondering whether or not I should send them back. I would like to have a pair and I am tired of sending back tubes that may be perfectly good!
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post #2 of 17
9/25/09 at 3:54pm
Hi Shelly

I have the same tester and I remember when you purchased yours. I have had that problem with a couple of 6SN7s, however, when I sent them back they did in fact have a small short as the seller re-tested and sent me a new pair.

I don't know if this would be your issue or just sensitivity.
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post #3 of 17
9/25/09 at 4:06pm
Thread Starter 
Thanks for the reply.

I remember sending back one pair of TS roundplates to get re-check for shorts and when they were sent back, there were no shorts (tested on an Amplitrex and on a TV7). I ended up not keeping them because I was too paranoid about putting any type of tube in my WA5-LE that ever showed any signs of shorts on any tester. However, I wonder if I am being too paranoid.

I wonder if a tube can acquire some intermittent shorts from shipping, like if small piece of metal that is broken off moves around and causes a short?

I am pretty confused to say the least.
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post #4 of 17
9/25/09 at 4:19pm
Hi Shelly:

I have 2 different B&K testers from you - a 650 (which is the big Mutual Transconducance type like yours) and a smaller 606. Occasionally a tube will test "barely" positive for shorts on one tester, and not the other. It can go either way. This is rare, but it happens. I have generally made the assumption that if only one tests positive for shorts, then it's the tester being overly sensitive. But I have never had one test strongly for shorts on one and not the other.

So I do think sometimes when the shorts indicator lights very weakly or intermittently, it's not really a short. But that's one reason I keep two testers. If it behaves that way on BOTH testers, I assume there is really a short.
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post #5 of 17
9/25/09 at 4:26pm
You are welcome..

Trust me, I wish it was a sensitivity issue with mine. I purchased a real nice lot of 6SN7s, all untested, so I gambled...Pulled some nice ones out of the group, but there were about five 6SN7Ws that I really wanted to be good. Love those tubes...Was able to salvage two....I was like keeping my eyes closed when I was testing for shorts and saying to the meter "don't move, c'mon steady"...lol

I have been in contact with a local head-fier who has a Hickok and we have tried to get together several times to compare, but schedules etc...have not been able to...

Hope you sort it all out.....I personally would not put any possible shorted tube in any amp, let alone your beautiful WA5-LE...
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post #6 of 17
9/25/09 at 4:37pm
Thread Starter 
Skylab: This is useful. Perhaps I should pick up another tester. By the way, do you have a meter, light or both. I have never had the shorts light go on (only had some make the meter move). It has to get to some level before the light will go on.

HeatFan12: LOL, everytime I get a new tube I hope the same thing... worrying that it will test bad. Although, since I never buy any tubes from ebay nor do I buy untested, I probably shouldn't be as paranoid as I am.
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post #7 of 17
9/25/09 at 4:39pm
Shelly, both my testers have only an indicator light for shorts - the meter is for leaks, and of course either the good/bad scale or the transconductance scale. Yours has a meter reading for shorts???
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post #8 of 17
9/25/09 at 5:08pm
Shelly, do you have the manual or at least a copy?

The way I read it, under the "Condense Testing Procedure" portion what you need to pay attention to is one of two things while testing for shorts.

1. If the SHORTS light illuminates, the tube is bad.
2. If the meter moves into the GRID EMISSION REJECT portion of the scale the tube is leaky and should be replaced but technically isn't bad yet.

The same applies for when you check the GRID EMISSION test.

It is very true that different tube testers have different thresholds for detecting leaky tubes. Leaky tubes generally still work but are very near the end of their life even though other tests might show them to be perfectly good. I can't speak about your B&K (not having owned that model) but my Jackson 648S is very good at testing for leakage and actually has a variable setting to help quantify just how bad the tube is. Either way, I toss leaky tubes as a rule. The consequences of a tube failure are not something I like to risk.

Hope that helps.

Nate

P.s. If you want a copy of the 747 manual shoot me a PM.
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post #9 of 17
9/26/09 at 12:03pm
Thread Starter 
Quote:
Originally Posted by Skylab View Post
Shelly, both my testers have only an indicator light for shorts - the meter is for leaks, and of course either the good/bad scale or the transconductance scale. Yours has a meter reading for shorts???
I thought there was both a meter and indicator light for shorts but now I am not so sure.

Nate: Thanks for your response, this helps clear things up a bit. I have a manual but thanks for offering to send me a copy of one.

In the comprehensive test section under "Shorts test" it says:

1. Press and hold down the SHORTS pushbutton 8
2. The tube is defective if the SHORTS lamp 3 lights or the meter deflects into the GRID EMISSION REJECT portion of the scale

Under the "grid emissions and gas test" it says the same thing:

1. Press and hold down the GRID EMISSIONS pushbutton 7
2. The tube is defective if the SHORTS lamp 3 lights or the meter deflects into the GRID EMISSION REJECT portion of the scale

I took this to mean that when you push the shorts button and the meter moves then the tube has a short. However, I guess it just technically says that it is "defective," not that it has a short. You are saying that it is "leaky" which perhaps makes more sense. It is hard for me to understand how a tube could have a minor short (an intermittent short I can understand). It makes sense that certain testers you be more sensitive to leaky tubes.

Edit: I just read the "feature" and under "shorts test" it says:

"A single pushbutton shorts test checks shorts or leakage between elements. Leakage paths up the 1 megaohm will light the SHORTS lamp, and leakage as low as 1/2 microampere can be detected on the meter. '

I guess I should have read the entire manual. Bad me. Now I will just have to figure out what in the heck a tube being leaky really means. It seems that all those tubes that I thought had shorts were actually "leaky." I guess in the end it doesn't matter since they are all at or near the end of their life.

I guess I will send back the tube for a refund since the seller was nice enough to offer this.

By the way, I have another question. If the meter moves (and there is no light lit), can I assume that the tube has no short? Could it damage the tester to test these tubes further?

Thanks again for your help Nate!!!!
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post #10 of 17
9/27/09 at 10:08am
Quote:
Originally Posted by shellylh View Post
By the way, I have another question. If the meter moves (and there is no light lit), can I assume that the tube has no short? Could it damage the tester to test these tubes further?
So this makes more sense, as it would be similar to the way my B&K testers both work - the meter is testing for gird emissions leaks.

A tube with actual shorts is a much bigger problem to use in an amp than a tube that tests positive for emissions leaks. There is even some debate as to how much emissions should be allowed and still have the tube be considered "good". The majority of the tubes I test do not move the leaks meter at all on my 650; a few I have tested pegged it, and those go right in the trash. Tubes that move the meter just a little, if they test OK otherwise (like no shorts and the transconductance is strong) then I usually keep them.

Unless the tube is very leaky, the odds of it hurting either tester or amp are very small, IMO. Shorts on the other hand can be a real problem.
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post #11 of 17
9/27/09 at 12:14pm
Thread Starter 
Yeah, it makes much more sense. I feel like an idiot now for thinking all this time all those tubes had shorts (thought I was having some really bad luck). But I am happy to know now that they were just a little leaky.

I am going to send back the Bad Boys since I would rather wait and try and get a pair that test good all around.
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post #12 of 17
9/27/09 at 1:37pm
sorry to hear about your troubles with the 6sn7 tubes. i usually toss 1/5 of the tubes i
purchase due to the same reason. a little leaky gas can lead to major shorts due to
increase in heat being generated. gas has a tendency to cause the tube to conduct
higher and hotter. almost all shorts are caused by excessive heat causing weak points
warping and touching of elements inside the tube. sometimes you can just tell if the tube
is leaky, also known as a minor-short, by looking at the flashing. if the flashing is
milky or whitish in color, to any degree, it's most probably leaking. also, white powder may
accumulate near the pin and glass envelope; this is high indication of leakage. also, if
your meter is bouncing and not stable it's a good indicator that the tube has leakage
issues, which will eventually lead to shorting, internal arc and death to many parts of
your amp.
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post #13 of 17
9/27/09 at 1:52pm
Thread Starter 
takezo: Thanks for your informative post. It wouldn't be so annoying if 6sn7's weren't so damn expensive!
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post #14 of 17
9/29/09 at 2:44pm
It can sometimes be worth the time to let a tube warm up a bit prior to testing. I've had tubes test positive for grid emissions initially, but later test completely normal. Often, the grid emission result doesn't reappear again.

The B&K 747 is very sensitive to grid emission. Some people think that it's overly sensitive, but I'd rather have a few false positives than let a bad tube get past the tester into an amp. I've also got a Heathkit TT-1A that I use if I've got any doubts about results from the B&K.
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post #15 of 17
9/29/09 at 4:26pm
Thread Starter 
Good advice.

I usually only let a tube warm up for about a minute. Should I wait longer than this - 5 minutes?

Quote:
Originally Posted by Hirsch View Post
It can sometimes be worth the time to let a tube warm up a bit prior to testing. I've had tubes test positive for grid emissions initially, but later test completely normal. Often, the grid emission result doesn't reappear again.

The B&K 747 is very sensitive to grid emission. Some people think that it's overly sensitive, but I'd rather have a few false positives than let a bad tube get past the tester into an amp. I've also got a Heathkit TT-1A that I use if I've got any doubts about results from the B&K.
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