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 Post subject: something about 6au6 cathode shorts
PostPosted: Jan Tue 30, 2018 3:57 am 
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Joined: Aug Sat 16, 2008 10:42 pm
Posts: 93
Found two shorted to cathode from heater 6au6s in a stromberg carlson FM AM tuner. I pulled six out of my NOS boxes and all of them show some level of short on my eico 667 tester. Other tube types are testing fine; I am quite confident this thing is accurate. Has anyone else noticed this happening with their NOS little pentodes? RCA, Westinghouse, DuMont, Sylvania, GE, Fisher, Heathkit...at least one sample of each of these is showing this condition. Most of them have lots of transconductance but I am reluctant to use them in a circuit. Thank you.

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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Jan Tue 30, 2018 10:04 am 
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Greetings to thomash85715 and the Forum:

I wouldn't be so quick to trust your tube tester. Shorts detection circuits are often overly sensitive.... many will show some level of short if the tube sockets get surface dirt on them.

If you really want to know, jig the tube up in a socket, apply DC to the heater and apply 100 volts DC between the heater and cathode. (Do not exceed the maximum values for heater-to-cathode voltage given in a tube manual.) Measure the current... it should be less than 100 uAmps.

The above data were taken from a table posted here on ARF in the following thread:

http://www.antiqueradios.com/forums/vie ... =8&t=81699

If your NOS tubes all show some HK leakage on the Eico, then it is probably the tester. The Eico tube tester is flagged in the same thread as being overly sensitive when measuring leakage.

Regards,

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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Jan Tue 30, 2018 2:50 pm 
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Joined: Nov Wed 07, 2007 12:44 am
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Location: Hawthorne, Ca
If every 6AU6 you test shows HK leakage, I would suspect an issue with your tester. I would recommend testing the tubes in question on another tester if one is readily available. Harry


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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Jan Tue 30, 2018 3:56 pm 
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Problems like this do come up, but the odds of finding two used tubes and six new ones all failing the same test are a lot lower than the odds of something being wrong with the tester. The fact that it tests other tubes okay may be a red herring; unless those other tubes use the same switch and control settings and socket as the 6AU6, the tester could have leakage across a switch or a bit of debris between to socket pins that only shows up when one kind of tube is tested. The first thing to check is whether or not the shorts light comes on if you set the tester up but don't insert a tube. The 6AH6, 6BA6, 6BD6, EF93, EF94, 12AC6, 12AF6, and 12CX6 are just a few of the tubes that have the same basing as the 6AU6, so the pin selector switches should line up the same (heater and other control settings will be different).

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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Jan Tue 30, 2018 5:00 pm 
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Unusual for several new tubes to show shorts but EICO 667 doesn't use a neon lamp for this test. It actually measures leakage on a meter.

Agree test other similar tubes and see what happens.

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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Jan Tue 30, 2018 7:42 pm 
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Yes this EICO uses a meter calibrated in ohms, not a neon light. And yes it is quite sensitive though I personally feel there is no such thing as TOO sensitive in a tube tester. I went to our local supply store Elliott's and they had a box of 6au6 for $5 each. One was open, one showed a significant leak on their old drugstore uncalibrated tester, one showed a steadily worsening leak and one was actually a 0A2 in 6AU6 box so at that point I gave up. I would submit there is no such thing as a "new" 6AU6 but just varying degrees of NOS. I did try testing a 6ah6 since the base is the same and as someone pointed out, a fault in the tester switching or socket would be revealed. And the 6AH6 tested fine with zero leakage. And some 6AU6 I had tested the night before now showed less leakage. So I am quite confused. Thank you all for weighing in.

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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Jan Wed 31, 2018 3:27 pm 
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My (unfortunate) experience with shorts and grid emission. I have a Heathkit TT-1. During restoration, I used contact cleaner on all of the switches. Unfortunately... the Grid Emission (or whatever it is called) should have had its contacts cleaned, not sprayed. Now, even with no tube in the tester, on a humid day, it will indicate grid emission.

Lesson Learned, too late of course...

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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Feb Thu 01, 2018 11:43 pm 
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Location: Independence, MO
Don, what type of contact cleaner did you use? I have heard that deoxit will do that on Sencor tube testers. Don't know what to clean them with that does not saturate the switch contact base.

Jim


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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Feb Fri 02, 2018 1:36 pm 
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rja2907@comcast.net wrote:
Don, what type of contact cleaner did you use? I have heard that deoxit will do that on Sencor tube testers. Don't know what to clean them with that does not saturate the switch contact base.

Jim

Jim, I am pretty sure that it was DeOxIt. I have been fooling around with electronics for about 60 years but never stop learning. My learning experience here was to NOT wet down a phenolic switch since it is porous. My game plan (eventually) is to remove the switch carefully and try to dry it out with denatured alcohol or acetone etc. Just haven't got "A Round Tuit" yet :wink:


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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Feb Wed 07, 2018 10:47 am 
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Location: OHIO 43074
Quote:
My game plan (eventually) is to remove the switch carefully and try to dry it out with denatured alcohol or acetone etc

It is probably not necessary to remove the switches. Spray them down with denatured alcohol (back in 'the day' i'd use a freon based spray) ... air blast dry with canned air (do not use your air compressor or you'll be chasing parts all over the shop) ... then bake with a heat gun, setting the entire tester out in the sun on a hot day, or convection oven at 100 degrees for a while.

That should take care of any "stuff" that may be in or on your switch wafers. Then you can use deoxit on a q tip for the contacts only, and some non migrating lube on the detents. If that fails, I'd repeat the entire process at least once before going to all the labor of un wiring the switches. Not sure removing them would produce any better results than the above.

Hope this works!

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 Post subject: Re: something about 6au6 cathode shorts
PostPosted: Feb Thu 08, 2018 2:21 pm 
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Barry H Bennett wrote:
...
It is probably not necessary to remove the switches...
Hope this works!


Thanks. It is only one switch. And yes, I have learned my lesson (the hard way) about spraying that stuff. Q-Tip application actually the way to go, and it saves precious product!

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