[Boatanchors] Boatanchors Digest, Vol 121, Issue 45

Wed Feb 26 15:44:08 EST 2014

Hi Bill, thanks for the reply.... The tubes are marked "Sonar", the maker of the radio.
In any case, I was just curious 'what' the Grid Leakage part of the tube tester was checking. 
There is not a hard failure in 1/2 of each section, of each tube. The portion of the tubes that 'fails', starts indicating 'leakage' as soon as the tube starts warming up. The leakage indication then fairly quickly goes to full scale (bad). As I mentioned, the problem is with the tubes. I just received some replacements that check perfect in both sections.

If the tubes were 'gassy',  BOTH sections should have checked bad, correct?

So, my conclusion is that the tester is applying a bias to the grid to see if it is capable of blocking the electrons from the plate.This would explain why the 'leakage' increases as the tube come up to temperature. There might (??) be internal damage to the grid structure of the 1/2 of the tube that checks bad.
Probably due to heat/stress as you mentioned, because these tubes run very hot!

The radio works with the 'bad' tubes, but it is much more sensitive and has much better audio with the new tubes. I am constantly amazed how available these 'odd' tubes are on eBay, and how cheap, too.

I know that some of the R-390A experts will hand pick tubes to find the ones that deliver the ultimate sensitivity. They are looking fore every db possible.

Thanks again,

ron
N4UE

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Sent: Wed, Feb 26, 2014 1:55 pm
Subject: Boatanchors Digest, Vol 121, Issue 45

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Today's Topics:

   1. tube question (bonddaleena at aol.com)
   2. Re: tube question (Bill Cromwell)

----------------------------------------------------------------------

Message: 1
Date: Tue, 25 Feb 2014 13:39:06 -0500 (EST)
From: bonddaleena at aol.com
To: boatanchors at mailman.qth.net
Subject: [Boatanchors] tube question
Message-ID: <8D1006FFF6BA189-1334-25CC9 at webmail-m220.sysops.aol.com>
Content-Type: text/plain; charset="us-ascii"

Hi, I think I knew the answer to this question, but old age must be catching up 
to me!

I am restoring an older tube-type VHF (FM) receiver. The kind that covers both 
VHF low and high. It was made by Sonar and during clean up, I ran the tubes 
through my Sencore tube tester. (I have several testers, but this is my 'go to' 
tester.

I found 2 'bad' tubes, which I need help understanding. The tubes are a 6BE8, 
which is a High Mu Triode - Sharp Cutoff Pentode.
The other is a 6KE8 which is also a dual device (Medium Mu - Sharp Cutoff 
Pentode).
Anyway, when testing the tubes, one section of both tubes tests fine for 
emission (I know how that part works) and NO Leakage.
However, when I set the tester to test the other 1/2 of each tube, the emission 
is fine, but the 'grid leakage' starts off low (not zero) and slowly climbs to 
the extreme right of the meter.

I'm trying to understand 'what' is being tested and how 1/2 of each tube can be 
OK, and the other 1/2 'leaky'.
It's NOT my tester, because I found a new 6KE8 in my stash, and both 1/2s test 
fine.......

Excuse my ignorance....

ron
N4UE

------------------------------

Message: 2
Date: Tue, 25 Feb 2014 14:39:57 -0500
From: Bill Cromwell <wrcromwell at gmail.com>
To: boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] tube question
Message-ID: <530CF18D.1050805 at gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

On 02/25/2014 01:39 PM, bonddaleena at aol.com wrote:
> Hi, I think I knew the answer to this question, but old age must be catching 
up to me!
>
> I am restoring an older tube-type VHF (FM) receiver. The kind that covers both 
VHF low and high. It was made by Sonar and during clean up, I ran the tubes 
through my Sencore tube tester. (I have several testers, but this is my 'go to' 
tester.
>
> I found 2 'bad' tubes, which I need help understanding. The tubes are a 6BE8, 
which is a High Mu Triode - Sharp Cutoff Pentode.
> The other is a 6KE8 which is also a dual device (Medium Mu - Sharp Cutoff 
Pentode).
> Anyway, when testing the tubes, one section of both tubes tests fine for 
emission (I know how that part works) and NO Leakage.
> However, when I set the tester to test the other 1/2 of each tube, the 
emission is fine, but the 'grid leakage' starts off low (not zero) and slowly 
climbs to the extreme right of the meter.
>
> I'm trying to understand 'what' is being tested and how 1/2 of each tube can 
be OK, and the other 1/2 'leaky'.
> It's NOT my tester, because I found a new 6KE8 in my stash, and both 1/2s test 
fine.......
>
> Excuse my ignorance....
>
> ron
> N4UE
> ______________________________________________________________
>
Hi Ron,

Are the tubes made by the same manufacturer? You may not know.

I am guessing if the tubes ARE bad that with heating something in the 
structure warps and touches something else that it shouldn't. Maybe it 
got *bumped* while it was very hot. Dilbert pulls the tube out of the 
socket live. Dilbert notices his fingers are being scorched. Dilbert 
drops the tube. Oooops.

The best way to resolve this is to put the tubes into a *real* circuit 
and run them at full voltage under real world loads (suitable for the 
tubes) and see what happens. Make sure there are appropriate fuses 
involved. Over the years I have encountered a few tubes that tested (on 
my Sencore) "flaky" but were okay and some that tested (same tester) 
okay and were absolutely scrap. The *real* circuits sorted them for me. 
Others with different testers have reported the same result. If the cold 
tube has dead shorts where there is no intended internal connection do 
NOT plug it into a circuit of any real value!! (for newbies who don't 
know that).

I look at tube testers as sorting out *gross* defects. That's useful but 
not the holy grail. If a radio works well with them they are probably 
good <wink>. Some of them will work well in one application and fail in 
another. You can sort them to use in the appropriate application but 
they usually won't stay sorted <evil grin>. Been there - done that.

73,

Bill  KU8H

------------------------------

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End of Boatanchors Digest, Vol 121, Issue 45
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