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 Post subject: Cathode Filament Tube Shorts
PostPosted: Jun Sun 02, 2019 6:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Detroit, Michigan
I have a Hickok 6000A tube tester, and it flags it's share of cathode to filament shorts. Question is... is it really necessary to replace the tubes that have this condition, if no problem is noticed in the operation of the radio or tv?


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 Post subject: Re: Cathode Filament Tube Shorts
PostPosted: Jun Sun 02, 2019 6:31 pm 
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Location: Dayton Ohio
Quite often they will work just fine. Especially if the cathode is grounded.

Rarely are they a dead short, and have significant resistance. Injection of hum and distortion is usually the issue.

-Steve

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 Post subject: Re: Cathode Filament Tube Shorts
PostPosted: Jun Sun 02, 2019 6:33 pm 
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***It Depends***

It all depends on what the circuit is. With a traditional basic radio, which one side of the filament circuit grounded, you will almost certainly have a significant hum. The only way this would NOT cause a problem is if the short were at the grounded end.
BUT--what else is tied to the cathode? If there is---eg---a cathode resistor, the the short will alter the bias no matter where it is.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Cathode Filament Tube Shorts
PostPosted: Jun Sun 02, 2019 10:53 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
The indication of a cathode filament leakage or short was a reason to sell a customer
a new tube in the days of radio and TV service.

There was a good reason. Part of your fee for labour was the time spent testing the tubes.


For a given type of tube the tendency for the tube to have hk (heater cathode) leakage
could be related to how hard the tube worked (how hot it got) in a given set.

TV repairman knew some things about customer complaints. A snowy picture was one.
When TV became popular, certain areas needed roof top aerials. A TV dealer had to include
an aerial installation, of the set just would not work. So snow was a big deal, and TV tuner tubes
did work hard.

Hk leakage goes back to how the tube was made, and one of the most critical parts of the
tube was the heater's coating.

Tube makers depended on standards for testing the chemicals. If a batch slipped through
that possibly cracked on the outer edge of the bundle that is inserted in the cathode
sleeve, then soon tubes of a certain date code would start to pop up. And if the tubes were shorting
hk and burning up resistors in TV tuners, TV servicemen would start to gripe loudly
and even badmouth the brand. Woe betide that particular plant.

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 Post subject: Re: Cathode Filament Tube Shorts
PostPosted: Jun Tue 04, 2019 2:08 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
There’s a big difference between a heater to cathode **short** versus heater to cathode leakage.

A heater to cathode short, meaning a few ohms or less between heater and cathode, is a major defect that usually occurs after a tube has arced or suffered some other catastrophic event. It will affect the operation of most circuits. It is not recoverable and the tube should be replaced immediately to prevent damage to other components.

Heater to cathode **leakage,** on the other hand, is inherent in all tubes. It tends to get worse (i.e. the leakage resistance decreases) with operating hours and temperature. In most cases it is not a problem as long as the leakage resistance remains high enough that it does not impact circuit operation. Generaly speaking, the higher the circuit impedances, the less leakage can be tolerated. How bad the leakage can get before the tube has to be replaced depends on the particular circuit.

You can experiment with any tube tester that measures leakage to find out how much leakage resistance it takes from the cathode to the heater to give a leakage indication. That should give you an idea of how much leakage a tube would have if it produces the same indication.

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 Post subject: Re: Cathode Filament Tube Shorts
PostPosted: Jun Tue 04, 2019 3:39 am 
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Joined: Jul Mon 08, 2013 2:48 pm
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... and your Hickok 6000A will not differentiate between a dead H-K short and leakage, the neon indicator will glow in both situations. As Chris said above, a tube with a dead H-K short should be discarded and no longer used in any equipment. This can be easily checked with a DMM (continuity tester). A tube with some high impedance H-K leakage could still be used but this is application-dependent. In low level audio circuits it might induce hum and visible hum bars in TV receivers. Most audio output power tubes can still be used without any noticeable effect. I have a lot of RCA 6973 output tubes which all light up the H-K short indicator on my Hickok and all works perfectly in the amplifier.
CAUTION !! It should be noted that all the above comments are only valid for H-K (Heater-Cathode) leakage, a tube with any other inter-electrode short or leakage must be considered as defective and should never be used in any electronic equipment.


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 Post subject: Re: Cathode Filament Tube Shorts
PostPosted: Jun Wed 05, 2019 12:03 pm 
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Location: Dayton Ohio
This is a reason why I really like the Jackson 648 series of tube testers.
Except for the very early models, most have a shorts/leakage sensitivity control with indications of what the leakage resitance might be.
I wouldn't say "calibrated" but its a ballpark reading.

Its easy to see the difference between leakage and a dead short. Leakage will make the neon indicator glow dimly and/or flicker and can be varied by the control. Dead short the indicator glows brightly solidly lit.

and yeah, it shows which elements have shorts/leakage.

-Steve

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-Pre-War FM
Consoles and floor models, the bigger, the better!


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 Post subject: Re: Cathode Filament Tube Shorts
PostPosted: Jun Wed 05, 2019 1:10 pm 
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Joined: Sep Tue 15, 2015 1:16 am
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Location: 18424 PA
You can also try cleaning the tube pins and Bakelite itself. I've seen used tubes that were very dirty and once cleaned passed with flying colors.


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 Post subject: Re: Cathode Filament Tube Shorts
PostPosted: Jun Mon 10, 2019 4:15 am 
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Joined: Aug Tue 24, 2010 8:56 pm
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Location: Northeast Florida
If you're seeing repeated indications of shorts, the first thing I would do is check the calibration on the tester. Many testers have an adjustable pot for setting the short threshold (even my cheapie B&K 606 has one). And I have found it necessary to recalibrate it from time to time.

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