Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently May Wed 23, 2018 12:24 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 22 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Vacuum Tube Failure
PostPosted: Jan Sat 13, 2018 10:54 pm 
Member
User avatar

Joined: Jan Thu 26, 2017 10:33 pm
Posts: 639
Do vacuum tubes short out over time if they have been used alot? I have a Zenith 6d525 that had two tubes that shorted out in it after about a year. The shorts were intermittent. All the capacitors and out-of-spec resistors were replaced.

_________________
No matter how smart you are, there is always someone smarter than you.

-Unknown


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Sat 13, 2018 11:15 pm 
Member
User avatar

Joined: Apr Sat 22, 2006 10:46 pm
Posts: 1712
Location: Waterloo, Iowa
They certainly can, though what specific tubes were they? Is the Rectifier one of them? (Likely) It's possible they were damaged, years ago "back in the day". This is one reason why vintage tube equipment isn't operated unless and until the power supply and coupling/bypass capacitors have been renewed.


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Sun 14, 2018 12:06 am 
Member
User avatar

Joined: Jan Thu 26, 2017 10:33 pm
Posts: 639
The shorted tubes were a 12SA7 and a 12K7. The 35Z5 rectifier had also blown long ago from a bad filter cap, but the cap as well as the tube were replaced to make the radio operational.

_________________
No matter how smart you are, there is always someone smarter than you.

-Unknown


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Sun 14, 2018 10:08 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1104
Location: Cedar Rapids, Iowa, USA
I think you meant 12SK7. That is used as R-F or I-F amplifier pentode, 12SA7 as mixer tube, pentagrid converter. They are not power tubes, and should last many years, and I can't think of any reason why they should be prone to shorting out from use. Rough handling maybe.


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Sun 14, 2018 4:37 pm 
Member
User avatar

Joined: Jan Thu 26, 2017 10:33 pm
Posts: 639
I did mean 12K7, the one with a grid cap. My Zenith 6D525 uses it. http://www.nostalgiaair.org/pagesbymode ... 025095.pdf

_________________
No matter how smart you are, there is always someone smarter than you.

-Unknown


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Sun 14, 2018 5:39 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7656
Location: Long Island
How are you determining that the tubes are shorted? Not for nothing there have been threads where people report that every tube they test appears to be "shorted" and it turns out that their tester is faulty!

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Sun 14, 2018 10:30 pm 
Member
User avatar

Joined: Jan Thu 26, 2017 10:33 pm
Posts: 639
The tube shorts out in the radio, it causes the heater string to draw too much current, causing the radio to quiet down and sometimes even stop playing. I found the problem tube by heating each tube individually with a heat lamp and then putting the tube back in the radio. When the radio shorted out right after I turned it on, I found the problem tube.

_________________
No matter how smart you are, there is always someone smarter than you.

-Unknown


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Sun 14, 2018 11:43 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12520
From info in your other thread, the 12SA7 has a intermittent open heater, it is NOT shorting out...

I ran a little AA5 Admiral nearly 24/7 for around 2½ years, except for the 12SA7 all the tubes had failures... Five times for 35Z5 no less... Three were high mileage used, a new one lasted 14 months, another new nine...

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Mon 15, 2018 3:07 am 
Member
User avatar

Joined: Jan Thu 26, 2017 10:33 pm
Posts: 639
Sorry, that's what I meant. Both tubes do that. Yes, they were both originals in the set with very high mileage. This radio has seen a lot of use. It makes sense.

_________________
No matter how smart you are, there is always someone smarter than you.

-Unknown


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Mon 22, 2018 7:36 pm 
Member
User avatar

Joined: Mar Sun 01, 2009 10:27 pm
Posts: 4485
Location: Ann Arbor, MI
A somewhat late reply. Open heaters are the most common failure for tubes that are not high power tubes. But an intermittent open heater is not so common. If you still have that tube, try re-soldering the heater pins. There could be a cracked solder joint where the wire from the heater solders to the pin.

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Tue 23, 2018 12:44 am 
Member
User avatar

Joined: Jun Wed 08, 2011 2:33 am
Posts: 6567
Location: Ohio 45177
I have heard of transmitting tubes shorting from being abused or run past their ratings or such. Not so much in receiving tubes. I have heard of bits of material getting loose in a tube and shorting them out but that would not be related to just age. And then there are supposedly some rectifier tubes of bad design that have a reputation. I would expect the failure mode in receiving tubes or audio tubes is just getting too weak to work well or getting gassy and being totally inop.


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Tue 23, 2018 2:07 am 
Member
User avatar

Joined: Jan Mon 18, 2010 2:13 am
Posts: 16221
Location: Dayton Ohio
wazz wrote:
And then there are supposedly some rectifier tubes of bad design that have a reputation.


Yes, the good ole 6X5

Its funny, the 6X5's predecessor didn't have so much trouble. 84/6Z4
Nor does its successor, the miniature 6X4

-Steve

_________________
Radio Interests
-Zenith
-Sparton
-Pre-War FM
Consoles and floor models, the bigger, the better!


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Tue 23, 2018 5:47 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4167
Location: Charleston, W.Va.
azenithnut wrote:
wazz wrote:
And then there are supposedly some rectifier tubes of bad design that have a reputation.


Yes, the good ole 6X5

Its funny, the 6X5's predecessor didn't have so much trouble. 84/6Z4
Nor does its successor, the miniature 6X4

-Steve

I am of the opinion that there is nothing wrong with the 6X5 tube per se, provided it was used within its original design parameters. The 6X5 has acquired its bad reputation primarily because some radio manufacturers (especially Zenith) used it with an undersized power transformer coupled with an over-valued input filter capacitor. On the other hand the 6X5 (when properly employed) was used very successfully by other radio manufacturers. It also served well in military applications for the US Navy during WWII.

Yes, there were some issues with the original 6X5 (ST) tube due to construction and/or quality-control issues of some (but not all) tube manufacturers. But these problems were fully corrected with the issue of the 6X5GT tube.

My conclusion is that the bad reputation of the 6X5/6X5GT is undeserved, and the fault for this can be summed up in only one word: ZENITH. :(

_________________
Poston


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Wed 24, 2018 12:36 am 
Member
User avatar

Joined: Jun Sun 03, 2012 12:40 am
Posts: 827
Location: Cromwell, Connecticut
In my opinion, I think that series string tubes take a beating. I’ve always seen one warm up before the others and bright flashes at power up from others now and then. It would be interesting to see how fast the filament voltages level out as they warm up.

_________________
Tony

People may not remember how fast you did a job, but they will remember how well you did it.


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Wed 24, 2018 2:18 pm 
Member

Joined: Jun Thu 30, 2011 9:03 pm
Posts: 230
5Y4GT wrote:
Do vacuum tubes short out over time if they have been used alot? I have a Zenith 6d525 that had two tubes that shorted out in it after about a year. The shorts were intermittent. All the capacitors and out-of-spec resistors were replaced.



I have one real world example of that with a 1950 General Electric where the local oscillator suddenly stopped working. I verified that with the second radio tuned to listen for the oscillator from the GE radio.

Pulling the 12SA7 I found shorted elements on my Hickok tube tester (yes the shorts test is accurate). Replaced it with a low hours replacement 12AA7 and the radio has been playing fine for the past two months since.

I have read in Getting the Most From Cacuum Tubes the author clearly states this can happen as there are materials heated inside a tube to near vapor point and over time small amounts evaporate and deposit elsewhere inside the tube, potentially causing internal shorts in height hours’ usage. It doesn’t happen every time but is a documented failure mode.

I can see in the failed 12SA7 a halo of dark material on the inside of the glass right around the top end of the heater where in use the glow of the tungsten is visible.



Joe


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Thu 25, 2018 12:24 am 
Member

Joined: Jun Thu 30, 2011 9:03 pm
Posts: 230
From the book I referenced (by Robert Tomer, Getting the Most out of Vacuum Tubes), which talks about the degenerative development of internal shorts in vacuum tubes:


...Natural mica has very excellent insulating properties and can be subjected to very high temperatures without break-down. However, in a vacuum tube, there are materials operating at temperatures near their vapor point. When these metallic vapors settle on the smooth mica surfaces, they produce a gradually decreasing resistance. To slow down this process, micas are coated with a finely divided substance that tends to break up the metallic vapor films. This results in a situation similar to that which occurs when raindrops fall on a clean automobile hood. Many small droplets appear first, widely separated from each other. As more rain falls, the drops become larger until finally one or two merge and then suddenly a whole series of drops become connected as a small stream of water starts running down the slope of the hood. In a vacuum tube, that small stream is one leakage path. The more it forms, the lower the resistance between elements becomes...
...
Insulation leakage caused by the deposition of metal films proceeds at a uniform rate in all tubes; however, it is greatly accelerated by such excesses as high heater voltages and high electrode dissipation. Cooling bulbs will often retard its development considerably because the vapors will usually deposit on the coolest surface. If this is the glass envelope, they will generally cause less harm than on the micas. As insulation resistance begins to decrease, it will progress very rapidly even though its initial build-up may be extremely slow. This is because even though the release of metallic vapors is more or less constant, the early effect is to produce islands of metallic film which are not connected with one another. As the islands become larger, the chances of their remaining isolated gets smaller, until finally, many islands connect and form multiple leakage paths almost simultaneously....


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Thu 25, 2018 2:39 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12520
Tbone wrote:
In my opinion, I think that series string tubes take a beating. I’ve always seen one warm up before the others and bright flashes at power up from others now and then. It would be interesting to see how fast the filament voltages level out as they warm up.

Yes they do... Cold resistance of the average AA5 string usually measures less that 120 ohms, at 120v that's a amp or more for a second or two... Current generally levels out fairly quickly but getting slammed by a freight train to start things moving eventually takes its toll...

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Thu 25, 2018 1:40 pm 
Member

Joined: Jun Thu 30, 2011 9:03 pm
Posts: 230
I have a Zenith H511 with a tube saver feature. It really helps make for a slow warm up, takes a minute or so for the heaters to ramp up. Just got it last year, will see if it makes a difference to tune life...


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Thu 25, 2018 2:19 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12520
jdivito wrote:
I have a Zenith H511 with a tube saver feature. It really helps make for a slow warm up, takes a minute or so for the heaters to ramp up. Just got it last year, will see if it makes a difference to tune life...

Just take comfort in fact a thermistor in string does potentially increase tube life... Unless one had probably a dozen radios with & without thermistors you'll never know... Those radios would have to be operated for same length of time, at same line voltage and even have tubes from same runs for meaningful results... At 65+ years later with tubes of different amounts of useage, how long it operates before the next one fails doesn't have any real meaning...

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Thu 25, 2018 2:56 pm 
Member
User avatar

Joined: Jan Thu 26, 2017 10:33 pm
Posts: 639
I actually made a mistake. It was just the 12SA7G tube that failed. The 12K7GT was not intermittent. The problem in the radio turned out to be a bad solder connection.

_________________
No matter how smart you are, there is always someone smarter than you.

-Unknown


Top
 Profile  
 
Post New Topic Post Reply  [ 22 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Adam Vaughn and 6 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB