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 Post subject: Does running tube filaments on DC reduce their durability?
PostPosted: Feb Fri 16, 2018 4:14 pm 
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I read on this forum a long time ago that in the 40s or before army radio equipment had filaments being run on DC. To keep the lifespan of the tubes some of these devices included a polarity switch at the front of the radio to be reversed every use. Apparently the issue was that the filament would open on one side. If this was true for tubes of that time, does it still apply for the later generations of tubes?

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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Fri 16, 2018 4:51 pm 
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Edison observed his light bulbs would have deterioration of the filaments on one end when run on DC. It still applies to tubes.

However, modern tubes have an indirectly heated cathode so the filimant does not emit electrons and is also coated.

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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Fri 16, 2018 5:17 pm 
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It's for the large directly heated triodes tetrodes and pentodes. Lets say you have a 813 with 10 volts DC on the filaments, since the filaments are also the cathode, the more positive side of the filament will be working harder than the more negative side and will wear down faster. Reversing the filament voltage will even out the filament wear.
01As should have this problem, but ECC83s wouldn't.


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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Fri 16, 2018 6:36 pm 
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Filament "notching."

Still an issue, even on small lamp bulbs.

Quote:
Notching

Notching is the phenomena in which a saw-toothed surface appears over portions of the filament. The notching grows due to the electro-migration of tungsten ions and becomes more noticeable after long operation. Notching depends on the lighting condition, either DC or AC. At AC voltage, the notching occurs near the area where the filament is supported by the anchor or is connected with the lead-in wire. These areas have a temperature gradient. At DC voltage, since tungsten ions move in only one direction, the notching occurs over the entire filament. Therefore, the lamp life at DC voltage becomes shorter than at AC voltage because of more severe notching. In order to reduce the notching, a rhenium tungsten filament, which has a higher temperature recrystallization point, can be used.


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http://www.tlt.co.jp/tlt/english/produc ... g/info.htm

Rich


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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Fri 16, 2018 9:48 pm 
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So, in the case of a 20s battery set that is still using batteries, just swap the A+ and A- leads every so often to prolong the life of the 01a's? Could that induce issues or is that safe to do?


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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Fri 16, 2018 9:52 pm 
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Can't usually just swap filament leads on battery radios. This changes bias for grid #1. You could change the way grid #1 gets bias but that's work.

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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Sat 17, 2018 1:20 am 
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Dang. What's the average life of an 01a?


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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Sat 17, 2018 1:37 am 
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Silvertone3004 wrote:
Dang. What's the average life of an 01a?
IMHO about 5k hours, give or take abuse. The filament will remain after all the thorium is exhausted. Average 1kh hours between rejuvenations about 5x rejuvenations. Filament will emit poorly once thorium is gone. Users would elevated the voltage to get tungsten to emit then filament would erode and part...

Why worry? '01a's are plentiful and cheap, will not spoil like the caps in a modern power supply. Stock-up while you can.

YMMV

Chas


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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Sat 17, 2018 4:41 am 
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Thanks for the replies. I guess it's not so much to do with the current passing through it as much as it's relation to the plate.


Rich, W3HWJ wrote:
http://www.tlt.co.jp/tlt/english/products/industrial_light/miniature_lamp/catalog/img/p_pro1_info_04.gif


Not relevant to later tubes right?

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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Sat 17, 2018 8:40 am 
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Greetings to the Forum:

When I first went to work for CBS, we were running Harris-Gates BT-18 TV Transmitters that used 8807's in the final stages (see my avatar).

These tubes have a Thoriated-Tungsten mesh filament which runs at 9 volts at about 140 amps.

The normal filament supply for these as designed by Harris-Gates was AC from a transformer, with a hum null control to minimize any hum coupled into the output of the transmitter. CBS insisted that that their standards would not allow this design and required Harris-Gates to modify the design of the transmitter and install a filtered DC filament supply.

During my tenure at the KCBS transmitter, the whole time we ran these tubes, we had quite a few failures of various types. One time, when the transmitter was blowing the screen supply breaker, upon opening it up and reaching in to extract the tube, I got the plate structure and most of the ceramic insulator section between the plate and the screen ring. The rest of the tube stayed in its socket. I got a good look at the internal structure.

However, none of these failures were due to an open filament. So, in this case, DC as opposed to AC made no difference at all.

The applicability of filament characteristics of an 8807 to the receiving tubes under discussion here is probably non-existent. However, I thought I'd throw this in, just to muddy the waters. :D

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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Sat 17, 2018 4:19 pm 
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hi guys,

interesting...

I have dimmed down a few sets of vintage xmas lights and my traffic light with a 1N5408 or 4007.

does this "notching" apply even when the lights are run at 60 VAC ?

viewtopic.php?f=2&t=332311

viewtopic.php?f=2&t=332796

will i "save bulb life" on running them at 1/2 voltage but destroy the filament ?

i nearly always have my cake and eat it too and hope i can this time as well.

LOL...

thanks.

steve

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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Sun 18, 2018 6:40 am 
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Dutch Rabbit wrote:
hi guys,

interesting...

I have dimmed down a few sets of vintage xmas lights and my traffic light with a 1N5408 or 4007.

does this "notching" apply even when the lights are run at 60 VAC ?

http://antiqueradios.com/forums/viewtop ... 2&t=332311

http://antiqueradios.com/forums/viewtop ... 2&t=332796

will i "save bulb life" on running them at 1/2 voltage but destroy the filament ?

i nearly always have my cake and eat it too and hope i can this time as well.

LOL...

thanks.

steve

Putting a diode in series does not cut the voltage in half; it cuts the power in half. So the voltage with the diode is 85V for a line voltage of 120V. Also, the diode makes the voltage DC, not AC since it always flows in the same direction. Running the bulbs on reduced voltage, AC or DC, considerably extends their lives except perhaps for halogen lamps that have to operate at high temperature for the halogen cycle to work.

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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Sun 18, 2018 2:49 pm 
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hi jim,

yes, 1/2 power, half wave is what i meant to say, AC to DC and all that... 8) .

glad to have my cake and eat it too again :D .

steve


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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Wed 21, 2018 1:29 am 
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I'm not sure if this helps or not, but of course typical car radios used DC with no polarity reverse switch. I don't know if tube failures were common or not, I'm sure the rough environment didn't help.

Also, many hi-fi designs used DC in preamp applications, the Dynaco PAS2 & 3, Heathkit AA-11, and a few Pilot models are some that I'm aware of. These are all over 50 years old now, so there should be some data out there about tube life.


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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Wed 21, 2018 5:22 am 
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classicelectronicsguy wrote:
I'm not sure if this helps or not, but of course typical car radios used DC with no polarity reverse switch.

Same with battery farm and battery only portable sets as well.

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 Post subject: Re: Does running tube filaments on DC reduce their durabilit
PostPosted: Feb Fri 23, 2018 3:44 am 
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As Chas said, 01A'a are cheap and plenty around. I doubt you'll play the radio enough to burn out one good tube.

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