Life span of a tube: How long?

Discussion in 'Ham Radio Discussions' started by W5INC, Nov 9, 2014.

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  1. W5INC

    W5INC Ham Member QRZ Page

    How long is the useful Life span of a tube? Let's use some well known tubes in AR circles as examples, a pair of 6146 final PA tubes or some 4CX1500Bs as examples. If these tubes are used correctly, operating environment maintained to specs properly, how long/hours of use are these tubes good for? Does Life span of the tube actually decrease significantly if you leave the tubes on 24/7/365 and in use such as in broadcasting application? I'm sure broadcast applications are more then bulletproof in their design and the use of the tube(s) in the actual Xmtr. I don't ever turn off my S Lines of any other tube/hybrid gear I am using during the Winter here in Tx. I see stories of folks using original tubes, in TS-830S rigs, FT-101s and other final PA tubes that came in these type of radios from the factory 25 to 30 + years. I believe K9STH posted a story about a Motorola radio that was forgotten about in a building and uncovered 20+ (?) years later and still worked when found.

    I ran across 1 of those pig in a poke type of deals. I bought 3 @ 4CX1500B tubes all used in some type of 4KW Xmtr. All 3 tubes were used in the Xmtr at the same time, so I am thinking the tubes weren't driven to hard in their lifetime. 1 tube has a stamp of 10,000 hours of use on it, which to me doesn't sound like much if the tube was never pushed to it's max for long periods of time. All of the 3 tubes are supposed to be full power out, according to the seller. The cost of all 3 tubes was 75.00 which seemed like a deal to me as I now have plenty of backup tubes for my 30S-1 amp. :)
     
  2. WA6MHZ

    WA6MHZ Ham Member QRZ Page

    I have 01As dating back to the 20s still good! And my 1908 Audion is also still good!
     
  3. K9STH

    K9STH Ham Member QRZ Page

    Tube heaters / filaments usually open when the voltage is first applied. As such, leaving them on full time should increase the life. However, there is a trade-off in that the expense involved in providing power full time has to be considered.

    When run conservatively, the 6146 can last for decades. If you want to increase that, then use the 6293 which is a 6146A/8298 manufactured specifically for pulse operation at up to 1000-watts input. When run at the same level, the 6293 generally lasts between 5 times, and 10 times, as long as the 6146 series.

    In the "goode olde dayes", when tube type equipment was the norm, commercial two-way base stations, which were turned on full time, would maintain specifications for well over a decade. There was one situation, where a base station had been installed in the attic of a building in the early 1950s, that it was forgotten. Around 2010, the equipment was discovered, still plugged in. Although the equipment had not met the specifications for use today, when checked out, the equipment was still meeting the original specifications.

    I have numerous tubes, in my antique / vintage radio collection, that were made in the early 1920s that still meet specifications today. There are a couple that have stickers which say that they were guaranteed for 3-days after being sold!

    Glen, K9STH
     
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    Depends, one in our group uses a home brew 4CX1000 amplifier his original tube had a broken locating pin and that tube has been in that amp since about 1971.
    My SB-220 has the original tubes from when it was built in 75..
     
  5. KA0AAM

    KA0AAM Ham Member QRZ Page

    One of the last things I look for on a boat anchor, is a bad tube.. . .It is probably 98 percent the components around it. Life of a tube, is decades and decades, as long as you keep the filament within specs, and the plate dissipation within its manufactured limits.

    I usually find that the filaments go bad. . .and those that have a extra long filament, the filament will sometimes ground out to the grid or something if the tube is on its side.

    the answer to your question? They last a LONG LONG time if you just take care of them. . . As some folks have said. . .tubes in the 1920ś are still working. I have tubes in the 1950ś on my amp, that are still putting out full power.
     
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    My experience comes from commercial operations with vacuum tube equipment such as 5 to 30 kW SSB transmitters in the 80's.
    Life spans could vary widely between different types, tube manufacturers and operating profiles.

    Thoriated tungsten cathodes usually lasted about 10000 filament hours, oxide cathodes between 5000 and 30000 hours, in some cases up to 60000 hours.
    Intermittent operation increased the life somewhat, but long cutoff periods with heated oxide cathodes could damage them.

    "Infant Mortality" sometimes claimed lower quality tubes.
    To avoid this, it was common practice to "burn in" tubes of 4CX250B size and upwards for 50 hours.

    Tubes of good quality can be in operation for decades if well designed and suitably derated.
    When carrier telephony systems were finally phased out in the mid-80's some tube amplifiers with early-40's tubes were found still operating.

    It is hard to estimate the tube life in 4CX1500B:s. If the cathode is of similar quality as in the 3CX5000 or the P290A filament hours in the region of 30000
    may be reasonable.

    The operational profiles that radio amateurs can generate are unlikely to wear out the cathodes, and if cold-start transients are handled properly by
    step-start, NTC resistors and proper dimensioning of the transformer the filaments are quite safe.

    73/
    Karl-Arne
    SM0AOM
     
  7. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Low cathode emission, not open filament or heater, is the most common cause of tube failure. Years ago I used to run my VFO, my 75A-4 and the 10-watt hi-fi amplifier I used for outboard audio, 24 hours a day, so I wouldn't have to wait for tube warm-up and for the PTOs to fully stabilise whenever I wanted to use the station. I noticed every few months my receiver performance would fall off and the VFO would sometimes become flaky and unstable but the problem would be cured by a new oscillator tube. I started routinely testing the tubes every year or so, and each time several would be weak. I went through a lot of 6BA6s and 6BA7s in the receiver and 6V6s in the audio amplifier over the years as my supply of tubes dwindled and I decided this shouldn't be happening. I checked a.c. line voltage and it was high, enough that the tube filament voltage read around 6.7 volts. I added a bucking transformer in series with the a.c. mains outlet, dropping the voltage enough to make the filament voltage read exactly 6.3 volts. That seemed to extend tube life a little longer, but still, I was replacing several tubes every year, more often than I thought I should. Finally I decided to stop running everything 24/7 and to completely shut down overnight and while I knew I wouldn't be operating the station. This also saved electricity and didn't leave the equipment vulnerable to unexpected lightning storm damage. My tube failures went way down.

    Running thoriated tungsten filaments for prolonged periods without plate voltage will reduce the life of the tube. I once read an explanation for this: energetic electrons escape from the hot filament in the vacuum and generate an electron cloud in the near-vicinity of the filament. The fluctuating magnetic field generated by the a.c. filament current causes the electrons to vibrate at 60 Hz. The vibrating electrons in the cloud literally sand-blast away the thin layer of thorium oxide coating on the tungsten filament, which is only a few molecules deep. Depletion of the thorium is what causes the emission of those tubes to dwindle. RCA recommends in their transmitting tube manual to reduce voltage to thoriated tungsten filaments to 80% the rated value during long periods of stand-by, and to cut the voltage off completely if the tube is likely to remain idle for more than 2 1/2 hours. Also, under-voltage on those filaments is just as damaging to tube life as over-voltage. It is recommended to keep the filament voltage as close to recommended as possible, and in no case allow them to deviate more than plus or minus 5% from the rated voltage.
     
  8. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    My Swan Mark II still has the original 3-400's in it, I purchased it new in the mid 70's and it is used almost on a daily basis plus a few all weekend contests every year, they still have full output. I had to replace the caps but the tubes are still going strong. My Knight Kit Span Master still has the original tubes and their fine. On my Hallicrafters S119 the filament went in a 6BA6 and the 6BE6 tested weak so I replaced both a month ago, both receivers are from the early 60's. All tubes in my BC454 are still good, it's from the early 50's. The original 6146 tubes in my DX60 are still at full output, I built it in 1964.
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think the DX-60 only has one 6146...

    But anyway, tubes can last a really long time and I'm not sure leaving them turned on all the time extends their useful life. For some designs, maybe it would; but I've never done that.

    My 57 year-old 75A4 (receiver) has all its original tubes. I bought it used in '72 and I've never changed any, and I'm pretty sure they're original because they all have "Collins" printed on them (as opposed to "RCA" or whoever actually manufactured them). It works exactly the same today as it did in '72, and probably as it did since it was manufactured. Nowadays it's only powered up maybe every couple of weeks, but back in the 70s it was powered up every day, sometimes for 24 hours or more at a time, so they have a lot of hours on them.

    The 4-250 in my homebrewed AM rig (built in 1967) has its original Eimac in it, and that was used when I bought it! Although I don't really know "how" used, I got it from a ham who worked in broadcasting, it was probably a pull from a transmitter of some sort. The 4-1000A in my six meter amp (built in 1984) was also a used "pull" when I bought it for $100 in 1984, and it still delivers over 2kW output power if I drive it hard (into a dummy load). It's usually operated at 1200W output power and I've put thousands of hours on it. Eimac made some good stuff back in the day.

    I have some very old 4CX250Bs (also Eimac) from the 60s which still work perfectly in my 2m amp. Key there is to keep grid and screen currents very low. But I've sure exceeded their plate current rating by quite a bit, many times. Doesn't seem to bother them, with enough air blowing through them.

    AFAIK the Transatlantic cable, which is old as I am, still works and it contains lots of tube repeater amplifiers.

    So, I'd "guess" tubes may actually outlast transistors in many applications, since they're more impervious to transients, EMP, etc. I guess we'll have to see if the power transistors in modern SS transmitters are still working 100 years from now; nobody could know that, yet.

    The first all solid state 100W HF amateur transceiver was the TR-7, I think. Not positive, but it's the first one I can remember, and I bought mine new in 1978. Still have it, still works perfectly, never had to replace anything but the pilot lamps. But still, that's only 36 years...not so long.
     
  10. N7ZAL

    N7ZAL Ham Member QRZ Page

    Years ago the view of tube life was tied to turning it off and on. FYI
     
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