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Author Topic: 811A's, 3 second warmup?? 3 SECONDS?  (Read 2711 times)
N4ZAW
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« on: September 20, 2009, 12:17:58 PM »

I just got an Ameritron linear. Just reading-thru the manual, it claims on page 2 in the "features", that these tubes exhibit "FAST WARMUP TIME: The 811A tubes take approxmimately 3 seconds to warmup"... This HAS to be a mis-print. I've never heard of a vacuum tube only taking 3 seconds to warm up.. The filiment needs longer than that to settle-in after power is applied, don't it?
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WB2WIK
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« Reply #1 on: September 20, 2009, 01:35:06 PM »

No.

They're right.  Warm up is probably actually faster than 3 seconds.

There are many hundreds of "instant on" tubes with directly heated cathodes.  Some were designed for mobile operation where, to save power, the heaters were turned off for receiving and then turned back on when you hit the PTT button on the microphone.  Those "warmed up" in less than 1/2 second to full emission.

811A, 572B, 3-500Z, 4-400A, 4-1000A and many other tubes in popular use in amateur amplifiers are essentially "instant on."

WB2WIK/6
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WB2WIK
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« Reply #2 on: September 20, 2009, 01:35:15 PM »

No.

They're right.  Warm up is probably actually faster than 3 seconds.

There are many hundreds of "instant on" tubes with directly heated cathodes.  Some were designed for mobile operation where, to save power, the heaters were turned off for receiving and then turned back on when you hit the PTT button on the microphone.  Those "warmed up" in less than 1/2 second to full emission.

811A, 572B, 3-500Z, 4-400A, 4-1000A and many other tubes in popular use in amateur amplifiers are essentially "instant on."

WB2WIK/6
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KB1LKR
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« Reply #3 on: September 22, 2009, 10:56:12 AM »

Fast warm-up filaments: think of incandescent light bulb filaments, they warm up in a few hundred milliseconds or less -- similar technology.

Too, in the transmitter tube case it also helps that it's a relatively low mass direct heated filament that also serves as the cathode not a relatively higher mass indirectly heated cathode structure wrapped around a separate heater.
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K4DPK
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« Reply #4 on: September 25, 2009, 01:54:50 PM »

Yes. you can actually turn them on while you are talking and talk them up.

Phil C. Sr.
k4dpk
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K7KBN
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« Reply #5 on: September 25, 2009, 07:18:57 PM »

If three seconds isn't FAST enough, you could always go with a solid state amplifier.... ;-)

73
Pat K7KBN
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73
Pat K7KBN
CWO4 USNR Ret.
KH6AQ
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« Reply #6 on: September 27, 2009, 06:59:27 AM »

The reason the 811 cathode emits electrons as soon as it nears full temperature is because the filament and the cathode are one and the same.

You might be thinking of indirectly heated cathodes (think 4CX250) where there is a heater inside of the cathode. The proper name for the heating device is heater and not filament.

The 811 has a thoriated tungsten filament. The truely instant on tubes mentioned by another use directly heated oxide cathodes. Barium oxide (with a bit of Stontium oxide) coats the filament. The work function of an oxide cathode is much lower than that of a thoriated tungsten cathode, allowing a much lower operating temperature and even faster warm up.

The indirectly heated cathodes have a tungsten heater inside of a (usually) nickel cathode coated with oxide.

The 'latest' thing in cathodes are scandate cathodes but I don't expect we will see them for the tubes we amateurs use.
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N4ZAW
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« Reply #7 on: September 27, 2009, 10:36:54 AM »

Thanks, folks. My father was a TV repairman, and I grew up thinking all tubes were fragile during a minutes-long warmup.
It's nice to know these transmitting tubes are different than the old horizontal sweep tubes.
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N2EY
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« Reply #8 on: September 30, 2009, 08:23:35 AM »

Yep, those directly-heated thoriated-tungsten transmitting tubes heat up fast!

Oddly enough, they're older than TV sweep tubes.

Mercury-vapor rectifiers need at least 30 seconds warm-up time (I use 60) even though they are directly-heated, but that's to get the mercury boiled.

73 de Jim, N2EY
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KH6AQ
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« Reply #9 on: October 02, 2009, 02:59:33 PM »

There is nothing wrong with a horizontal sweep tube. They are the most advanced glass tube. Very high plate current and transconductance. Especially high in proportion to the plate dissipation.

The ability to output RF well beyond what the plate can handle, along with a low price when they were manufactured, tempted some company called Swan to really push them.
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N3JBH
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« Reply #10 on: October 03, 2009, 05:57:14 AM »

I do not think Swan was alone with that idealization...
Yaesu SBE and i fairly certain others followed the same thought process for awhile. Heck i believe are own beloved W8JI was at one point on that band wagon with the AL-84 Please accept my apology in advance if i am wrong Tom... Jeff
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W8JI
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« Reply #11 on: October 03, 2009, 06:57:31 AM »

Ameritron made a sweep tube amp called the AL84.

We bought new RCA 6MJ6 tubes in bulk, they were $2.30 each. We had under $10 in a set of four tubes. They were much shorter life than an 811 tube and less power, but they were 1/5 the price.

Now sweep tubes, when you can find them, are two or three times the price of an 811 tube. For two or three times the price you get less life and less power. Not a good deal.

Tom
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KH6AQ
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« Reply #12 on: October 03, 2009, 07:27:16 AM »

The Svetlana EL509 is the only 'sweep' tube still in production.

The Dentron GLA-1000 amp can be modified to use these.
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KB1LKR
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« Reply #13 on: October 03, 2009, 12:00:17 PM »

Sweep tubes would have made sense for ICAS service in a day and age when they were cheap and plentiful -- perhaps as close as the local drug store w/ a tube tester, particularly for price sensitive applications. Jump ahead 30-40 years and it's a strange new world.
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