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Author Topic: "Pulse Rated Tubes" and AL-80B advertized output...  (Read 1971 times)
KT0DD
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Posts: 277




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« on: January 17, 2013, 09:54:48 AM »

Okay, I'm not an engineer, and not real good at tube theory, but after reading the discussion in "Legal Limit Amplifiers - NOT" and so much discussion of the output of an Ameritron AL-80B ( I just bought one) It jogged my memory of something I saw on the RF Parts site.

RF Parts sells a "SELECTED - 3-500ZG - PR"...(PR stands for pulse rated) at a little higher price than their standard 3-500ZG's. I also know that alot of amplifier owners using the 3CX800A7's are now switching them to the 3CPX800A7's...P of course is the designator for pulse rated.

Is this more marketing hype on the "pulse rated" issue, or if I want reliable, linear operation at 1KW on my AL-80B, should I get the pulse rated 3-500ZG?

Thanks.

Todd - KT0DD
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KD0REQ
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Posts: 931




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« Reply #1 on: January 17, 2013, 10:08:31 AM »

I am not an engineer, but I saw one slip on the ice in a parking lot once.

the idea behind beefing up the connections and elements of a tube and rating it for pulse service comes from bombardment shock.  a pulse duty service like radar or sonar may have an average current across the tube that falls within ratings, but the instantaneous current can be hundreds of times higher for some number of microseconds.

normal ratings are based on sustained average power dissipation over time, something that is nicely represented by the sine wave average of current across the elements in an AM transmitter, for instance.

the things pulse-rating bring to the table for amateur service would be better shock resistance and probably a thicker emission surface promising some longer life.  there will not be more amps in the antenna, and some yahoo driving a kilowatt amplifier with a kilowatt amplifier is still going to be doing their showering in sparks.

I would not automatically assume that a selected tube from normal production being "pulse rated" guarantees those advantages.  like a Zenith, it should be designed in, not added on.
« Last Edit: January 17, 2013, 10:12:02 AM by KD0REQ » Logged
N6AJR
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Posts: 9908




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« Reply #2 on: January 17, 2013, 10:17:44 AM »

Are these also  advertised as medical pulls and will they work OK in my alpha 87a ??  I have been looking for a "cheep" tubes as spares.
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K6AER
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Posts: 3515




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« Reply #3 on: January 17, 2013, 10:20:27 AM »

Pulse Rated tubes have greater spacing from the cathode to the anode. This enables the design to run higher voltage on the anode for higher pulse wattage out of the tube. The wattage dissipation still has to be within tube specifications. Along as the duty cycle is pulse and the output dissipation is within tube specification the tube will last. I SSB mode the duty cycle is between 20-30%. Most pulse rated tubes are ceramic such as the 3CPX800A7 and the CPX1500A7. As a result some amplifier designs run higher anode voltage with the pulse rated tubes for increased output on CW or SSB. Key down dissipation is still the same.

In an amplifier designed for a set voltage like the AL-80B, no advantage is made by switching to pulse rated tubes. Your anode voltage has not increased.  On the down side pulse rated tubes generally require more drive due to the increased anode to cathode distance. Switching to a pulse rated tube could actually result in lower power output.

Also tubes don't see high spike current in a RF amplifier or DC service. In a pulse application the tube is still a sine wave conduction times the number of sine waves given the duration of the pulse. Depending on the class of service A, AB or C or other the tank circuit will complete the sine wave with what is call the flyback effeect.You cannot pull 10 amps out of a tube designed for 1 amp of service with out greatly increasing the anode voltage. Then you run into Anode Cathode break down voltage.
« Last Edit: January 17, 2013, 10:31:55 AM by K6AER » Logged
KB5UBI
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Posts: 97




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« Reply #4 on: January 17, 2013, 10:58:29 AM »

Here we go again...  It would be difficult to harm the 3-500 running a KW of SSB in an AL-80B. On the other hand, with high duty cycle modes such as RTTY or AM, one could easily exceed the time-averaged dissipation limit if the 3-500. Continued excess Anode dissipation will kill a tube like the 3-500, but not low duty cycle over excursions of plate current.

Check out W8JI's page about power grid tubes. He designed the AL-80B.
http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm

KB5UBI

 
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W1QJ
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Posts: 1445




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« Reply #5 on: January 18, 2013, 05:49:22 AM »

Here we go again...  It would be difficult to harm the 3-500 running a KW of SSB in an AL-80B. On the other hand, with high duty cycle modes such as RTTY or AM, one could easily exceed the time-averaged dissipation limit if the 3-500. Continued excess Anode dissipation will kill a tube like the 3-500, but not low duty cycle over excursions of plate current.

Check out W8JI's page about power grid tubes. He designed the AL-80B.
http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm

KB5UBI

 
Correct!!  Many do not seem to grasp the concept of "time averaged dissipation".  Thats why they can't accept the fact that a single 3-500 can easily do 1KW PEP SSB.  Add speach processing to that and it now cuts down the talk time a great deal because as we know processing increases the average power.  To compensate you have to cut time way back.  More people should read the information at this link!!!
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K9MRD
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Posts: 331




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« Reply #6 on: January 18, 2013, 07:26:30 AM »

http://www.radiomuseum.org/tubes/tube_701a.html

Note the pulsed plate current/voltage and the average dissipation.  I built a CW amp with one of these 701A's around 1959.  Had to wind the filament transformer and fabricate a tube socket using plate caps.  Easily ran KW input.  Loved to watch the 872 mercury vapor rectifiers flash as I keyed the transmitter! The good old days when plate transformers weighed 100+ lbs.
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KF7CG
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Posts: 835




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« Reply #7 on: January 18, 2013, 07:49:08 AM »

Deja vu, but at higher power! I remember back to when the argument was about sweep tubes and their longevity and the like in amateur transceivers when running higher power. The sweep tubes were all pulse power tubes with well known pulse limits, and modest average dissipations. But everyone attacked the design since their peak inputs were higher than the average dissipation would allow. Similar to the great 811 flap.

KF7CG
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