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Author Topic: "Legal Limit" Amplifiers-NOT!  (Read 14524 times)
AD4U
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Posts: 2186




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« Reply #30 on: January 09, 2013, 06:20:24 AM »

Once again this discussion has NOTHING to do with exceeding the plate dissipation ratings of a single 3-500Z(G) tube.  With PROPER COOLING a single 3-500Z tube is rated to dissipate 500 watts of power (600 watts claimed for a G) in CONTINUOUS duty CCS service - which means key down non stop forever until the emission of the tube finally goes to zero.  As I remember EIMAC stated that one could expect something over 10,000 hours (or was it 30,000 hours) life MTBF out of their 3-500Z when operated at 500 watts dissipation assuming adequate cooling. When operating a single 3-500 tube in typical amateur SSB or CW duty cycles, plate dissipation in the tube will not even come remotely close to 500 watts.

Ths issue (again IMO) is exceeding the manufacturer's PUBLISHED MAXIMUM plate current rating, which is 400 mA per tube, in order to get 1000 watts (or 1100 or even more as some claim) out of an amp running a single 3-500 tube.

Driving a single 3-500Z or G tube beyond the manufacturer's maximum published plate current of 400 mA will more than likely run the tube out of its linear operating region.  This often results in splatter and a "dirty" signal. Anybody know what a tube's operating curves look like?  Anybody ever wonder why they call it a LINEAR AMPLIFIER?  I never said that one could not drive the amp in question to 600 mA plate current or that said amp would not achieve 1000 watts output in SSB service.  It will.

Most will agree that the EIMAC 3-500Z tube is better than the imports used today.  When EIMAC introduced the 3-500Z many manufacturers made "2KW input legal limit" amps that ran 2 each EIMAC 3-500Z tubes. Examples are the Drake L4B, Heathkit SB-220, Swan Mark II, Henry 2K4, and others.  EVERY ONE of these manufacturers stated in their manual that MAXIMUM plate current for 2 each 3-500Z tubes in any mode was 800 mA (400 mA per tube).

Only one current manufacturer of amps states that a single 3-500Z(G) tube can be driven to much more than 400 mA (600+ mA as some claim) in order to achieve 1000 (or more) watts.  What has changed?  IMO it is advertising hype.  Maybe PT Barnum was right.

Dick  AD4U

PS:  "No matter what", this will be my final comment on the matter.

« Last Edit: January 09, 2013, 06:59:20 AM by AD4U » Logged
W8JX
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« Reply #31 on: January 09, 2013, 07:04:31 AM »

I agree. A single 3-500's 400ma plate current specification is for continuous commercial duty and not for an amateur SSB duty cycle. A single 3-500 amp such as the AL-80B will output the rated 1 KW PEP SSB and 800 watts CW under amateur duty cycles, just as represented by Ameritron. The power supply is the limiting factor in the AL-80B, probably as Ameritron intended.   

The "problem" is at 1kw plate current is exceeded a good bit and the tube is not being properly cooled for "rated" loads and tube life will suffer.
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KB5UBI
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« Reply #32 on: January 09, 2013, 02:18:14 PM »

I agree. A single 3-500's 400ma plate current specification is for continuous commercial duty and not for an amateur SSB duty cycle. A single 3-500 amp such as the AL-80B will output the rated 1 KW PEP SSB and 800 watts CW under amateur duty cycles, just as represented by Ameritron. The power supply is the limiting factor in the AL-80B, probably as Ameritron intended.   

The "problem" is at 1kw plate current is exceeded a good bit and the tube is not being properly cooled for "rated" loads and tube life will suffer.

A 3-500 in continuous 400ma commercial service will see much higher peaks in IP when in use.  Intermittent excursions to 500+ma range allow 1KW from an AL-80B in SSB service, without harm. With anything more than SSB's short duty cycle, Ameritron's power supply will be the limiting factor as designed. Tom did a great job with this design.   
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W8JX
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« Reply #33 on: January 09, 2013, 02:25:58 PM »

I agree. A single 3-500's 400ma plate current specification is for continuous commercial duty and not for an amateur SSB duty cycle. A single 3-500 amp such as the AL-80B will output the rated 1 KW PEP SSB and 800 watts CW under amateur duty cycles, just as represented by Ameritron. The power supply is the limiting factor in the AL-80B, probably as Ameritron intended.   

The "problem" is at 1kw plate current is exceeded a good bit and the tube is not being properly cooled for "rated" loads and tube life will suffer.

A 3-500 in continuous 400ma commercial service will see much higher peaks in IP when in use.  Intermittent excursions to 500+ma range allow 1KW from an AL-80B in SSB service, without harm. With anything more than SSB's short duty cycle, Ameritron's power supply will be the limiting factor as designed. Tom did a great job with this design.   

It is a good amp but they should have spent another 30 or 40 bucks on design and used a chimney for proper cooling of tube. (they did on AL82) You actually need to de-rate a 3-500z without proper cooling.
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K0CWO
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« Reply #34 on: January 10, 2013, 06:40:32 AM »

I have a 22 year old AL-80A with the original Eimac 3-500 tube which I frequently run at 1KW SSB output.  I also had an AL-80B with an import 3-500 tube that I used at 1KW SSB output.  I have yet to receive complaints about splatter running these amps at that output level and I have not had any of these tubes fail running them that way.  I know quite a few amateurs that use these amps at the advertised maximum output and I have not heard of any frequent tube failures from them either.  I am not saying that failures don't occur but I must say that I don't think the AL-80 series amplifier is widely known to blow tubes when operated in accordance with the operators manual.  From my hands on observations over the last ten years these amplifiers are reliable when used at the 1KW SSB PEP output level.  Your mileage may vary.   
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AD4U
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« Reply #35 on: January 10, 2013, 08:00:05 AM »

I said I would not post any more comments on this thread, but I guess I lied.

I do not own an AL-80 series amp.  I have never used an AL-80 series amp.  I do not have a  manual for an AL-80 series amp.  Also I NEVER said that an AL-80 series amp would not provide 1000 watts PEP output on SOME bands.  It will.  Also I NEVER said the AL-80 series of amps were not good amps.  They are good amps.  I just think their capabilities are just a little exaggerated.

What I said was doing so would require the single 3-500Z(G) tube to be driven to much more than the manufacturer's published maximum plate current of 400 mA.  Assuming a loaded (key down) plate voltage of 3KV and 400 mA = 1200 watts INPUT.  In order to get 1000 watts OUTPUT the amp would have to be over 83% efficient.  I doubt that even Ameritron (MFJ) can achieve that kind of efficiency in a GG amp.

For all who posted or emailed me directly that a single 3-500Z(G) tube in an AL-80 type amp can be driven to a plate current in excess of 400 mA on peaks in SSB service......

I ask any of you to provide us JUST ONE link to any legitimate published data sheet for a 3-500Z(G) tube which states in writing that a single 3-500Z(G) tube may be driven to a plate current over 400 mA - even on SSB peaks.

Then I will shut up.   Wink

Dick  AD4U

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K0CWO
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« Reply #36 on: January 10, 2013, 01:45:50 PM »

Dick,

I'm not 1/100th as smart as you and I have seen the various 3-500 spec sheets.  I was merely reporting hands on practical observations.  No harm intended on my part.

73,
BJ k0cwo
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KD8MJR
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Posts: 2706




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« Reply #37 on: January 10, 2013, 03:16:11 PM »

For all who posted or emailed me directly that a single 3-500Z(G) tube in an AL-80 type amp can be driven to a plate current in excess of 400 mA on peaks in SSB service......

I ask any of you to provide us JUST ONE link to any legitimate published data sheet for a 3-500Z(G) tube which states in writing that a single 3-500Z(G) tube may be driven to a plate current over 400 mA - even on SSB peaks.

Then I will shut up.   Wink
Dick  AD4U

Just Crickets chirping so far Grin
Dick your 100% right on this. All other arguments are like so many I hear in other hobbies I have been in.  Race Cars: "Oh yeah with Mods you can get 750HP out of a stock Mitsubishi Evo 8 Engine!"  2 months later:  "My Engine blew up!"
It's a never ending list, some companies and many people out there some how forget that Life expectancy is inversely proportional to how hard you push any device.
« Last Edit: January 10, 2013, 03:51:12 PM by KD8MJR » Logged
KB5UBI
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Posts: 97




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« Reply #38 on: January 10, 2013, 05:57:41 PM »

I among others do not dispute the 400 ma Ip continuous rating, but my interpretation of it's meaning differs. 1000 watts PEP voice output from a single 3-500ZG may average around 250ma plate current and peak at 3 to 4 times that amount. Using your interpretation of 400ma Ip as an absolute peak, a 3-500 amp by any manufacturer would be limited to 250 watts PEP or less. The FCC approved the AL-80B rated at 1000 Watts PEP and the tubes were warrantied by Eimac and Amperex for use in the AL-80B.
The life of a thoriated tungsten cathode tube like the 3-500 is directly related to long term dissipation and not peak envelope current. Excessive cathode/anode current does not shorten the life of a 3-500ZG, but excessive continued dissipation will.

 
 
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K7NSW
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« Reply #39 on: January 22, 2013, 09:40:10 AM »

OK!! W7APM nailed it - thank you!  Ten-Tec is a good example of the truth he is pitching.  A pair of 3-500Z tubes in a well built conservatively designed safe amplifier that will  last you your lifetime.  The max output is 1,200 watts.  YOU WANT MAX POWER?  Better go see RF Concepts/Alpha.  There is a reason they use those big ceramic tubes and a power transformer that is shipped separately on its own pallet.
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ZENKI
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« Reply #40 on: January 22, 2013, 01:37:33 PM »

Regardless of who is right or wrong,  when you drive a 3-500 this hard the IMD performance is much worst.  Then you throw in overdriving, ESSB, poor radios and you have a mess on the band.

The most conservative way to run 1 kw is with a pair of 3-500's like the drake L4B and SB220 does. If  you need that small amount of extra power out  by driving a tube to its limits, you really need  better antennas.

Regardless what Ameritron says driving single 3-500 to their stated output power levels is poor engineering practice for single 3-500. This practice just encourages the all knobs to the right mentality
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W8JX
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« Reply #41 on: January 22, 2013, 02:16:54 PM »

The most conservative way to run 1 kw is with a pair of 3-500's like the drake L4B and SB220 does.

When they were built the legal limit was lower too. The power supply is the limiting factor here not the "valves".  With a stiff supply and 3500v or more and proper cooling with forced air up through sockets and thru chimneys around tubes, legal limit is very doable with a pair of 3-500z's and with a long life too. You need to de-rate dissipation capacity at least 20% (likely closer to 30%) when simply using air blow over valves. The old L4B RF deck is well designed with proper cooling and with a upgraded power supply will easily do legal limit too. 

While I am not the biggest fan of Ameritron amps, the did do it right with AL82 as it has a stiff 3.6kv supply and proper cooling too. 
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AD4U
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« Reply #42 on: January 23, 2013, 05:50:45 AM »

Here I go again.  If I could make a few changes in the AL-80 series of amps to make it a "true" 1000 output amp without exceeding any of the published specs for a single 3-500, this is what I would do.

Give it a stiff 4KV PS.
Cool the 3-500 with a pressurized chassis (blower not fan) and use a chimney (ala AL-82)
Add some zener cathode bias (to keep idling plate current to acceptable levels at 4KV).
Run the amp at 400 mA as published in the tube data sheets.

This would equate to 1600 watts input.  Assuming around 65% efficiency one would get an honest 1000 watts out and not exceed any of the manufacturer's published specs for a single 3-500.

Of course doing so would increase the cost of the amp considerably, and one feature is the AL-80 amps is their relatively low cost per watt.

Dick  AD4U
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W8JX
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« Reply #43 on: January 23, 2013, 09:14:33 AM »

Here I go again.  If I could make a few changes in the AL-80 series of amps to make it a "true" 1000 output amp without exceeding any of the published specs for a single 3-500, this is what I would do.

Dick, where in my post did I say a AL80B was reliable 1KW out amp? My comment was on a pair of 3-500z's properly feed and cooled.

I will say this though 1kw out a a AL80B is less of a stretch than 800 watts from a AL811H
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AD4U
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Posts: 2186




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« Reply #44 on: January 23, 2013, 10:14:43 AM »

Via W8JX:

Dick, where in my post did I say a AL80B was reliable 1KW out amp? My comment was on a pair of 3-500z's properly feed and cooled.

AD4U Replied:

You never said that.  I am the one who posted around 5 comments on this thread that question the wisdom of driving a single 3-500 to 600mA or more in an AL-80 amp in order to get 1000 watts out.  I posted that doing so would greatly exceed ANY AND ALL published plate current specs for a single 3-500.  I think you and I are in complete agreement that an AL-80 WILL put out 1000 watts on some bands, but not unless the published specs on the single 3-500 are greatly exceeded.

Dick  AD4U


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