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Author Topic: "Legal Limit" Amplifiers-NOT!  (Read 14459 times)
QRP4U2
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Posts: 130




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« Reply #15 on: December 31, 2012, 09:23:42 AM »

Quote
My math shows an AL-80B with 3300 volts and 400 mA can only produce 1320 watts INPUT.  In order for that amp to produce 1000 watts OUTPUT, one of two things must happen:


At 65% efficiency this equates to 858 Watts out.

But how long can it sustain that output at say for AM or continuous carrier ?

I think MFJ makes some decent amps, but in their manuals and internet site they throw out all kinds of power ratings which can be confusing to a Ham with little electronics experience or math.

For example, for their AL-82, they discuss their 1800 Watt capable Hypersil transformer.

http://www.ameritron.com/pdffiles/AL-82Q.pdf

Now they do disclose that efficiency is 65% CW and 62% SSB (crest).  

They also state the AL-82B will output 1500 Watts out for 30 minutes but according to my calcs, that would be 700 miiliamps at 3300 volts with 65% efficiency.

So yes, buyer beware and look at the capabilities of the amplifier, and make some simple power calculations before buying.

Phil - AC0OB
« Last Edit: December 31, 2012, 09:43:40 AM by QRP4U2 » Logged

AC0OB - A Place Where Thermionic Emitters Rule!
WX7G
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Posts: 6319




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« Reply #16 on: December 31, 2012, 02:49:05 PM »

QRP4U2,
the latest edition of the AL-80B manual says to load it to 600 mA plate current. At 3000 volts this is 1800 watts input. The manual says the efficiency is 65% so that works out to 1170 watts output.

For AM the manual says to tune for maximum power then back off the RF drive to obtain 250 watts carrier output. During 100% modulation the RF output will hit 1000 watts.

« Last Edit: December 31, 2012, 03:17:57 PM by WX7G » Logged
QRP4U2
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Posts: 130




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« Reply #17 on: December 31, 2012, 05:22:19 PM »

Are you looking at a different manual?

Page 9 of this manual says to load the amp  below 450mA plate current since that is the max plate current rating of the tube:

http://www.ameritron.com/pdffiles/AL-80B.pdf

Phil - AC0OB
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AD4U
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Posts: 2186




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« Reply #18 on: December 31, 2012, 06:12:43 PM »

QRP4U2,
the latest edition of the AL-80B manual says to load it to 600 mA plate current. At 3000 volts this is 1800 watts input. The manual says the efficiency is 65% so that works out to 1170 watts output.


EIMAC the company that "invented" the 3-500 tube states in their data sheet (I have one in front of me at the moment) that plate current on a single tube is 400 mA MAXIMUM - PERIOD - DO NOT EXCEED.

IMO driving a single 3-500 past its rated plate current of 400 mA may cause the tube to operate out of its linear region which usually causes  dirty signal and SPATTER all over the band.

But then I never said an AL-80B amp would not OUTPUT 1000 watts.  I said that doing so requires the tube to be pushed WAY beyond its published plate current maximum.  That is all.

Dick  AD4U
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N3QE
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Posts: 2421




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« Reply #19 on: January 01, 2013, 08:35:28 PM »

Seriously, I have debated on this forum whether or not an AL-80B amp can get the claimed 1000 watts out of a single 3-500 without greatly exceeding the EIMAC rating of 400 mA maximum plate current per 3-500.

It's kind of historical, but remember that the old FCC rules for grounded grid amps, required that the driver power be added to the measured average DC plate power in computing total input power. This, ahem, mythical interpretation of the conversation of energy, influenced the specsmanship of all the grounded grid amps for decades and still continues today for the 3-500Z class amps even though we no longer rate them by DC plate input power.
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KB5UBI
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Posts: 97




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« Reply #20 on: January 01, 2013, 09:52:23 PM »

The current production AL-80Bs are supplied with a 3-500ZG which has a 20% higher Anode dissipation rating than the 3-500Z.   
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AA4HA
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Posts: 1640




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« Reply #21 on: January 02, 2013, 12:59:25 AM »

I think MFJ makes some decent amps, but in their manuals and internet site they throw out all kinds of power ratings which can be confusing to a Ham with little electronics experience or math.
...
Now they do disclose that efficiency is 65% CW and 62% SSB (crest).  

They also state the AL-82B will output 1500 Watts out for 30 minutes but according to my calcs, that would be 700 miiliamps at 3300 volts with 65% efficiency.

So yes, buyer beware and look at the capabilities of the amplifier, and make some simple power calculations before buying.
This is the kind of question that should be in the exam pool. How to calculate amplifier power if you know the incoming AC electrical service requirements, plate voltage/current, filament voltage/current, efficiency or RF power output. You can also apply this to solid state devices. It could be worked backwards to figure out efficiency as well.

What a way to get across that a licensee understands ohms law and efficiency.

Manufacturers have been using poetic license with specifications for decades. They get away with it because many of us are too ignorant to hold their feet to the fire. The much vaunted ARRL is in bed with the manufacturers because they are the advertisers. Nobody wants to issue a declaration on industry standard measurements, some amateurs might actually read them.
« Last Edit: January 02, 2013, 01:03:18 AM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AD4U
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Posts: 2186




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« Reply #22 on: January 02, 2013, 05:39:55 AM »

The current production AL-80Bs are supplied with a 3-500ZG which has a 20% higher Anode dissipation rating than the 3-500Z.  

This has nothing to do with plate dissipation of the tube.  How the tube is cooled (removing the heat) is what is most important here.  

Under the EIMAC numbering system a 3-500Z tube means: 3 = TRIODE; 500 means 500 watts safe CCS plate dissipation assuming ADEQUATE cooling; and Z = zero bias under CERTAIN operating conditions.  

Whether a tube plate is metal or graphite may (or may not) increase the dissipation capabilities but it does not increase the MAXIMUM plate current the device can be driven to and still remain LINEAR.

EVERY tube has a set of operating curves that define what the device will do and how it will operate given certain conditions.

A LINEAR amplifier means the tube operates in the LINEAR portion of these operating curves.  GENERALLY the linear region of these operating curves is NOT at the top end, but in the center.  

When a tube is driven out of the LINEAR area of the published operating curves (usually past the maximum plate current ratings), the amp is no longer a LINEAR amp.  Granted the wattmeter indicates even more RF output, but the question remains whether or not the output is "clean" and whether or not the manufacturer's maximum published plate current (cathode current for the purists) is exceeded.  This is 400 mA for a 3-500Z(G)

Dick  AD4U
« Last Edit: January 02, 2013, 05:43:41 AM by AD4U » Logged
WX7G
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Posts: 6319




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« Reply #23 on: January 02, 2013, 05:41:03 AM »

See the AL-80B manual, tuning steps 11a, 11b on page 29 and step 2 on page 21. These say to tune to up 600 mA. 400 mA applies to continuous key down such as RTTY.

The Eimac 3-500Z datasheet shows the plate characteristics looking quite linear to over 800 mA.

http://www.umich.edu/~umarc/station/docs/3-500z.pdf

Does running a 3-500Z at 800 watts in OOK CW or 1000 watts PEP in SSB do any harm? It does not exceed the plate dissipation rating (about 220 watts in CW and SSB), it does not cause excessive distortion (see below), and it does not reduce cathode life. No harm is done.

The AL-80B is specified to give -35 dB or better IMD at rated output. This is 1000 watts. So don't worry, tune per the instructions, go on the air and have fun with an amp that is as clean as many of the exciters exciter that drive it.


« Last Edit: January 02, 2013, 06:11:33 AM by WX7G » Logged
KU3X
Member

Posts: 145




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« Reply #24 on: January 02, 2013, 10:40:22 AM »

The current production AL-80Bs are supplied with a 3-500ZG which has a 20% higher Anode dissipation rating than the 3-500Z.  

This has nothing to do with plate dissipation of the tube.  How the tube is cooled (removing the heat) is what is most important here.  

Under the EIMAC numbering system a 3-500Z tube means: 3 = TRIODE; 500 means 500 watts safe CCS plate dissipation assuming ADEQUATE cooling; and Z = zero bias under CERTAIN operating conditions.  

Whether a tube plate is metal or graphite may (or may not) increase the dissipation capabilities but it does not increase the MAXIMUM plate current the device can be driven to and still remain LINEAR.

EVERY tube has a set of operating curves that define what the device will do and how it will operate given certain conditions.

A LINEAR amplifier means the tube operates in the LINEAR portion of these operating curves.  GENERALLY the linear region of these operating curves is NOT at the top end, but in the center.  

When a tube is driven out of the LINEAR area of the published operating curves (usually past the maximum plate current ratings), the amp is no longer a LINEAR amp.  Granted the wattmeter indicates even more RF output, but the question remains whether or not the output is "clean" and whether or not the manufacturer's maximum published plate current (cathode current for the purists) is exceeded.  This is 400 mA for a 3-500Z(G)

Dick  AD4U
WELL PUT !
Manufactures will tell you anything to make a sale.
When I became a ham over 35 years ago, DB gain figures were advertised in QST. Why would anybody buy a Mosley full size three element mono band beam when they could buy a TA-33 and have the same gain, same front to back figures, two more bands, smaller in size and have less wind load?
Just watch TV. Do you really think by drinking a certain kind of beer loads of women will be tearing off their close just to be near you?
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K9MOV
Member

Posts: 48




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« Reply #25 on: January 04, 2013, 12:14:27 AM »

A little off topic, but, I have many local hams that are on the DX-honor role and have never run more than 100 watts plus dipoles at 30 to 35 feet. Also very modest equipment. ( not the 6000 to 12,000 dollar radio's) I also have many local hams running big rigs, big power and they will never get on the honor role. The biggest gain in DB is between the ears, everything else is 2nd.
Lane--k9mov
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KD8MJR
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Posts: 2701




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« Reply #26 on: January 06, 2013, 06:25:56 PM »

Just watch TV. Do you really think by drinking a certain kind of beer loads of women will be tearing off their close just to be near you?

Well that Depends on where your drinking the Beer and what kind of Women your talking about.  Grin Grin
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W8GP
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Posts: 224




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« Reply #27 on: January 07, 2013, 07:25:47 AM »

I run my SB-220 at the legal limit, it's legal and I run it to it's limit!!!! Realistically, this thread is kind of academic as this type of "legal limit" debate has ben going on for some time.The difference between my 1200 watts and true legal limit is negligible at the receive end. So why would anyone want one of the amps capable of 2500+? Aside from RTTY or other high duty cycle modes, the answer is to use it, of course! As a low-bander there are many times when I thought it would be useful to have a a little extra oomph. And you know I'm not alone.I hear one well known DXer state his power as 450 watts.Maybe or maybe not.And I get a kick out of the guys that that are 40 over state something like "I"m using a two hole 8877, but I'm only running a KW out because I don't want to hurt it". OK......
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K1DA
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Posts: 538




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« Reply #28 on: January 08, 2013, 08:27:09 AM »

A pair of 3-500s with air system sockets and a stiff supply will do 1500 watts out "all day long"  without exceeding the published EIMAC ratings (which you could look up) .  The Al 1200 was sold before the "new" power regulations were imposed and it has a 1200 watt dissipation tube and a good, if loud, air system socket cooling system.  Many were uprated with Peter Dahl transformers.  THere are others which have good rf decks but marginal power supplies.
An L4 BASHER is a good amp and the outboard supply is easy to upgrade. Just doesn't do 160, as is also true of many Henry amps.  The SB 220, and the L7 have M. Mouse cooling systems,  and power supplies, as does the otherwise well made Kenwood TL 922, and the EIMAC specification for 3-500s used in non-airsystem sockets is to DERATE to 400 watts. 
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KB5UBI
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Posts: 97




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« Reply #29 on: January 08, 2013, 03:30:31 PM »

A pair of 3-500s with air system sockets and a stiff supply will do 1500 watts out "all day long"  without exceeding the published EIMAC ratings (which you could look up) .  The Al 1200 was sold before the "new" power regulations were imposed and it has a 1200 watt dissipation tube and a good, if loud, air system socket cooling system.  Many were uprated with Peter Dahl transformers.  THere are others which have good rf decks but marginal power supplies.
An L4 BASHER is a good amp and the outboard supply is easy to upgrade. Just doesn't do 160, as is also true of many Henry amps.  The SB 220, and the L7 have M. Mouse cooling systems,  and power supplies, as does the otherwise well made Kenwood TL 922, and the EIMAC specification for 3-500s used in non-airsystem sockets is to DERATE to 400 watts. 

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.   
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