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Author Topic: "Legal Limit" Amplifiers-NOT!  (Read 12361 times)
W7APM
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Posts: 25




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« on: December 21, 2012, 10:33:11 AM »

Since when did SB-220's and L4B's and similar amps become legal limit amps? If you are new to amps and a seller tries to sell such as a legal limit amp I would be very wary. Either the seller has overdriven the amp ( grid current ratings of the 3-500's exceeded) or has a bad wattmeter or is untruthful, or all of the above. These amplifiers were designed when the legal limit power was 2 kw input to the finals and at 60% efficiency or so you can expect around 1300 watts output.

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W8JX
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« Reply #1 on: December 21, 2012, 11:19:07 AM »

Actually more than one L4B has been upgraded to legal limit and beyond and it as not really too hard either. The L4B has a sturdy RF deck and good cooling design it just lacks a better power supply. Increase supply voltage 500 v or so and stiffen it up too and it will easily make legal limit and then some. Given that L4 uses a external power supply it is easier to do than on a SB220 which also lacks in supply area as well for true legal limit.
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AA4PB
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« Reply #2 on: December 21, 2012, 11:25:25 AM »

The difference between the old legal limit and the present legal limit is 0.6dB. Not worth worrying about - just run the amp as it was designed.
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W8JX
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« Reply #3 on: December 21, 2012, 12:39:08 PM »

The difference between the old legal limit and the present legal limit is 0.6dB. Not worth worrying about - just run the amp as it was designed.


This is quite true but some like to run well beyond limit too but never talk about it. There is one such person on 40m regularly and splattering +/- 5+kc too.







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ZL1BBW
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Posts: 355




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« Reply #4 on: December 25, 2012, 03:40:34 PM »

Our new legal limit is 1000 watts PEP output  any mode,   so that will require a step up to get 1000 watts cw output.  Not sure if it is at the amplifier terminals or the antenna terminals, because that could make a fair bit of difference.
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
KU3X
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Posts: 141




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« Reply #5 on: December 29, 2012, 09:15:20 AM »

I saw one guy selling an original AL 80, not the A or B model, and claimed it would run 1500 watts out.
Another HAM claimed 1400 watts out of a Collins 30 L 1, not the S model.

I guess you no longer have to worry about grid current? .....Right !

Barry
www.ku3x.net
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G3RZP
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« Reply #6 on: December 29, 2012, 09:55:07 AM »

Maybe he meant '1400 watts out of the AC line'?
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KU3X
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Posts: 141




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« Reply #7 on: December 29, 2012, 11:02:50 AM »

Maybe he meant '1400 watts out of the AC line'?
No, he was referring to output power of the 30L1.

Barry
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AA4HA
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Posts: 1378




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« Reply #8 on: December 29, 2012, 01:23:54 PM »

 Wink Maybe they were talking about legal limit on 30 or 60 meters or techs on 10 meters.
« Last Edit: December 29, 2012, 01:33:52 PM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WB2WIK
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Posts: 20547




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« Reply #9 on: December 29, 2012, 03:53:07 PM »

As a point of clarification, in the U.S. there was never a regulation that I know of that limited power to 2000W DC input power.  That was a marketing deployment by various manufacturers.

Prior to the "1500W PEP output" regulation, the previous regulation was 1000W DC input, with absolutely no stipulation for "PEP input power."

When the regs changed, everybody gained except AM operators, who actually "lost."  1kW DC input with 100% modulation and 65% efficiency would be 2600W PEP output power.  The new limit is lower. Tongue
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QRP4U2
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Posts: 108




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« Reply #10 on: December 29, 2012, 04:15:39 PM »

Quote
These amplifiers were designed when the legal limit power was 2 kw input to the finals and at 60% efficiency or so you can expect around 1300 watts output.


And your point is?
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W8GP
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Posts: 190




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« Reply #11 on: December 30, 2012, 11:03:53 AM »

QRP4U2:
      I think you're in the wrong forum! Cry
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W7APM
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Posts: 25




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« Reply #12 on: December 30, 2012, 11:16:34 AM »

As the starter of this thread my point is if you are buying an HF amplifier and are new to the game to beware of advertizing claims. I see sellers on various forums and Ebay trying to inflate the value of what they are selling by claiming they are Legal Limit Aplifiers. If they will lie to you about the power out capabilities they likely will lie about other operational capabilities too.

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QRP4U2
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Posts: 108




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« Reply #13 on: December 30, 2012, 08:09:14 PM »

Quote
As the starter of this thread my point is if you are buying an HF amplifier and are new to the game to beware of advertizing claims. I see sellers on various forums and Ebay trying to inflate the value of what they are selling by claiming they are Legal Limit Aplifiers. If they will lie to you about the power out capabilities they likely will lie about other operational capabilities too.


Thanks for the clairificaton Allen and I apologize if the comment came out as a bit critical,  but I have seen people comment like this as if they were the power police.

Quote
QRP4U2:
      I think you're in the wrong forum! 

This was my original eham name and I have yet to change it since at the time I was doing a lot of QRP.

Phil - AC0OB

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AD4U
Member

Posts: 2153




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« Reply #14 on: December 31, 2012, 05:28:16 AM »

If Ameritron (MFJ) can get 1000 watts OUT of a single 3-500, then why can't Heathkit get 1500 watts out of TWO 3-500's.  My SB-220 runs 3300 volts on the tubes, which is about the same as the AL-80B.   Roll Eyes

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.

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:

Either the single 3-500 tube must be driven WAY past the rated plate current

OR

The efficiency would have to be almost 76%.  I seriously doubt that even MFJ can get that kind of effiency.

Dick  AD4U
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