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Author Topic: Choosing a Legal Limit Amp  (Read 21074 times)
W6UV
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Posts: 540




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« Reply #15 on: April 17, 2013, 11:57:42 AM »

Thanks for the info, Jonathon.

I don't do 6m, so that's not a consideration. Are the THP2933 MOSFETs used in the HL-2500kfx more rugged (able to withstand greater SWR) than the ARF-1500 in the HL-2.5kfx?
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N6AJR
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« Reply #16 on: April 17, 2013, 12:20:30 PM »

I have had many many amps over the years. everything from sweep tube amps, to clipperton L to alpha 76a's, to ameritron 811's, 811H and als500, als600, als 1200 , IC 2-KL's tl922's and so on. The one I kept and use is an alpha 87a. full automatic operation, self tuning, autoband switch  and solid as a rock.  I got it for $5500 used, and never had a problem in 6 or 8 years I have had it.  Alpha may even have a used one or to in stock.  call alpha first and see what they have.  they are the absolute best I have ever owned.  Nice to look at too. good value for the $$$. great service.
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W6GX
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« Reply #17 on: April 17, 2013, 09:19:16 PM »

Thanks for the info, Jonathon.

I don't do 6m, so that's not a consideration. Are the THP2933 MOSFETs used in the HL-2500kfx more rugged (able to withstand greater SWR) than the ARF-1500 in the HL-2.5kfx?

The ARF-1500 has a very high source-to-drain breakdown voltage of 500v.  The HL-2.5kfx operates at a drain voltage of 125v.  The issue is not how rugged the PA is but rather how good the protection circuitry is.  No matter how good of an operator you are you will make mistakes and cause a high SWR event.  Or you could experience an equipment failure that can also cause a high SWR event.  In solid state amps. the deciding factor is whether the protection circuitry could protect a catastrophic SWR event like a shorted antenna lead.  I know of someone who blew a pair of ARF-1500s when his antenna matching network arced and caused a short on the antenna lead at full power.  THP has made some production changes to the protection circuitry in the HL-2.5kfx and all newly produced units have the latest protection circuitry.  The PA's are always at risk with repeated high SWR events, no matter how good the protection circuitry is.  Running legal limit requires that you have a very robust system- coax, lightening arrestor, matching network, antenna switch, antenna balun, antenna traps, etc.  A blown balun or trap will cause a high SWR event.  A failed or poor quality antenna switch could cause a short on the antenna lead.  Under normal use the HL-2.5kfx could withstand a SWR of 1.8:1 without harm.  I hope this helps.

73,
Jonathan W6GX
« Last Edit: April 17, 2013, 09:24:15 PM by KF7BBJ » Logged
W6UV
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Posts: 540




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« Reply #18 on: April 17, 2013, 09:52:00 PM »

Running legal limit requires that you have a very robust system- coax, lightening arrestor, matching network, antenna switch, antenna balun, antenna traps, etc.

Point noted. I recently rebuilt my station from the ground up and everything is designed for the legal limit as a minimum, with most things designed for 2.5-4KW.
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NK2F
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Posts: 33




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« Reply #19 on: April 18, 2013, 02:11:03 AM »

You may want to take a look at Alpin 200, also a Bulgarian-made amp, on the same platform as the Alphas and Acoms. The Alpin brand is unknown in the US, but it is in Europe, with raving reviews in the German and UK publications. Distributed by EU dealers, direct to the rest of the world. CE tested and listed so you know you are not buying an IMD nightmare. Pi-L circuit has the ability to match loads to 3:1 so no need for an antenna tuner. Tons of high end protection circuits so the tubes will last forever. I measured over 2400W on 15m with about 70W of drive power. Best tuning indicator I have used, making manual tune as fast as automatic.

Take a look at http://www.alpinamplifier.com/index.php?option=com_content&view=article&id=3&Itemid=10  Note that the price for the US is 20% less (no sales tax) and shipping, at least to NYC area, is around $400, delivered in less than a week with no import duties or taxes of any kind.
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N4FBW
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Posts: 34




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« Reply #20 on: April 18, 2013, 05:29:54 PM »

Getting service for a THP amp is not a problem. AVSL services THP products, just stop by the THP website for more information.
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AD4U
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Posts: 2180




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« Reply #21 on: April 18, 2013, 05:35:59 PM »

There is nothing more dreamy than a set of 3CX800A's tuned up. Get an Alpha 89

Except possibly a pair of 8877's in an Alpha 77SX.  I know because I have one.   HI   Wink

Dick  AD4U
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W8JX
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Posts: 6482




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« Reply #22 on: April 18, 2013, 05:37:48 PM »

There is nothing more dreamy than a set of 3CX800A's tuned up. Get an Alpha 89

Except possibly a pair of 8877's in an Alpha 77SX.  I know because I have one.   HI   Wink

Dick  AD4U

And why would you have a pair of them unless you like running well beyond legal limit.
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N4ATS
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Posts: 849




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« Reply #23 on: April 18, 2013, 06:14:57 PM »

Simple.....Head room

Unlike most over-rated amps...

Amateur Radio...Think Simplistic
« Last Edit: April 18, 2013, 06:17:01 PM by N4ATS » Logged
W8JX
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Posts: 6482




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« Reply #24 on: April 18, 2013, 06:40:04 PM »

Simple.....Head room

Unlike most over-rated amps...

Amateur Radio...Think Simplistic

Head room, not likely when a single tube can close to 3kw with proper power supply.  Run as much power as you can afford and get away with it seems
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N4ATS
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Posts: 849




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« Reply #25 on: April 19, 2013, 04:05:30 AM »

Opposite , run as easy as you can while maintaining full legal limit.
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AD4U
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Posts: 2180




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« Reply #26 on: April 19, 2013, 05:21:53 AM »

There is nothing more dreamy than a set of 3CX800A's tuned up. Get an Alpha 89

Except possibly a pair of 8877's in an Alpha 77SX.  I know because I have one.   HI   Wink

Dick  AD4U

And why would you have a pair of them unless you like running well beyond legal limit.


Why do you drive a car that can go faster than the legal speed limit?  It is because you drive 100 MPH every where you go??  I bought the 77SX in 1991 for $2000 because the owner had to sell it and I wanted to buy it.  I have ben offered and refused $5000.  Good investment??
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N4ATS
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Posts: 849




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« Reply #27 on: April 19, 2013, 05:45:36 AM »

"Why do you drive a car that can go faster than the legal speed limit?  It is because you drive 100 MPH every where you go??  I bought the 77SX in 1991 for $2000 because the owner had to sell it and I wanted to buy it.  I have ben offered and refused $5000.  Good investment??"

Exactly and well put. Most of the amps on the market (cheap stuff like MFJ) may say legal limit or rated at 600 watts and yes , they can reach legal limit or 600 watts , but hold that power long? Most will melt down because to reach that limit , you have to go balls to the wall.

Alpha , set it at 1.5 KW , key it on FM, walk away and come back an hour later , still 1.5K

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AD4U
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Posts: 2180




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« Reply #28 on: April 19, 2013, 06:47:07 AM »

"Why do you drive a car that can go faster than the legal speed limit?  It is because you drive 100 MPH every where you go??  I bought the 77SX in 1991 for $2000 because the owner had to sell it and I wanted to buy it.  I have ben offered and refused $5000.  Good investment??"

Exactly and well put. Most of the amps on the market (cheap stuff like MFJ) may say legal limit or rated at 600 watts and yes , they can reach legal limit or 600 watts , but hold that power long? Most will melt down because to reach that limit , you have to go balls to the wall.

Alpha , set it at 1.5 KW , key it on FM, walk away and come back an hour later , still 1.5K



Agreed..........If anybody wants to run with the BIG GUNS in HAM radio, you DO NOT do it with an Alpha 77SX, except perhaps as a driver for the big amp.
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NN4X
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Posts: 45




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« Reply #29 on: April 19, 2013, 07:34:28 AM »

Dishtronix Prometheus DX2400: It's the modern-day equivalent of the Alpha 77DX.
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