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Author Topic: How much power is enough?  (Read 1530 times)
KA0UK
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Posts: 6




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« on: March 03, 2004, 09:06:51 PM »

I have the opportunity to purchase either a 1000 watt linear or a 1500 watt linear. By my calculations the higher power amp has a gain advantage of 1.76 db. Given 20 meters as a reference band how much improvement in my signal strength should I expect assuming band conditions are consistent? Thanks,
David KA0UK
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NI0C
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« Reply #1 on: March 03, 2004, 09:46:33 PM »

What you want to know is: Would anyone notice the 1.76 dB difference?  Also, you want to know if YOU will notice the difference-- in terms of beating out the competition in a pileup, or in extended QSO times under marginal conditions, etc.  The difference is certainly marginal, only slightly noticeable.

Keep in mind that by going from a barefoot 100 watt rig to 1000 watts, your improvement will be already 10 dB.  I'd advise looking at other differences between the two amplifiers you are comparing-- price, reliability, availability of replacement tubes, QSK or not, cooling fan noise, etc.  These other considerations may well make the 1.76 dB difference not so important.

73 de Chuck  NI0C
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WB2WIK
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« Reply #2 on: March 04, 2004, 12:41:15 PM »

Yep, the difference is 1.76 dB.

However, I think the *real* reason to use a 1500W PEP output amplifier is that it will sound better and deliver a cleaner signal when operated at the 1000W PEP output level.  "Headroom" is a wonderful thing, when the goal is linearity.  The same reason people buy 1000W(pk) audio amplifiers for their living rooms.  Nobody could actually tolerate listening to a 1000W audio signal generated in the same small room they're standing in, and this power would blow out most speaker systems, anyway.  But when operated at comfortable listening levels, the "big" amplifier will be so clean and distortion-free, it will be a pleasure to listen to.

I normally wouldn't operate a "1000 Watt" RF amplifier at 1kW output power unless I was on a pretty empty band, like ten meters when the band's closed, or 160 meters during the day...but there are possibly some "1kW" rated amplifiers that are scrupulously clean at full output, especially if there was designed-in derating for the Marketing department!

WB2WIK/6
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K8AC
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« Reply #3 on: March 04, 2004, 06:01:54 PM »

What are you thinking when you say: "assuming band conditions are consistent"?  The power difference won't mean a thing when conditions are good.  When conditions are marginal or very poor, and your signal is at the threshold of readability, it can mean the difference between being copied, or not.  Most hams buy the line that an S unit is 6 dB - ever measure your receiver?  I have, using an accurate attenuator and found it not unusual for an S unit to be as little as 1.6 dB on the lower portion of the scale, and that's on a fairly expensive transceiver.  Anyway, the answer to your question depends upon which bands you operate and whether you chase DX or participate in nets on 40 SSB.  If you're chasing a new one on the other side of the world on 10 meters and signals are down in the noise, it might make all the difference in the world.  If you're checking into a net on 40 and all the signals are at S9 or better, it won't matter in the least.

One thing for certain, the difference between 100 watts and 1000 watts will usually be very, very noticeable.  Don't be fooled by the old line: "it's ONLY a 10 dB difference!"
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W5HTW
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« Reply #4 on: March 05, 2004, 12:14:35 AM »

Agreed - don't be fooled. Theory is one thing, actual use may not always be that clear cut.  I have, in checking into a net on 75 meters, been completely in the mud to NCS, unreadable.  Though I prefer not to use an amp, jumping from 100 watts to 1000 watts brought me up to what he described as "armchair copy."  

But I recall way back in the mid sixties when SSB was just becoming popular enough I (and many others in like boots) wanted on it, and get rid of our AM rigs, a fellow I knew had a nice SSB transceiver.  I would guess it put out 100 watts, though I do not recall what brand it was.  He also purchased a new National NCL-2000 amp.  And he checked into our local 3905 net and talked with a couple of the guys who had sideband.  He tried some "no amplifier" and "with amplifier" tests and they could not tell the difference.  Nor could I.  And of course not!  He was less than a mile away, on 75 meters daytime.  He had to already be S9 plus 20 or so, as just about everyone was.  Putting the amp on made no noticeable difference.  The point of this is, when the 100 watt signal is marginal, or slightly worse, the 1000 or 1500 watt signal can make a major difference.  But when 100 watts does the trick, the 1KW may be such a waste it won't even be noticed!  

I also recall, a couple of years ago, a fellow trying to demonstrate to me (I think he was in New England) the difference his 1.5KW amp made on 40 CW, over his 100 watts.  He was perfect copy with 100 watts, and I doubt I saw a single S-unit difference when he put on the amp.  Kind of a wake-up call for me, that convinced me, as I had suspected, using an amp on CW is not worth the effort.

But do remember -- practice may not always walk in step with theory.  Too many variables: QSB, QRM, your own perception.

73
Ed
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KA0UK
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« Reply #5 on: March 05, 2004, 01:14:43 AM »

Thanks Guys, I had assumed that an amplifier rated at 1KW would put out a clean 1KW on all frequences - a bit naive perhaps. I understand that the change from 100 to 1000 watts is worthwhile and that an increase above that within legal limits is much less likely to be so. I would like to put out 1KW so will choose the higher power amp. Thanks to all for their insights.
David KA0UK
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W7DJM
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« Reply #6 on: March 07, 2004, 02:53:15 PM »

""advise looking at other differences between the two amplifiers you are comparing-- price, reliability, availability of replacement tubes, QSK or not, cooling fan noise, etc. These other considerations may well make the 1.76 dB difference not so important.""


ABSOLUTELY

"" will sound better and deliver a cleaner signal when operated at the 1000W PEP output level""

MAYBE.  Maybe not.  depends on the amplifiers you are comparing, the tubes, whatever  Two many variables in all the different amps to make a statement like that.


Some more things to consider, is, whether you need 160 meters--many older amps do not cover 160.

The general reputation of the amp for reliablility and tube availability.

Are you going to run AM (linear) or RTTY?  Both these modes require a good solid amp.

The fact is, MOST of the time--for "normal" SSB use you could run a "1000" watt amp at 600 watts and nobody would notice.
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W8JI
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« Reply #7 on: March 17, 2004, 06:47:19 AM »

1 db is all the difference in the world when you signal is NEAR noise or QRM level. It can often be the difference between OK copy and no copy at all.

When signals are 10-20dB out of the noise, it is meaningless.

As for IM distortion, lower power is not always cleaner, and most good amps are cleaner than radios even beyone full rated power. Anyone who has actually made IM measurements and watched products change with drive power knows this. Radios, for example, often have a "sweet spot" between very low power and full power. So you can turn things down "too much" at times.

Stick with triodes if you are concerned about distortion products. Grid driven tetrodes have poorer IM performance. Check amplifier reviews QST to confirm this. You'll find the tetrodes in the -30dB IM3 range and triodes mid -30's to mid -40's.
 
Also tetrode tubes are often Russian Military surplus, eventually the sources will disappear. (Like the 4CX1600B did.)

73 Tom


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N7DM
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« Reply #8 on: March 30, 2004, 08:52:52 PM »

Interesting....

I have run a few Rock-Crushers in the 'old' days, myself. 813's... 304TL, etc
Further, in my Phone Patching days, I had a SB-220 that definitely made the All-Heart-Phone-Company run right along, from overseas!

For the past dozen years I have been running a Ten-Tec Hercules 2... at 450 watts.  Now, that may sound like over-grown flea power, in some circles, but mark my words, for DXing with the Eu guys, the 450 watts always yields good results.  And you know what?  I don't miss 'peak the grid, dip the plate, touch the loading' for a second!  450 watts of instant-on , full QSK power with no tuning...bandswitching ganged to the main rig... is...
'nice'...  

Think *I* will stick with this...
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KD7YQM
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« Reply #9 on: February 21, 2010, 10:20:53 AM »

I think this falls in with too much sex or money ;-)
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AD6KA
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« Reply #10 on: February 21, 2010, 01:30:32 PM »

"Are you going to run AM (linear) or RTTY? Both these modes require a good solid amp."

If you are going to be involved in RTTY contesting, I would chose the 1.5 kw amp. Almost all of the new
1 kW amps on the market advise not to run their amps at full power with %100 cycle duty modes.

Your antenna situation (and future antenna plans)
play a factor in this decision as well.
I know this may be off topic, but sometimes investing money in improving your antennas will pay off in more QSO's that the amplifier....in the long run.

Good luck es 73, Ken  AD6KA
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K4FX
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« Reply #11 on: February 26, 2010, 04:04:41 AM »

WB2WIK made a good point about 1KW amps, you looked at a AL80 series, one 3-500Z, rated at 800w & CW 1000w SSB, that IMO is pushing that tube hard, but on the other hand the Acom 1000 series of amps uses a 4CX800 (Russian) equivalent tube and it is rated at the same 1KW SSB, now I would say that tube would easily run over 1KW. Acom has a very realistic rating on that amp. I know people running a pair of those valves who can do 2.5K with 100w drive. But like Tom said, the 4CX800s are getting scarcer and scarcer, as expensive as new amps are, could you imagine not being able to get any tubes for one? Talk about a bummer.

Like Matt KK5DR says, the best tube is probably the 8877 since it is still in service and we can expect it to be around for 10-15 more years. Plus one of them has the overhead at 1000w that Steve so correctly mentioned. Or maybe a deal on a 3CX3000F7 for a homebrew, no socket, zero bias, 225w grid, lotsa wattsa,

I learned a long time ago to look at the tube manufacturers specs, (max drive, max grid and plate current, etc) and run the tube accordingly.

K4FX
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K2MK
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« Reply #12 on: February 26, 2010, 06:26:49 AM »

The posting was from 2004. It's likely that he has made his choice by now.
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KG4RUL
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« Reply #13 on: February 26, 2010, 08:24:31 AM »

If the antenna is not melting, you don't have enough power!!!!!
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K4RVN
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« Reply #14 on: February 27, 2010, 06:51:33 PM »

K2MK, so you think after approx 6 years he has bought an amp? I missed it also until I saw your reply.
Good for you! There were some good answers anyway.
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