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Author Topic: 600W Amp vs 800W Amp  (Read 7319 times)
KB1TJY
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« on: July 28, 2011, 06:04:51 AM »

Greetings!

I have an Ameritron AL-811 rated at 600W. I am beginning to wonder if I would have done better to have spent the extra couple hundred dollars and bought the AL-811H which is rated at 800W. In terms of signal strength is there really that much of a difference?

Many thanks and 73's,

Lyman
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KA5N
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« Reply #1 on: July 28, 2011, 06:37:42 AM »

Well figure it out 800 Watts /600 Watts = 1.33333  and 10 Log ratio =1.25 dB 
Considering that one dB is about the limit of decernment the change would be just noticeable
and less than one S unit.
 
However if you improve your antenna 3 dB (fairly easy to do) that would be a worthwhile
improvement.

In other words wait till you decided (later on) to run some power before spending more
on a slightly higher output amp.
Allen
« Last Edit: July 28, 2011, 07:49:35 AM by KA5N » Logged
N4RSS
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Posts: 260




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« Reply #2 on: July 28, 2011, 07:12:53 AM »

Well figure it out 800 Watts -600 Watts = 200 watts /600 = -.33333 ...
10 log ratio = .523 dB apx. 

That's not correct.  Back to the beach no time to type further

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KD8GKR
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Posts: 47




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« Reply #3 on: July 28, 2011, 07:32:52 AM »

Short answer, your the only one that will notice while watching the watt meter. The 811(non H) is probably the most used amp, due to price. Take the few hundred you saved and due some upgrade to antennas.
« Last Edit: July 28, 2011, 07:37:50 AM by KD8GKR » Logged
W8JX
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« Reply #4 on: July 28, 2011, 08:21:43 AM »

Short answer, your the only one that will notice while watching the watt meter. The 811(non H) is probably the most used amp, due to price. Take the few hundred you saved and due some upgrade to antennas.

The 811 amps are a bit over rated. The 3 tube is pretty solid at 450 watts or so. Driven to run at rated 600 watts it will have much shorter tube life. 4 tube is pretty solid at 600 watts.
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W8JI
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« Reply #5 on: July 28, 2011, 09:41:08 AM »

Short answer, your the only one that will notice while watching the watt meter. The 811(non H) is probably the most used amp, due to price. Take the few hundred you saved and due some upgrade to antennas.

The 811 amps are a bit over rated. The 3 tube is pretty solid at 450 watts or so. Driven to run at rated 600 watts it will have much shorter tube life. 4 tube is pretty solid at 600 watts.

This actually all goes back to duty cycle vs. average power, not absolute power, so any "sound byte"  answer is very incomplete.

The averge dissipated power in the tubes, over any 15 second or longer period, should be kept below 240 watts (average heat) with 800 watts PEP output the RF peak limit for acceptable IMD (-35 dB PEP) on SSB.

That would be about 250 watts long term average power output on voice and 300 watts average long term on CW, which if you do not run heavily clipped or processed voice would easily be 800 watts PEP continuous voice or 600 watts normal CW.

The real reason some people kill tubes is how they tune, and some are just bad tubes that will go anyway no matter what.




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KD8MJR
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« Reply #6 on: July 28, 2011, 12:44:45 PM »

Short answer, your the only one that will notice while watching the watt meter. The 811(non H) is probably the most used amp, due to price. Take the few hundred you saved and due some upgrade to antennas.

The 811 amps are a bit over rated. The 3 tube is pretty solid at 450 watts or so. Driven to run at rated 600 watts it will have much shorter tube life. 4 tube is pretty solid at 600 watts.

I agree with W8JX and for the extra $150 your better off with the 811H and the extra 1db+ of gain
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AA4HA
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« Reply #7 on: July 28, 2011, 12:46:37 PM »

600 watts = 57.78 dBm
800 watts = 59.03 dBm

Power difference = 1.25 dB

Each S-unit is approximately a 6 dB difference in received signal level.

Antenna changes will have a much bigger effect. A 3 element beam vs. a dipole is 6 dB (or approx 1 S-unit)

Just by changing out the RG-8 to LMR-400 (at 30 MHz, 100 feet) will give you around 1.25 dB of less loss. The same as if you went from 600 to 800 watts on the transmitter. Your receiver will also pick around a 1/4 of a S-unit.

From both the antenna and feedline change you get the additional benefit of improving your received signal (no transmitter amplifier that you are running is going to help you there). (net improvement from both 1.25 S-units on receive and transmit)

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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W8JI
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« Reply #8 on: July 28, 2011, 12:49:50 PM »

While it is only about 1 dB, the AL811H is a much better amp because it is neutralized and has 25% more tube dissipation available.

The ALS600 is fine though.
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KD8MJR
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« Reply #9 on: July 28, 2011, 03:35:43 PM »

600 watts = 57.78 dBm
800 watts = 59.03 dBm

Power difference = 1.25 dB

Each S-unit is approximately a 6 dB difference in received signal level.

Antenna changes will have a much bigger effect. A 3 element beam vs. a dipole is 6 dB (or approx 1 S-unit)

Just by changing out the RG-8 to LMR-400 (at 30 MHz, 100 feet) will give you around 1.25 dB of less loss. The same as if you went from 600 to 800 watts on the transmitter. Your receiver will also pick around a 1/4 of a S-unit.

From both the antenna and feedline change you get the additional benefit of improving your received signal (no transmitter amplifier that you are running is going to help you there). (net improvement from both 1.25 S-units on receive and transmit)

For most people an Amp upgrade is much easier than an Antenna upgrade. I am just saying that given the choice, for an extra $150 getting 150 extra watts is a no brainer, of course a better antenna is the best route but as I said for most people that may not be possible.
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W8JX
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« Reply #10 on: July 28, 2011, 04:33:48 PM »

Short answer, your the only one that will notice while watching the watt meter. The 811(non H) is probably the most used amp, due to price. Take the few hundred you saved and due some upgrade to antennas.

The 811 amps are a bit over rated. The 3 tube is pretty solid at 450 watts or so. Driven to run at rated 600 watts it will have much shorter tube life. 4 tube is pretty solid at 600 watts.

This actually all goes back to duty cycle vs. average power, not absolute power, so any "sound byte"  answer is very incomplete.

The averge dissipated power in the tubes, over any 15 second or longer period, should be kept below 240 watts (average heat) with 800 watts PEP output the RF peak limit for acceptable IMD (-35 dB PEP) on SSB.

That would be about 250 watts long term average power output on voice and 300 watts average long term on CW, which if you do not run heavily clipped or processed voice would easily be 800 watts PEP continuous voice or 600 watts normal CW.

The real reason some people kill tubes is how they tune, and some are just bad tubes that will go anyway no matter what.


W8JI, I have a lot of respect for your knowledge and comments on amps but I still feel that 811 amps are overrated. My "logic" here is the rated dissipation of tubes. The continuous rated safe dissipation of tube or tubes should easily exceed 1/3 of rated output power. (assuming approx 66% efficiency though it will likely be less than that) That being said a 811 is rated about 45 watts continuous and 65 for very brief periods. with 66% efficiency you will need approx 680 watts input for 450 watts out or 230 watts of plate dissipation key down and  beyond the 195 watt intermittent rating of tube (65 x 3)  Take output up to 600 and plate dissipation is up to 300 watts or 100 watts per tube. (well beyond ratings) One could argue that for intermittent SSB that this is acceptable but very short tube life will result. Unlike 572's (which have about 3x rated dissipation and have far greater thermal mass to absorb overloads) 811's can toast pretty easy as many owners have found out hard way. If this rated dissipation was not so important twin 3-500z,  single 3cx1500, single 3cx1200 and twin 3cx800 amps would not be so sought after. These amp (with proper cooling and power supply) can make legal limit year after year without complaint because rated dissipation easily exceeds what is needed. It is a shame that Ameritron never scraped 811 amp and made a modern version of heathkit SB-200 using two 572b's which can safely make more power than four 811's with proper power supply.
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W8JI
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« Reply #11 on: July 29, 2011, 02:44:56 AM »

Tube life caused by anode over temperature failures is a function of avarage dissipation over a period of time, NOT absolute output power. TIME is just as important as power in elevating anode temperature.

Power output is a meaningless guideline. An AL811H can have the tubes ruined in less than a minute with zero watts output, or last 10 years at 800 watts PEP on voice amateur service.

http://www.w8ji.com/al811h_schematic.htm
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W1QJ
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« Reply #12 on: July 29, 2011, 05:07:08 AM »

Tom, as someone who understands EXACTLY what you are talking about I meet up with the same resistance as W8JX explains.  You have given the reason in your explanation.  What I have discovered over this issue is to play the hot potato game.  I simply throw the "pulse rating" of a tube right back.  When you look at the pulse rating on a tube like a 3CPX800A7 or similar all the math that W8JX can throw at you can not explain the rating UNLESS "time" becomes a factor.  That is the key ingredient everyone misses in the equation because it's not in there.  In reality it comes into play however.  Lou
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AA4PB
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« Reply #13 on: July 29, 2011, 06:33:03 AM »

The original question: In terms of signal strength is there really that much of a difference?

The answer to that question is NO. As Tom points out, there may be other reasons to consider the larger amp., but not in terms of increased signal strength on the receive end.
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W8JI
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« Reply #14 on: July 29, 2011, 06:38:47 AM »

Tom, as someone who understands EXACTLY what you are talking about I meet up with the same resistance as W8JX explains.  You have given the reason in your explanation.  What I have discovered over this issue is to play the hot potato game.  I simply throw the "pulse rating" of a tube right back.  When you look at the pulse rating on a tube like a 3CPX800A7 or similar all the math that W8JX can throw at you can not explain the rating UNLESS "time" becomes a factor.  That is the key ingredient everyone misses in the equation because it's not in there.  In reality it comes into play however.  Lou

That's right Lou!!  People look for and sometimes stubbornly insist on a simple answer even if the simple answer is wrong. I think that goes to people being able to understand a simple statement, but not being able to or wanting to take the time to understand the way something really works.

Voltage failures are instantaneous, but current or heating failures always have a time and/or duty-cycle function. The vast majority of failures in good 811 tubes are user controllable. Operational failures are determined by the TIME and dissipation as a matter of seconds up to a minute or so,  and absolutely NOT by the output power as some would wrongly tell others.

This is why systems like this have a variety of ratings, including IVS service. The AL811H is fine at 800 watts PEP voice and 600 watts on CW with OOK (off on keying) so long as the amp is tuned correctly and the duty cycle is normal voice or CW. There will be no shortening or difference in life dropping power so long as the time-averaged duty cycle of periods less than a minute or more than 10-15 seconds does not exceed 60 watts. That dissipation could be exceeded at one watt output or at a kilowatt output,  because output power does not determine anode heat.

Dissipation averaged over time determines anode temperature, not output power.

73 Tom
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