Linear amplifiers, Advertising blurb vs reality.

[QRZ-934]

Ok, so my first ever linear amplifier was purchased online from a well known supplier last week.
Heres my experiences with it so far.
Its an RM KL503, output as advertised is 300 watts max output for a drive power of up to 6watts.
Ive been using 4 watts to keep within the maximum specified.
Page six of the manual states: " The amplifier should give full output ( I assume 300 watts?) with approximately 4-5watts input."

Its a fairly heavy piece of kit and requires a substantial power supply to run it, 35 to 40 amps, so ive used a nice sized car battery to ensure it gets all the amps it needs.

Its proven useful on several occasions where noise or distance gets in the way of a contact, but with my swr meter being what it was reading up to a maximum of 100 watts and the needle not exactly hitting the stop with much conviction, I was not particularly convinced that the advertised output was as described so shelled out yet more money for a swr meter that could handle up to 1KW.
On hooking up that meter I was able to read the following power outputs.

AM.
1)23
2)50
3)60
4)75-80
5)90
6)130-140.

FM.
1)23/25
2)45-50
3)50-60
4)70-80
5)70-80
6)130 ish.

SSB.
1)30
2)65/70
3)70/75
4)105
5)105
6) 130/140.

As you can see its absolutely nowhere near the claimed output power!
I paid for up to 300 watts yet im seeing less than half that advertised.
Antenna swr was set at 1.1:1, battery power was 12.81 volts so within the specified range to drive it, and input set to 4 watts.
Ive contacted the supplier regarding this and received a reply stating this is what would be expected and that the 300 watt figure is obtained at theoretical maximums.
That not what Ive paid for however, I bought it due to price, and advertised output power not for "theoretical maximums".
Those words dont appear anywhere in the advertising or in the manual.
Theres clearly an issue with the power output on the selection between 4+5 as very little changes also.

I have since sent another mail and we will see whether that yields any further info, Ive also mailed the manufacturer.

Seems ive been hoodwinked at this point into believing the manufacturer hype, Time will tell.

[InTheClouds]

RM themselves are saying it's a 250W amp not 300W.

https://www.rmitaly.com/en/product/kl503-2/

And yes it is common for advertised RF power output to be hyped. This is fairly common knowledge amongst CBers, but to someone new could catch them out. Maybe they test them with absolute peak power and use that spec (irrelevant of how your modulation would sound) I would say you are getting what is to be expected from your amplifier. Factor in that cb power meters are not the most accurate devices in the world and it seems your amp is working well.

I would say yes it is a little annoying for you but don't stress too much, 130W/140W is a very punchy amount of RF and you have a good set up/ low SWR so probably best to enjoy your amplifier, don't overdrive it, aim for 100W out to preserve its life (there will be barely any difference between 100/130W at the RX end). Wherever you bought it, it would put the same RF power out.

PS you will get a little less power out from a car battery at 12V compared to a high amperage 13.8V PSU. You might get an extra 20W.

[QRZ-934]

Ok, well Ive received an email back and with some suggestions that could be the cause of my lower than expected output.

First up is the battery, ok its up there in terms of voltage and current draw but its suspected to be creating a lot of sag in output as the volts drop away under load.

To rectify that Ive run the radio off the battery and attached the amp to the PSU ( a watson w-30am).

Setting the power to what would be expected from a vehicles alternator 14.45 volts and attaching the amp yields the following.

FM test.
1) 35 up from 23.
2) 90 up from 70.
3) 105 up from 75.
4) 116 up from 80.
5) 180 up from 80 and a huge improvement.
6) 200 up from 130 again a big improvement.

Volts under load stayed at 14.43.

Although its still not hitting the advertised output its a heck of a lot better than previously, I suspect itll get a tad warmer too so a fan is next in the offing to keep it cooler.
To be honest I only really need around 100 watts to go most anywhere, although the extra 100 would be useful for when conditions arent so good I suppose.
If one things clear its to use the biggest darn supply you can find to run the amplifier on.

I want to thank the supplier (Moonraker) for getting back to me so quickly and for their useful suggestions, Thanks guys, I shall be buying from you again.
All I need now is a psu to run the radio on.

[NightProwler]

I love a happy ending

[InTheClouds]

Congrats on solving it.

[SgtBilko302]

Knights advertise it as 250w am/fm and 450w ssb. Have you tried putting 6w in on FM and 12w in on ssb,

Paul.

[NightProwler]

Knights advertise it as 250w am/fm and 450w ssb. Have you tried putting 6w in on FM and 12w in on ssb,

Paul.

That's what I thought. It will make a difference.

[ch25]

T It will make a difference.

Biggest in signal quality. RM amps driven hard (still under claimed limit) are outputting garbage.
Chris.

[gmham]

Ok, well Ive received an email back and with some suggestions that could be the cause of my lower than expected output.

First up is the battery, ok its up there in terms of voltage and current draw but its suspected to be creating a lot of sag in output as the volts drop away under load.

To rectify that Ive run the radio off the battery and attached the amp to the PSU ( a watson w-30am).

Setting the power to what would be expected from a vehicles alternator 14.45 volts and attaching the amp yields the following.

FM test.
1) 35 up from 23.
2) 90 up from 70.
3) 105 up from 75.
4) 116 up from 80.
5) 180 up from 80 and a huge improvement.
6) 200 up from 130 again a big improvement.

Volts under load stayed at 14.43.

Although its still not hitting the advertised output its a heck of a lot better than previously, I suspect itll get a tad warmer too so a fan is next in the offing to keep it cooler.
To be honest I only really need around 100 watts to go most anywhere, although the extra 100 would be useful for when conditions arent so good I suppose.
If one things clear its to use the biggest darn supply you can find to run the amplifier on.

I want to thank the supplier (Moonraker) for getting back to me so quickly and for their useful suggestions, Thanks guys, I shall be buying from you again.
All I need now is a psu to run the radio on.

How you getting on with the linear ? I notice this one states a max of 12 watts input on ssb, unless you have a newer ssb set with variable power then guess in a lot of cases your going to be on the limit or overdriving this linear ? Wonder why they choose a max input of 12 watts ? Older ones were a max of 20 watts input were they not ?

What input on ssb were you running into the amp for these particular ssb output power readings ?
Cheers.

[wa10]

gmham,
No there were not 20w ssb, that's just rmitaly BS lies

RM and Zetagi just double the FM numbers up because it sounds better than the truth, even their FM ratings are way higher than ANY reputable manufacturer would claim,

look at this article, THE TRUTH about rm amps from a guy that does know what he's talking about, He designed some popular ham radio solid state & tube amplifiers https://www.w8ji.com/rm_hla-150_test.htm

[gmham]

Okay thanks so much will have a read of that later. Have you personally had a chance to look at the new black front / heat sink KL503 amp yet ? According to the adverts here replaces the Kl500 model. Any links to any mods worth doing on these amps to improve them in any way ? Again appreciate your advice as I know from here on TM1 in the past your certainly know your stuff on linear amps ! ..use the term “ Linear “ very loosely !!

[wa10]

No not seen inside one,

the original kl503 is a 1 x 8 mosfet amp, like a kl60 driving 2x kl203's, they do about 250w,
driving the nuts off them will get a little more and smoke soon

KL500 is a 4 X bipolar transistor amp like 2x kl400's or 2x HLA150's without the filters
they do about 300w flat out any mode without mods,

or almost 200w rating if it was made by a reputable company that had to meet acceptable IMD specs & not have resistors burning up & falling from the board & popping transistors if your vswr is not very low.

[gmham]

Great stuff thanks again appreciate the info.

[QRZ-934]

Ok, well Ive received an email back and with some suggestions that could be the cause of my lower than expected output.

First up is the battery, ok its up there in terms of voltage and current draw but its suspected to be creating a lot of sag in output as the volts drop away under load.

To rectify that Ive run the radio off the battery and attached the amp to the PSU ( a watson w-30am).

Setting the power to what would be expected from a vehicles alternator 14.45 volts and attaching the amp yields the following.

FM test.
1) 35 up from 23.
2) 90 up from 70.
3) 105 up from 75.
4) 116 up from 80.
5) 180 up from 80 and a huge improvement.
6) 200 up from 130 again a big improvement.

Volts under load stayed at 14.43.

Although its still not hitting the advertised output its a heck of a lot better than previously, I suspect itll get a tad warmer too so a fan is next in the offing to keep it cooler.
To be honest I only really need around 100 watts to go most anywhere, although the extra 100 would be useful for when conditions arent so good I suppose.
If one things clear its to use the biggest darn supply you can find to run the amplifier on.

I want to thank the supplier (Moonraker) for getting back to me so quickly and for their useful suggestions, Thanks guys, I shall be buying from you again.
All I need now is a psu to run the radio on.

How you getting on with the linear ? I notice this one states a max of 12 watts input on ssb, unless you have a newer ssb set with variable power then guess in a lot of cases your going to be on the limit or overdriving this linear ? Wonder why they choose a max input of 12 watts ? Older ones were a max of 20 watts input were they not ?

What input on ssb were you running into the amp for these particular ssb output power readings ?
Cheers.

Hiya, well Ive been running this amp for a while now and its working well for me.

Input power I tend to keep am/fm to 4 to 5 watts and around 10-11 on ssb, which gives me 200 watts indicated on the zetagi power meter.
I dont tend to run more than 100 anyways as it seems its really not necessary even for distances of 1000 miles, Ive tested several long distance stations by going higher and it seems to make little difference to either RX strength or clarity to them.
It also means the amp doesnt heat up much even on FM, it gets warm but certainly not high enough to do an egg on it!
Having the extra capacity is a bonus if things ever got really bad the extra power might help but otherwise Im happy and im sure the amp is too on just 100watts out.

[gmham]

Yep they always say the first 100 watts is what matters and then you would have to raise to 400 watts to notice any real world difference.