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Author Topic: Amplifier "headroom" question  (Read 1813 times)

Posts: 416

« on: August 05, 2006, 06:01:37 AM »

Hello, Just how important is "Headroom" in a properly designed amplifier? I am looking for something that will run a clean 600-800 watts and have narrowed my choices to the Ameritron ALS-600, AL-811H and the AL-80B. It seems that Ameritron ratings are pushing the maximum limits of their transistors/tubes according to the specs. Of course, amplification is just a function of money - Hi Hi, so I am wondering if the extra $400-500 for some headroom is worth it for just a ragchew & casual DX operator? I am not into contesting or other heavy duty operations. Thanks in advance for your time. 73, Todd

Posts: 94


« Reply #1 on: August 05, 2006, 06:48:17 AM »

I would think for casual ssb operation the amplifiers you mentioned would be fine. The duty cycle for ssb is only about 20 percent, so you already have built in headroom. If you're going to work RTTY or analog sstv then I would go looking for more headroom.

Posts: 13

« Reply #2 on: August 05, 2006, 08:17:32 AM »

I recently purchased a Tokyo Hy-power HL-1.5KFX 1KW solid state amplifier.  When one of my ham friends asked about the headroom, I told him I'll find out.  That evening I looked up the specs on  the SD2933 X4 MOS FETs by ST micro. The SD2933 is rated at 300w min with a 400w typical output. That equates to a 200% minimum to a 600% typical headroom rating for the amplifier...Those MosFets should last a while...


Posts: 1

« Reply #3 on: August 05, 2006, 08:17:45 AM »

In a tube amp, it more or less has some built in head room by virtue of the abilitly of tubes to be abused a little without permanent damage.  In a solid state amp, they have no head room in my opinion and I always operate any power transistor device at least 25% below its ratings.  No one will tell the performance difference between 25% anyway but the rocks (transistors) will.  And I didn't say "pills" because I am not a cber.

Posts: 501

« Reply #4 on: August 05, 2006, 10:13:03 AM »

I have owned the AL-80B and AL-811 (3tube) amplifiers.  Both fit my casual operating requirements just fine.  I ran the AL-80B at 800 watts, and the 811 at 400 watts (in my opinion about what they are good for).  I have a friend who runs an ALS-600 at 500 watts.  We both use open wire line fed doublets on 75 meters.  The difference in power output on the receiving end between 400 and 800 watts is negligible to me. They are reliable, clean sounding amplifiers at those power levels.  Although I have no personal operating experience with the ALS-600, I would recommend any of the three.  I really enjoyed the AL-80B and sold it to replace it with a QRO HF-2500DX which I run at 1250 watts (plenty of headroom).  In practice, with average propagation, I doubt anyone I talk to regularly would notice the difference between the AL-80B at 800 watts and the QRO at 1250 watts, or an AL-811H at 600 watts for that matter.   For the price, tube availability at this time, and reliability, the 811H is very hard to beat.  I hope this info helps.  73 and good luck, k0cwo

Posts: 1435

« Reply #5 on: August 05, 2006, 12:34:11 PM »

What do you mean by clean watts?
What do you mean by headroom?

Posts: 4914

« Reply #6 on: August 05, 2006, 12:56:49 PM »

There is a bit of confusion about head room and duty cycle. They are not the same. The reason today’s amplifiers are capable of 2500 watts out is in order to meet the more stringent IMD numbers the FCC requires (43dB down from a two tone modulation peak), you need almost 3 dB of headroom at your rated power output. If you want to run 1500 watts PEP on SSB the peak power out at 1500 watts needs an amplifier that is linear at least 2 dB above that level to keep the tube from hard compression. This is different than tube dissipation of CW-100%, Morse Code-50% and SSB-15 to 24% duty cycle. You can be dissipating only 25% of the tube rating and still be in hard compression on SSB.

If you amplifier is capable of 800 watts PEP you should derate the output on SSB to 600 PEP watts in order to have some headroom for an occasional aggressive voice peaks and there for keep from splattering. Also setting the ALC and output from you transceiver so is not to overdrive the amplifier is a must.

Controlling the modulation background noise into the microphone to the transceiver is also helpful for background noise is rich in distortion products and this will add to bandwidth and possible spattering if the transceiver ALC can not control the SSB modulation and power output. Your SSB audio should have at least 35 dB of dynamic range and with some equipment and room acoustics up to 46 dB of dynamic range is possible.

It is a joy to hear a station with wide dynamic range and a clear articulate modulation. This is not the same as wide band audio from DC to 5 KHz in bandwidth. Many of the Hi-Fi stations heard on the air although they have hugh frequency response, have poor room acoustics and are over driving their amplifiers. This not only sounds terrible but the signal  have lots of  IMD products and excessive wide signals. This is due partly to the audio bandwidth but also their amplifier is no longer linear and is being overdriven producing IMD (intermodulation distortion) products.

Why have an amplifier to help you in a DX application when you transmit you own QRM with your audio. You can hear what I am talking about by putting you receiver AGC on fast and listening to station where their peak signal and no voice modulation levels are at. A typical good signal would be S9+20 voice peak and a valley (no voice modulation) of S7. When you hear a signal that is over S9+20 with the valley of only S9+10 tune around the station and notice how wide and the splatter products can be heard up and down the band. In each case it may be a function of transceiver IMD and or the amplifier not having any headroom left in its operation.

I know this is getting as little off the subject going on about audio dynamic range but amplifier head room, audio bandwidth and audio dynamics go hand in hand to produce a clean SSB signal. If you are not interested in all three why worry about amplifier head room.

Posts: 9930

« Reply #7 on: August 05, 2006, 01:49:25 PM »

I have had several 811's and 2 811 H's and an als 500. currently I have the als 600 on my ts 2000.  

The 811 is about $675 for 600 watts and you have to manually tune it. the als 811 h is 800 watts for about a hundred bucks more and you have to manually tune it,  the als 500 is 500-+- watts and you need an 80 amp power supply to use it but band switch only no tuning.I forget the price ( about $800 I think new)

the ALS 600 come s with its own power supply ( 50 volts) and puts out ^00 +- watts  and you only have to band switch it. new about $1200 but used in the $600-$800 range.

I use mine as a second backup, ( orion/alpha87a first, then an Ic 746 pro /at-500/Ic-2kl amp) so I just need something convienent.

if you just rag chew, or dx and don't jump around a lot the 811's are a good buy for the $.  if you do contesting or any thing with a lot of moving up and down the band then the no tune options are better.

they all work well and I always set my amps for max smoke output ( tune for resonance) then back the input down a couple of watts ( 5 w?) so they are never run on the ragged edge but slightly lower.  they will last for ever this way.

if you are normally running 100 watts and jump to 400-500 watts it will bring you up out of the noise. so I would run either 100 watts, 400watts or full legal limit. the difference between 1500 watts and 500 watts on recieve is about a half an sunit +-. hard to hear on the other end.

just getting above the "masses" by a couple hundred watts will make those DX call go from 2 hours to 20 minutes of calling, usually, but if you get a 500 watt amp and them improve the antenna , a bit you can double your apparent transmitted output and als it lets you hear better whhich an amp doesn help with.

first good radio, next good antenna, then the medium amp,, then a great antenna ( steppir comes to mind) them the killer amp.

tuning is a pain so I like autotune or bandswitch only amps.

If you do a lot of CW you will want one with full break-in ( qsk) ability.

if running psk31, you don;t need an amp and if running rtty, drop the power on the amp down 50% as it has a hig duty cycle and heats stuff up fast.

Posts: 1435

« Reply #8 on: August 06, 2006, 03:24:52 PM »

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