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Author Topic: Why are there recommendations both for/against use of ALC with amps?  (Read 18039 times)
KD0ZGW
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« on: June 16, 2015, 12:38:30 PM »

Slightly confused rookie here.  Have done a lot of reading and seen advice several times that ALC is not needed and can cause problems.

If I understand it's function properly ALC provides a feedback signal from the amplifier is used to automatically limit the exciter output when the amplifier is at it's maximum output.  Seems like a good idea but there are an awful lot of amps and exciters out there and I assume they can't all be readily interfaced.

What am I missing?

Thx in advance for advice

'KD0ZGW
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W1BR
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« Reply #1 on: June 16, 2015, 01:14:08 PM »

http://www.w8ji.com/alc_exciter_power_overshoot.htm
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WA7PRC
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« Reply #2 on: June 16, 2015, 01:25:20 PM »

W8JI explains it well:
Quote
ALC overshoot, or power overshoot, is caused by the basic flawed design of ALC circuits and RF power control systems. Normal ALC is like closing the barn door after the horse has escaped.

The problem is rooted in group delay times as the signal makes its way through the radio to the output port.
IMO, the best way to prevent overdriving an amplifier and without using ALC is to do the limiting AHEAD of the rig's microphone input. I use a Symetrix 528E 'Voice Processor' that has a compressor/limiter. After the amplifier is adjusted for best power AND linearity, I set the output level of the 528E to the point just below where the amplifier produces flat topping.

vy 73 es gl,
Bryan WA7PRC
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WB2WIK
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« Reply #3 on: June 16, 2015, 01:47:32 PM »

The power overshoot article is interesting but doesn't really answer the question.

This discourse addresses the actual question a bit better, I think; it's a discussion of one specific product that has an ALC subsystem that actually works well (I have one, and do use it all the time -- ALC is always connected).
Quote
I read on the Ameritrons' AL80B technical info that on this Amp is
> installed a "electronic bias" that is a known circuit and a " Dynamic ALC"
> that allows "high level, low distortion RF processing", doubling the
> average SSB power.
Hi Vince,

That's advertising fluff. Because the ARRL Handbook says
RF compression causes up to 3 dB improvement in "talk power", that
got put in the ad.

How much improvement it really makes depends on the ALC response time
of your rig, and how you run the rig.

Here is what the circuit does. Instead of sampling RF voltage at the
tube cathode, it samples grid current. Grid current is referenced
against an unregulated sample of rectified and filtered power
transformer secondary voltage.

One advantage of this system is if the transformer or power line
sags, the peak grid current is reduced.

The other advantage is if you set the ALC control to allow (for
example) 150 mA of grid current, you can never exceed 150 mA of grid
current under any tuning condition. That reduces your worry or
concern about arcing the tank circuit, overdissipating the grid, or
overdriving the PA from nearly any cause.

Any time the grid current starts to approach 150 mA (the circuit has
built-in hysteresis), negative ALC voltage appears.
Quote
> I believe that could be some sort of amplified grid current feedback to
> transceiver, instead of the standard capacitor partition of input signal.
That is correct.

That was written by W8JI several years ago.

Not all transceivers even have an external ALC input, and not all amplifiers have an ALC output; some work quite well, some don't.

The AL-80B's circuit seems to work well with anything I've ever connected to it.
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KM1H
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« Reply #4 on: June 16, 2015, 01:50:30 PM »

Quote
W8JI explains it well:

But Ameritron still pushes it; sounds like double talking to me.

ALC was fine for vacuum tube rigs where only the final amp was controlled with a negative going voltage into the tubes bias circuit. Simple and usually effective. Its either the Chinese or Europeans who woke up and dropped it in some products.

I have a Daiwa processor now that does well and way back I had a modified Comdel into the CE-100V (A pair of very low distortion USA TungSol 6550's) and NCL-2000 that kept the 8122's under control. The built in scope on the 100V and a CE MM-2 after the amp helped; I still use all but the Comdel.
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W8JX
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« Reply #5 on: June 16, 2015, 03:10:13 PM »


IMO, the best way to prevent overdriving an amplifier and without using ALC is to do the limiting AHEAD of the rig's microphone input.


Not a good way at all and not sure why some think it is. When you suppress audio you also reduce peak to zero signal ratio. SSB output is usually rated that carrier is suppressed "x" amount of db below peak output at max output. As you reduce audio drive you reduce peak audio but carrier floor is same so you reduce effective talk power and compression. Best way is to NOT mess with audio and limit RF output power. It is not hard to do on a modern rig.
« Last Edit: June 16, 2015, 06:53:18 PM by W8JX » Logged

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Ham since 1969....  Old School 20wpm REAL Extra Class..
WA7PRC
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« Reply #6 on: June 18, 2015, 12:11:44 PM »


IMO, the best way to prevent overdriving an amplifier and without using ALC is to do the limiting AHEAD of the rig's microphone input.


Not a good way at all and not sure why some think it is. When you suppress audio you also reduce peak to zero signal ratio. SSB output is usually rated that carrier is suppressed "x" amount of db below peak output at max output. As you reduce audio drive you reduce peak audio but carrier floor is same so you reduce effective talk power and compression. Best way is to NOT mess with audio and limit RF output power. It is not hard to do on a modern rig.
We used limiters in broadcast engineering. There was no feedback from the PA to utilize. I presume that's still the case.
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W1BR
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« Reply #7 on: June 18, 2015, 12:28:20 PM »

You shouldn't use the mic gain to reduce power, but to limit peak power it is probably effective.  True, if one tries to reduce his rig from 100 watts to 10 watts using the mic gain, the carrier level to PEP output ratio is reduced, as well as spurious products from the mixing chain, etc.  But, using a limiter is a different animal.
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KD0PO
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« Reply #8 on: June 18, 2015, 12:29:10 PM »

FWIW alpha doesn't put an ALC jack on the 9500...
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KM3F
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« Reply #9 on: June 18, 2015, 12:58:57 PM »

There is a lot if dynamics to all this.
You cannot mix up audio peak limiting with Auto Limit Control even though they work together but in a separate ways and different in timing of the events.
I tune for max power PEP from the amplifier, then over Load for a small drop in power, then introduce ALC to see drops when peaks are reached.
This is seen when then ALC meter function rises off zero just a small amount on peaks.
You cannot possibly overdrive with this setup.
I uses an 8 band in the audio chain to boot and works very well.
No one has ever saw artifacts from my signal at 800 watts using an AL80B and Kenwood TS2000 that would even approach splatter or QRM off frequency..
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KD0REQ
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« Reply #10 on: June 18, 2015, 02:57:12 PM »

~PRC in broadcast had a fixed system.  the feed to the class-A line or the digital stream from the studio goes into a speech amp at the transmitter with fixed, carefully adjusted gain into the modulator, carefully adjusted to tickle the RF chain just so, which is held very closely on the same frequency into the same rugged antenna field.  broadcasters are commercially monitored (at least in my day) on an average monthly for frequency, modulation, quality of signal, and regularly for coverage.

the only thing that's a wild card is the programming leaving the studio.  Joe Fogey's Sinatra Hour is going to be barely tickling 0 dB.  Wild Harry's Rockin' Saturday, the VU meter is going to be bouncing off the peg because it sounds best on the studio speakers, and it makes the bass bodacious on every thump.  for that boy, you really need audio shaping and a good limiter.  stations make their sound by choice of mike and adjustment of the Orban.  I remember the kid's AM station when I was in college would splat out multiple times whenever they played "How Can I Mend A Broken Heart."  confirmed a semester later when one of their jocks was in my classes they had a bad power supply, and it took a while to locate parts.  they could have dialled down the processing, but no... they had That Sound to preserve.

hamming, nothing is stable.  antennas fluctuate, spinning up and down the dial, no visual signal big as life to indicate how you're modulating, working the mike all over.  you need oopsie control on the power and shaping control on the mic amp.  the first is ALC and the second is your compression.
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KK5DR
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« Reply #11 on: June 18, 2015, 05:01:26 PM »

I've been saying it for decades now, that not using ALC is a receipt for damage to your gear.
In today's fully solid state amateur stations that run up to 1kw, not using ALC is going to come back to bit you some day. ALC in this type of station does a number of function besides power limiting. It is tied to SWR, excess device collector/drain current, combiner/divide imbalance, imbalance device pair current, device gain imbalance, incorrect band selection, etc. any one of these parameters has the potential to fail, or be tripped. ALC is used to "throttle back" the exciter output to a safe level that can save the amp from further damage or misoperation.
In regards to a solid state exciter driving a tube type amp, I say use ALC here as well. The only exception being an amp that can handle the full instantaneous RF power output of the exciter. If you use an amp that has very high gain tube(s) in it, those tubes can and will be damage over the long term by excessive drive. In a conversation I had with Reid Brandon (Chief engineer with CPI/EIMAC), he told me that 90% of all tubes that fail in amateur service, do so because of chronic overdrive. He said they aren't complaining at EIMAC, it's more business for them.
The simple use of a properly adjusted ALC system would have saved at least 75% of that 90%.
A properly adjusted ALC feedback system will not cause and harm of any kind to the transmitted signal.
You ask, "How is ALC adjusted properly"?
You simply tune your amp as normal, to the desired full output. Leave the exciter drive level set to the point where that output was tuned to.
Raise the ALC adjustment by turning the screw on the back of your amp(front on some), turn it till you see the full power output of the amp just begins to drop by a few watts.
That's it, your done, the threshold is set.
Mind you, the exciter drive level most be left in that same spot at all times. It's only at this tune point that the system is calibrated. Change the exciter output above that tune point, the system goes into ALC loop instability which causes RF compression, and this isn't the linear RF compression used in early stages of the exciter PA. No, this kind of compression is very nonlinear, and distorted.
Keep the exciter drive in the calibrated state, and the transmitter is as clean as the exciter/PA were designed to make it. There will be no detectable artifacts on the stations transmitted signal if the system is well calibrated.
This does not apply to mic gains set too high, heavy audio compression, etc. we are discussing only the RF power output of the amp.
I personally never operate any of my Amps without a properly calibrated ALC system in operation. I've never had any problems with distorted RF power out, when my station is operating normally. I'm usually QRO when on phone.
Here is a fact many hams may not know: All commercial and military high power HF comm systems use ALC. it's design in such a way that should there not be a detectable ALC loop current, the HPA is disabled and an alarm is indicated on the exciter display. These systems are fully automated and preset in ROM in the exciter. When the HPA is connected by the proper cable connection, the system "hand shakes" with the processor in the HPA and the system is set. No operator need interfere with it.
Commercial and military gear are designed and built to far higher standards than any ham gear could ever be made. Firms like; R&S, RFHarris, Rockwell/Collins, Thales, Marconi, Racal, etc, all have this type of ALC system for their HF comm systems.
If you decide to not use it after reading all that I have said here, then you get whatever comes with that decision.
« Last Edit: June 18, 2015, 05:17:08 PM by KK5DR » Logged
W8JX
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« Reply #12 on: June 18, 2015, 05:43:27 PM »

Sorry but pretty much be. ALC is not needed if you know how to properly use your equipment. I have never used it and never will. It is a throw back from tube radio days when there no easy way to limit drive.
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KM4AH
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« Reply #13 on: June 18, 2015, 07:25:30 PM »

Depends on the tube or tubes and depends on the rig. If you are driving a single 8877 tuned for 50 watts PEP and the rig is hitting 150 watt spikes or more at the beginning of a syllable you may not like it much long term. Processing is fine as long as you are not the one listening to it.
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W8JX
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« Reply #14 on: June 18, 2015, 07:35:15 PM »

Depends on the tube or tubes and depends on the rig. If you are driving a single 8877 tuned for 50 watts PEP and the rig is hitting 150 watt spikes or more at the beginning of a syllable you may not like it much long term. Processing is fine as long as you are not the one listening to it.

I really like this 150 watt spike bs.  Next it will be 200. If this was remotely true there would be a lot of blown 8877's. I think some kinda make it up as you go to justify need for ALC and besides the ALC voltage developed is AFTER any spike dah...
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Ham since 1969....  Old School 20wpm REAL Extra Class..
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