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Author Topic: Why are there recommendations both for/against use of ALC with amps?  (Read 18027 times)
KB1GMX
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Posts: 1794




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« Reply #30 on: June 21, 2015, 03:20:52 PM »

On the Tempo-One at full drive it would overshoot and flat top. especially on
40M for some reason.  This is a glow in the dark radio with traditional ALC to
its finals.

The solution was grid and plate loading resistors to tame the gain so it was
not hitting the final so hard to start with.  Then the ALC was dial back so it
behaved smoother.  After that not an issue.

Most of the generation of solid state DC to daylight radios have far to much
RF gain in the transmitter lineup.  On a TS-440SAT also a alc spiker a
carefully placed 7db pad really calmed it down and with no other changes
got me better audio reports at full power.  Before it was mostly ok when run
reduced power.  Now its possible to flat top but only with excessive mic gain.
Likely that will save the finals as hammering them is not a good thing.

Far to many radio have power spikes most to their point of clipping but if you
winding back to 35 or 70W that's still a problem.   A far better way would
be a power attenuator to both cut the power while letting the radio run near
max and also assure it a excellent SWR into the amp.

I've done the above with many of my 100-300W Solidstate amps especially
the MOSFET amps as they do not tolerate excess power at all.  By excess
I mean a 350W amp that needs 8W for full power and 20W will fry the the
devices.  The goal being even if the radio spikes its not exceeding the
safe {no damage} input power.  Rolling the power back is both risky from
a possible spiking case but also operator error [oops forget!]/.

Allison

Allison
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KM3F
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Posts: 908




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« Reply #31 on: June 21, 2015, 11:13:00 PM »

The OP did not reference the equipment he uses.
Some radio models indeed spike, some a little some not at all.
I run both Kenwood TS2000 and 480 models with peak reading watt meters to observe and tune with and repeatedly have tested for spiking at full power and dialed back to 10 watts and rarely see a spike I would ever be concerned about.
I do not see it on the AL80B peak reading wattmeter either.
For some combinations of equipment  the spike could be a mike PTT key up response from the mike.
It could be a detection of audio from the radio speaker that may not cut off fast enough.
I use a home made PTT hand mouse to get away from some of these possibilities plus the delay time  selection in the menus.
I have driven 2m and 70 cm Transverters that only require less than a half watt and never lost them to spiking and drive into SS vhf amps.
Everyone is entitled to their opinion but not insist everyone else is wrong.
I think I have proven over time that ALC use, spiking in not an issue and other attributes work fine for me as I use them.
.
The use of audio limiters and compression in the audio  chain keeps the audio level under control.
Those who claim distortion results and those who miss use that function in total are misguided.
You hear it every time you turn on a commercial AM, FM or TV station and don't complain of distortion.
You do not use mike gain to control amplifier power as a normal way for transmitting.
If power level is decided on and the amplifier is able to do double that, then get a smaller amplifier and run it at is was designed for.
.
In the output chain, ALC can be used to attain max usable clean power with it's absolute limiting ability.
You have a spiking issue don't blame it on the ALC function.
Fix you spiking or get a different radio.
.
I run the AM mode as well and put up a signal comparable to the best out there  most of the time with the same equipment.
On AM mode I still use a little ALC but run the AM power full on and adjust amplifier drive and peak tune the amp with the carrier control at 400 watts.
The advantage is in a KW the audio  chain has it's own ALC  and will attack the modulation peaks if you don't take away that ability by using the carrier control as the rf drive control.
This is all verified by observing the AM peaks on a peak reading wattmeter.
If you don't do it this way, the voice peak pull the power back. Also seen on a watt meter in it's average reading function.
I set amplifier drive to a 100 watts carrier and see 100 x 4 = 400 x about 90& modulation or 360 watts AM peaks as I would expect to see..

Understand what you have and understand how to use it.
Good luck.

 
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W1BR
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Posts: 4179




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« Reply #32 on: June 22, 2015, 07:46:55 AM »

AM usually relies on peak limiters to avoid over modulation. ALC would seem to be a very poor choice for AM?  I also doubt that all peak detecting watt meters are the same in their ability to accurately measure power spikes with fast rise times of short duration. A scope is still the best method, for all modes.

Mike noise and other audio anomalies should be handled by a decent AGC system.  The is that the rigs with these problems will exhibit overshoot on CW as well as SSB.  I keep a monitor scope inline; and occasionally verify that nothing in my transmitter chain has changed for the worse.  Just my opinion.

regards

Pete
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SA4MDN
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Posts: 99




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« Reply #33 on: June 22, 2015, 12:45:39 PM »

well I disconnected my alc line a few months ago after reading negatives about using it, it is now connected again as in my haste to get a contact last night I forgot to turn the power down on the rig and pumped 200w into a 811h, got halfway through saying my call sign when I noticed the meters on the amp and the led bars on the ldg 1000 pro 2 tuner going nuts, lucky for me everything has survived my blunder, but for me that's a plus for using alc, makes me feel a little safer now from my own stupidity sometimes.
« Last Edit: June 22, 2015, 01:20:33 PM by SA4MDN » Logged
KM3F
Member

Posts: 908




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« Reply #34 on: June 22, 2015, 03:10:18 PM »

If it means anything the designer of the AL80B says a scope is not useful for reliably  seeing fast spikes and a good peak reading meter will show it as a stored spike for long enough to display it on the meter after it passes so you can tell at least there was one.
I find that to be true for the most part.
A very fast rise time on a Scope screen may not be seen due to brightness limitation on many scopes.
It's not that the Scope can't detect it it can't always display it on screen..
I have the AL80B peak meter in the amplifier, the ATR20 tuner  Peak reading meter that is the same circuit and an older WM-1 computing meter. They all see the same thing when it happens.
Good luck.
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W1QJ
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Posts: 2945




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« Reply #35 on: June 22, 2015, 03:23:05 PM »

an analog peak reading meter is useless.  A digital peak meter such as the LP-100a or the Powermaster holds the highest peak reading for a few seconds so you can digitally see a read out of the high spike power amplitude.
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KM1H
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Posts: 5087




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« Reply #36 on: June 22, 2015, 06:08:19 PM »

Quote
If it means anything the designer of the AL80B says a scope is not useful for reliably  seeing fast spikes and a good peak reading meter will show it as a stored spike for long enough to display it on the meter after it passes so you can tell at least there was one.
I find that to be true for the most part.

To be honest it doesnt mean much at all considering.... The reality is that the right scope or the right digital display is required to get meaningful results.

Carl
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KB1GMX
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Posts: 1794




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« Reply #37 on: June 22, 2015, 07:16:14 PM »

I've always used a scope, done right its the best way.  It can be done wrong.

The procedure I use is first set the radio up on a load, and a Bird, tune up and drive it
with a 1khz tone to max power (SSB) then drop it to the desired power less than full. 
Use your ALC or whatever drive control and then use a keyer or 555timer to send
dashes repeatedly and sync the scope to the keyer. If the radio has spikes they will
be there.  A good radio keys like a CW rig, with reasonable risetime (also fall time)
and no overshoot.  Unreasonable rigs will have blips at the leading edge.  Those blips
are overshoot or, as called here, excess power spikes.    Its the ALC responding
too slowly, excessively, and often the overshoot has undershoot afterwards as well.

NOTE if the scope has memory (old storage tube scope, or modern digital) its much
easier to do.  But a repeated pattern of keying the transmitter with enough gap its
easily viewed on any old scope.  I've found a keying rate of 2-5 a second is enough
with a on time of maybe 100 to 300ms. 


Allison

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AD5X
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Posts: 1622




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« Reply #38 on: June 23, 2015, 05:13:35 AM »

You need a 'scope that can catch and hold the leading edge of the first dit (or dah) of a transmission only.  The leading edge of that first dit is where you can see the overshoot.  A repetitive keying pattern won't show this, as the radio's ALC takes over within 1-2 ms and holds the power level properly for about 5-seconds.  So except for the leading edge of the very first dit, you won't see any overshoot with a repeating pattern.

You can use single-sweep and trigger on the RF signal.  Or trigger on the amp-key output of the radio and offset the trigger or display to properly view the leading edge of the first dit of the signal (the amp-key output usually occurs 5-10ms or so prior to the signal, and the overshoot is maybe only 1-2us in duration).

Phil - AD5X
« Last Edit: June 24, 2015, 05:37:13 AM by AD5X » Logged
ZENKI
Member

Posts: 1621




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« Reply #39 on: June 23, 2015, 05:53:47 AM »

Shortly you will able to buy an excellent monitor scope for monitoring over driving and capturing keying transient problems.

http://www.telepostinc.com/LP-500.html

The LP500 will be Ideal for monitoring these problems.

ALC overshoot on Tetrode and Solid state amps cause tremendous  amount of IMD and keyclicks.  This is especially so if the  amplifier is a low  drive amplifier.
The Latest Radcom has a review of the  SPE1.3KFA, besides having  terrible IMD performance the amp was very sensitive too ALC  and over drive problems which would be a
real disaster for the ham bands.

The other problem on  many amps is the horrible EBS designs that dont work well. They contribute more to causing splatter and keying transients than even ALC incompatibility.
EBS on any amplifier is best disabled just like ALC. You can always use an  SWR  or over power Alarm too protect your amp. The Array Solutions power master and MeterBuilder meters offer these features.
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KM1H
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Posts: 5087




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« Reply #40 on: June 23, 2015, 05:46:35 PM »

Quote
AM usually relies on peak limiters to avoid over modulation. ALC would seem to be a very poor choice for AM?  I also doubt that all peak detecting watt meters are the same in their ability to accurately measure power spikes with fast rise times of short duration. A scope is still the best method, for all modes.

You can see the results of an ALC dominated AM signal on any rig with a panadaptor/bandscope; it aint pretty and it sounds bad also.
Most amps require about 20W tops for a 350-400W carrier. Properly setup you can watch the upward PEP swing on the meter since it is 4X the carrier power.

SDR rigs such as the Flex 5000 sound absolutely great on AM and you can fully tailor the audio.
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KM3F
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Posts: 908




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« Reply #41 on: June 23, 2015, 08:08:09 PM »

I feel that ALC does not do anything to create modulation flat topping as I perceive some here are thinking.
It does reduce transmitter driver power therefore amplifier output.
This is not flat topping or amplifier induced limiting because it all happen from the amplifier input stage not the output.
I set amplifier for 400 watts max power tuning for a more linear amplifier operation knowing my setup will not top about 360 watts PEP so should never reach ALC feed back levels and still stay within a more linear part of the amplifier operation.
Peak limiting back in the audio chain also helps with any tendency to overdrive any part of the chain plus it's other benefits.
It works for me without any obvious side effects.
Could it be better? sure but at what cost?
This is Ham radio not some commercial venture, so we use what we have.
You can peel  this onion as much as you like. Reality is we have what we have to work with.
100 different radios, 50 different amplifiers and endless combinations with different results.
 
Good luck.
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N3DT
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Posts: 1653




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« Reply #42 on: June 24, 2015, 11:40:30 AM »

I've got a TS2000 and AL80B with an LP100A wattmeter. I haven't looked at my signal on the scope yet (lazy) but I get lots of unsolicited compliments on my audio. I tune the AL80B to about 750-850W (depending on the band) on FM and drive it with about 70-75W from the TS2000. I do run up the mike compression a bit and I haven't tried playing with the ALC yet although it's hooked up but I've got the ALC set turned all the way to 10. I haven't seen any over shoot from the TS2000 even when running 10W into the LP100A. It seems to be a good combination, even with the HH mike. I can't complain and I only gave $900 for the AL80B.

I do notice the AL80B watt meter doesn't always agree with the LP100A but it's probably within 10-15%.
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W1BR
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« Reply #43 on: June 24, 2015, 12:30:43 PM »

Audio quality doesn't mean a thing. Nothing. You can be running the amp  into severe gain compression on peaks, and still have great audio reports, while you're buck shooting crap across most of the band. Lord forbid one uses ALC to ensure constant flat topping.  Instead of IMD bursts every few seconds on peaks, we are treated to a constant barrage of ALC induced products. ALC is not speech processing, nor an effective limiter.  It is designed to capture  random syllabic peaks.

Pete
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KM4AH
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Posts: 945




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« Reply #44 on: June 24, 2015, 01:10:03 PM »

Audio quality is subjective. I like somebody to sound as close as possible to the way they would in person, not the way they have God sounding in the Ten Commandments.
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