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Author Topic: Why are there recommendations both for/against use of ALC with amps?  (Read 18132 times)
KM4AH
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Posts: 963




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« Reply #15 on: June 18, 2015, 09:20:41 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...

Maybe you did just fall off a turnip truck last night. This is a well known problem with several rigs along with mods to try and correct it. And yes, there are a lot of blown 8877's because of it.
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G3RZP
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« Reply #16 on: June 19, 2015, 04:26:43 AM »

ALC spikes are a fact of life. The gain of the RF chain isn't wound back until the output (or whatever the threshold is) is reached. So it is shutting the stable door after the horse has bolted. How far the horse has gone is dependent on the design - if the gain control stage is between balanced modulator and filter in a 'classic' rig, there's the group delay of the filter in the mix, and if it's multiple conversion with several filters, the group delays add up and the spike can have appreciable width just because of the delay in the signal getting from the controlled stage to ALC detection. It can often be seen on the first dot of a CW transmission.

When I got my ACOM 1500 amplifier, it was very noticeable that even at 400 or so watts output, (about 15 or 20 watts from the transceiver) the power spike was enough to frequently trip the amplifier protection circuitry. So I've removed the ALC......Even 30 years ago, Yaesu were relying on ALC to set the tx gain, and so the spikes were there. With tube amplifiers such as the FL2100 , it wasn't so much of a worry, but SS amps can die that much faster when overdriven.

When using AM or partially suppressed carrier, ALC is a no-no, because of carrier 'pumping' during pauses in modulation. The old Marconi automatic tuning transmitters had a motor driven variable attenuator in the drive line, and during the tune up phase of the automatic tuning cycle, it would adjust for the correct output level and then sit there until the next tune up sequence. It would be easy to do that with a digital switched attenuator now, of course, although that wasn't really an option in the late 1950's.

ALC was originally used to help prevent overdrive on speech, and would detect the onset of grid current in the PA. A dual time constant was sometimes used as well, but as soon as compatible AM or CW was required, the complexities of power spikes and pumping came in. When ALC is used as the sole gain control, problems are to be expected.
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W8JX
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« Reply #17 on: June 19, 2015, 05:17:42 AM »

Maybe you did just fall off a turnip truck last night. This is a well known problem with several rigs along with mods to try and correct it. And yes, there are a lot of blown 8877's because of it.

The blown 8877's are not from spikes but rather operator error as it is easy to damage control grid if not tuned properly. Nobody ever talked about ALC overshoot until 590 had a issue that has long been fixed and now all some talk about is ALC overshoot until some new problem takes center stage. There seems to be a new breed of hams that do not know how to properly use equipment and they depend on it being plug and play and bitch when it is not.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W1BR
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« Reply #18 on: June 19, 2015, 10:40:27 AM »

I'm not sure about momentary transient current spikes on a 8877 grid, but I would be very leery of any overdrive on a 3CX800 tube.  the effects of overdrive, even momentary, would appear to be cumulative. That's one reason I like glass power triodes/tetrodes, like 4-400, 3-500, etc. They are much more forgiving... and tubes like 4-400 are still available and cheap.

Pete
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G3RZP
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« Reply #19 on: June 19, 2015, 01:36:03 PM »

For a home brew HF amp in the US, from a viewpoint of robustness and cheapness, an amp with three 4-400s in GG with about 3500 to 4000 on the plates is very attractive. The difficult part is  about 44 amps at 5.0 volts after the filament choke......Use 4, and series the filaments to need  28 amps at 10 volts, with some unbalance current in the neutral lead of the 5 - 0 - 5 volt filament transformer. If push comes to shove, you can use 4-250s and on SSB, you can over run them.

But it isn't going to end up small!

OK, I'm agreeing with Pete, K1ZJH.
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W8JX
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« Reply #20 on: June 19, 2015, 02:35:22 PM »

Use 4, and series the filaments to need  28 amps at 10 volts,

Or simply use 2 for 14 amps at 10 volts and give up only a few db.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
G3RZP
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Posts: 1319




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« Reply #21 on: June 20, 2015, 01:36:05 AM »

Quote
Or simply use 2 for 14 amps at 10 volts and give up only a few db.

Yes, you would give up less than 3dB and you can push a pair of 4-400 in AB2 grounded cathode to over 1500 watts at audio, so allowing for tank circuit losses, it should be possible to get 1500 watts at RF. Even in GG, you will get around a kW out of a pair with 4kV on the plates.
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N6PJB
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« Reply #22 on: June 20, 2015, 11:53:02 AM »

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.

I agree with this.  Grin
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W4KVW
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« Reply #23 on: June 20, 2015, 12:06:42 PM »

I for one do use the ALC adjustment on my ICOM PW-1 amplifier.Without the ALC the amplifier comes close to legal limit output even at 35 watts of drive.It is a simple adjustment & without doing so with these amplifiers the life span is short from what I have read & heard from those who failed too do so because they thought they were smarter than the folks who designed & built the amplifier.The amp works as it should at full output on all bands from 160 meters thru 6 meters with just 35 watts of drive.It has done so with two different transceivers (ICOM 756 PRO II & ICOM 7600) so that is how I will continue running it here.Whatever makes you happy with your gear but I'm leaving mine as it is.  Wink   Grin   Smiley

Clayton
W4KVW
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W1BR
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« Reply #24 on: June 20, 2015, 01:37:09 PM »

Yes,  that is where the problem comes in--when an exciter has considerably more output than is needed by the amplifier.

I don't run ALC on my SB-220, and according to my monitor scope my 100 watt exciters are not capable of driving in into compression.  That, with careful loading seems to keep things running smoothly.

Pete
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G3RZP
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« Reply #25 on: June 20, 2015, 02:51:06 PM »

I don't know about other rigs, but I do not like the Yaesu reliance on ALC.
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AD5X
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« Reply #26 on: June 20, 2015, 04:13:34 PM »

I don't know about other rigs, but I do not like the Yaesu reliance on ALC.

My ICOM IC-706MKII put out a 140-watt 2-ms spike regardless of the transceiver's power setting.  I.e., even at the normal 100-watt power level, there would be the 140-watt spike.  And if I set the transceiver to minimum power (about 5-watts), there would still be a 140 watt spike.  After the spike, the transceiver's ALC would control the output to the power set-point during the transmission period.  Once you had about 5-seconds of transmission inactivity, you'd get another 140-watt spike.  My IC-706MKIIG was a little better.  It would spike to 110 watts regardless of the power setting.

All easy to see on a good peak-reading power meter (I have an Array Solutions Powermaster) and/or a good oscilloscope (like my Tektronix TDS-2022B).

Phil - AD5X
« Last Edit: June 24, 2015, 05:41:05 AM by AD5X » Logged
KM1H
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Posts: 5533




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« Reply #27 on: June 21, 2015, 12:48:15 PM »

Quote
I've been saying it for decades now, that not using ALC is a receipt for damage to your gear.

Nonsense, there is no such thing as a blanket statement here.

Quote
n 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.

A lot more nonsense.
Most amps ALC are not compatible with SS and the rigs built in loops are self contained.

OTOH rig overshoot is a huge problem that is the real cause of tube failures and the only cure is to get a new rig that works properly. If Joe Sixpak is driving his 8877 with 150W and excessive grid current then the a'hole deserves to lose the tube. OTOH if he has it set at 50W and low Ig and his piss poor designed rig nails the amp at 200W then he should sue the ass off the rig manufacturer.

Quote
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.

And I havent used ALC since I sold the Drake C Line around 83 for a TS-930. Monitors were made to allow users to set up their equipment. I started with an old O'scope for AM, graduated to a CE-100V with a built in monitor and a CE MM-2 on the output of the NCL-2000 in 1965; I still have and use them. A SM-220 came next and I still have it plus I moved the old HP 141T spectrum analyzer to the operating bench when I upgraded the main service bench.

Quote
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.

You forgot to mention that military and commercial gear has no mike gain or power controls and the gear has to be compatible with a wide range of voice levels of all 3 sexes. IOW your comment is totally incompatible with the subject under discussion.

Quote
If you decide to not use it after reading all that I have said here, then you get whatever comes with that decision.

Not using ALC will likely help more than the other way around and with monitoring being built into more gear the whole issue will go away soon.

Carl
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KM4AH
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Posts: 963




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« Reply #28 on: June 21, 2015, 01:07:58 PM »

Maybe you did just fall off a turnip truck last night. This is a well known problem with several rigs along with mods to try and correct it. And yes, there are a lot of blown 8877's because of it.

The blown 8877's are not from spikes but rather operator error as it is easy to damage control grid if not tuned properly. Nobody ever talked about ALC overshoot until 590 had a issue that has long been fixed and now all some talk about is ALC overshoot until some new problem takes center stage. There seems to be a new breed of hams that do not know how to properly use equipment and they depend on it being plug and play and bitch when it is not.


That is interesting since Collins had a service bulletin dealing with ALC overshoot dated 9-2-60 .  Many rigs have had spikes including the IC-781 which I think may predate the TS590 a couple of days. My TS-940S would hit a 150 watt peak occasionally with the power set at 100 watts that was at a long enough duration that you could see it on  a PEP wattmeter.

And no, I don't use ALC amp control and never have so I am not defending anything. I use amplifiers with tubes that are more immune.
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KM1H
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Posts: 5533




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« Reply #29 on: June 21, 2015, 01:47:23 PM »

ICRAP lead the industry in ALC overshoot going back to the early 80's and their first SS xcvr. I'll never own an ICRAP after evaluating many models for the YCCC back then.

Currently use a TS-940 with lots of mods and a stock TS-950SD. Havent burped an amp yet including those with very touchy tubes such as NCL-2000, Alpha 76PA, Dentron DTR-2000L, MLA-2500, which are in regular use plus a SB-230 for portable use with a little TS-130S. Im sort of a confirmed Kenwood guy but they havent built anything since that interests me. Owned a FT-1000D for awhile but kept the 940 since it was gone in 90 days. Also tried some high end Ten Tec trash back then.

Guess that is why I went from GM Buick/Caddy to Ford/Mercury and the latest is a 2015 Fusion Titainium model.
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