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Author Topic: Retrofit ALC to LK-450 amp - possible?  (Read 2750 times)
K5WLR
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Posts: 114




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« on: April 04, 2013, 11:44:20 AM »

Has anyone tried to retrofit an ALC circuit to an Amp Supply LK-450 amplifier? Trying to keep my signal clean and don't want to go over 750 watts. My exciter is a Heath SB-401 and it cranks out 130 watts. This tends to overdrive the amp, creating splatter.  Shocked

Thanks for any possible help on this.

73!   Cool

Will Rogers
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W8JX
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« Reply #1 on: April 04, 2013, 12:02:41 PM »

You could use a 9 volt battery and a 30 to 50k pot to apply negative ALC bias to rig to limits its output to desired level.
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KM3F
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« Reply #2 on: April 04, 2013, 03:48:47 PM »

An ALC circuit could be put in the amplifier.
Requires a neg supply and diode conduction clamp, a time constant capacitor and level control pot.
Feed the signal from the amp input circuit so the drive  limit setting is fed back to the 104 when it detects it.
With the drive level you have , it would provide some RF compression and tend t make you a bit louder from higher average RF drive compression when the ALC signal is fed back.
Good luck.
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K0IZ
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« Reply #3 on: April 04, 2013, 06:52:00 PM »

I seem to recall reading that ALC in an amp is not recommended.  I run an LK800 and use my K3 (or KWM2) with ALC.  But no ALC on the amp (doesn't have, anyway). 
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W8JX
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« Reply #4 on: April 04, 2013, 06:54:22 PM »

I never hook up ALC from amp to rig.
« Last Edit: April 04, 2013, 06:57:19 PM by W8JX » Logged

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K5WLR
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Posts: 114




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« Reply #5 on: April 04, 2013, 07:30:22 PM »

OK! Lots of good information. The goal is to reduce the output from the rig to the amp; I got it down to 80 watts by setting the load at the lowest setting and redipping the plate. Still a bit much, so it looks like using the 9 volt battery with the potentiometer should get it a bit lower. This amp doesn't take much to get it going. I'll report back once I have tried this.

Thanks for all the help guys and 73!  Cool

Will Rogers
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W8JX
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« Reply #6 on: April 04, 2013, 08:37:38 PM »

Connect pot across battery and wiper to ALC input. Remember to apply negative bias.
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KM3F
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« Reply #7 on: April 05, 2013, 10:58:56 AM »

Just a reminder that ALC feedback from the amplifier input circuit does the same control action as a fixed ALC supply.
Only difference is the fixed is fixed and not change with the radio drive levels at the varying audio rate.
How this all works out in the end depends on how the ALC control circuit in the radio, works.
You could fix set the ALC with an external supply and still have to reduce the mike gain or compression 'might' take place in the radio with it's effects,
or if the ALC feeds back to the mike circuit, it will all be under control in a full loop.
The other item is mistuning the radio tube final could have the same effects on IMD, and harmonics that misstuning the amplifier at low power has, then passing the crap on to the amplifier.
Don't know in this case how any of it will work out, but be awhere it may be something to deal with after operating and getting some signal reports.
I feel 'dynamic' ALC is the best overall setup so the radio final can be tuned as it should be and full control of the drive level smoothly is attained. Your only after ALC action beyond the max drive level the Amplifier needs for full output so it does not over-drive into it's non-linear area more than need be.
What you have is a setup that needs a lot of understanding to learn what it takes for tuning up and changing frequency on the band and going to another band and doing it all over again each time.
Sorta like the old days. Hi hi.
Good luck.
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K5WLR
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« Reply #8 on: April 06, 2013, 02:09:23 PM »

Well, the battery with potentiometer works fine. I can adjust the voice output from 0 to full output.  Cheesy  It looks good on the scope; no flattening of the trapezoidal pattern at just under a KW.

Thanks for the help and 73!  Cool

Will Rogers
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W8JX
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« Reply #9 on: April 06, 2013, 06:35:46 PM »

Well, the battery with potentiometer works fine. I can adjust the voice output from 0 to full output.  Cheesy  It looks good on the scope; no flattening of the trapezoidal pattern at just under a KW.

Thanks for the help and 73!  Cool

Will Rogers

You might add a off switch to extend battery life. Should last a few months or more that way. 
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K8AXW
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« Reply #10 on: April 06, 2013, 09:00:36 PM »

I do basically the same with a 2db attenuator between the exciter and my amps. 

Not only does it cut the drive back to where I want it but it also eliminates overshoot and adds a little matching to the input to the amp.

A relay inside the attenuator case bypasses the attenuator if I wish.  The 2db doesn't drop the receive signal hardly at all.
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