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Author Topic: Amplifiers and exciter ALC  (Read 3724 times)

Posts: 246


« on: April 04, 2011, 10:47:10 AM »

Here is one for the amplifier kings.

I went over to a fairly new (licensed less than two years) hams house yesterday to help him get his brand new AL-80B working with his FT-2000. I have been using amps of all many for years and have never used any form of ALC feedback between the amp and the exciter. These have mostly been dual 3-500Z amps, and my current AL-1500. I have just tuned things up and used them. (Of course limiting the exciter drive on the AL-1500 to 45W)

This new ham is going through the AL-80B manual's tune up procedure step-by-step, and there must be 5 or 6 steps in the manual that discuss about how to precisely set the front panel ALC control. He is painstakingly going through the manual steps one-by-one, but there is no ALC connection between the AL-80B and the FT-2000 (per my recommendation).

OK, this is a simple single 3-500Z amp rated for a maximum input power of 100W, being driven by a radio that will put out a maximum of 100W, so I told him to ignore all the ALC stuff, and just watch the plate and grid current meters while he is tuning for maximum power. How can he overdrive it enough to require ALC, I thought....

Now I am wondering, did I steer him wrong?

Mike, W7VO

Posts: 2408

« Reply #1 on: April 04, 2011, 11:21:45 AM »

I don't know anything about an AL-80B amp, but a single 3-500 tube does not require 100 watts drive unless there is some kind of RF swamping circuit in the amp's input.  I have to take issue with Ameritron (MFJ) on that one.

I have been hamming since the 1960's and I have never hooked up the ALC on any amp I ever used (SB-200, L4B, SB-220, 30L1, Alpha 77SX).  But then I know how to properly drive and tune an amp.

In order to keep an amp from being over driven, (IMO) the proper way to do so is by reducing drive, not by adjusting the ALC.

I think you gave the new ham good advice.

Dick  AD4U

Posts: 21686

« Reply #2 on: April 04, 2011, 02:22:01 PM »

The dynamic ALC circuit in the AL-80B is a good one and actually produces higher average output power when used with compatible transceivers.

It doesn't require 100W drive, it runs maximum output power with about 65W drive (I have one).

I do use the ALC connection, when using the amp with a rig that has a compatible ALC input.  Even my old TR-7's ALC is compatible, and that rig is 33 years old this year.

The instructions on how to adjust it aren't very clear.  Obviously, you can adjust it until the cows come home and if the cable connection to the transceiver isn't made, it doesn't do anything at all!

With my rigs, I leave the rear panel control set to CCW, connect the ALC cable, and adjust the front panel ALC control so it limits grid current to 150mA maximum and cannot be driven beyond that no matter how much power the transceiver produces.  It works.

I have true PEP wattmeters (Autek WM1s and Bird 43Ps, and they generally agree with each other).  The AL-80B will produce 1 kW PEP output power.  Without the ALC connection made, flipping the meter to AVG you'll see maybe 150W on voice peaks (while actual PEP is 1 kW).  Then, plugging in the cable you see 300W AVG on the same meter (while actual PEP is still 1 kW).  I've done this while stations are listening and when I plug in the ALC connection, the typical comment is "wow, you're definitely stronger now!"  In reality, PEP power is the same.  Average power was enhanced so it sounds stronger.

Not all ALCs are this good, but the one in the AL-80B seems to be.

Posts: 1155

« Reply #3 on: April 04, 2011, 02:24:30 PM »

Other side of the coin guys. With the newer rigs with the power out set on a menu, it is a bit of a chore to turn the power level up and down all the time.

With proper care in setting ALC levels one can switcch from Amp on to Amp offline simply by using the standby switch. Therefore you can run barefoot  with the amplifier warmed up and in standby until it is really needed then turn it on with a minimum of fuss. Less time with the amp on when it is not needed, thereby more closely following the rule of minimum needed power.


Posts: 752

« Reply #4 on: April 09, 2011, 09:20:49 AM »

In my operation it gets a bit hairy at times and I get into trouble not having ALC in some amps being keyed from the same radio.
I run an AL80B, a 6m 600 watt with 5 watt drive level and a solid state 2 meter amp driven by a transverter.
Forgeting to change all the setting in a hurry causes issues.
I am in the process of building in an attenuator and ALC circuitry in the 6m amp to get around grey matter fade especially during contest time when changing bands in a hurry and forgetting something.
The 2m amp will get protection from a return power coupler to kick out the amp in case the correct the antenna does not get switched in.
The more automation the easier it is on the equipment and the hassle of repairs.
ALC is a good thing to have. It even gives you a measure of RF compression that does not cause any detectable distortion on a signal, just a slightly louder signal.

Posts: 83


« Reply #5 on: April 13, 2011, 08:38:28 PM »

As long as he understands the necessity to keep an eye on the grid current, he should be fine. As another poster indicated, with a healthy 3-500 installed, it won't take much more than 65 watts to reach full output. That being said, once the amp's rear panel pot is set so that the maximum ALC voltage doesn't exceed the maximum tolerated by the exciter, the front panel ALC setting is easy to work with.

I never bothered with the ALC connection with my AL80B as mine had the grid protection board installed, making it pretty much foolproof since it would immediately dump into standby if the grid current exceeded 125ma. I used the AL80B with my Pro III for years and it never dumped out unless I was checking the operation of the protective circuit. Later, I drove the amp with my Flex 5000A which has no ALC provision and it still never dumped out. Since the overload board is cheap and easy to install, it might be a good project for him to get one. It's cheap and seems to be foolproof.

« Last Edit: April 13, 2011, 08:44:34 PM by W1AEX » Logged

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