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Author Topic: Keeping a clean signal  (Read 2711 times)
KB7FSC
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Posts: 37




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« on: February 05, 2013, 07:51:17 PM »

I am new to the amplifier world.  I have spent a lot of time, however, reading these forums and W8JI's website.  I am now the proud owner of a used AL-811 amplifier.  I have owned it for about a week, and I have been playing around with tuning it into my dummy load.  I am planning on getting on the air with it this weekend, and I wanted to make sure that I am running as clean of a signal as possible and I don't want to create any splatter or damage my new amplifier.

I admit that I don't fully comprehend all there is to know about this new piece of equipment, but from my reading, this is what I believe to be true.  Please add or correct.

1 - Tune the amp into a dummy load at maximum possible exciter power while staying under maximum plate and grid current values.  Adjust Plate 1st for max output, adjust load 2nd for max output, and 3rd retouch the plate.

2 - Reduce only the exciter power to reduce output from the amplifier.  Do not retune the plate or load with the exciter at a reduced power level.

3 - I know to quit adding exciter power when tuning up when the plate or grid current exceed max specs, or if adding more exciter power does not increase the output of the amplifier.

4 - Tune at maximum values, but when in operation, take it easy and don't run the amp at the edge of its maximum ratings.

What else am I missing that will prevent a lesson from the school of hard knocks??

I just want to make sure that when I get on the air this weekend, I'm doing the best job possible.

Thanks,

Wane - KB7FSC
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WB2WIK
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« Reply #1 on: February 06, 2013, 09:33:12 AM »

I think you'll find Step 1 requires a bit more, since tuning is reiterative.  When you peak PLATE, then LOAD, and go back and peak PLATE again, you may not be fully loaded.  It can take 2-3 trips around both controls before you hit the proper LOADing point.

And after that, what I generally find is the amp still isn't really fully loaded.  I watch Ig and crank "up" the loading (reducing the LOAD capacitance) a bit more.  If the Ig goes down and the output power remains about the same, that's good.

Experiment with the dummy load and see what you find.

I always tune for "most output power with minimum possible grid current," and keep Ig well under the recommended maximum.  I don't watch plate current much.  While tuning, I don't watch it at all.  I'll take a look to see what it is after the tuning process is completed; however since I very rarely run a full-carrier mode (like RTTY, FM, AM, etc), no matter what the "peak" plate current is, the average is usually quite low and I've never lost a tube running them even 1.5x the max Ip rating on short duty-cycle modes like SSB  (or even CW).
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AA4HA
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« Reply #2 on: February 06, 2013, 12:42:20 PM »

I guess I would only add the other things that go into putting out a clean signal without an amplifier. Proper settings of microphone gain, compression, having a beefy, stable power supply for your transceiver, if doing phone decent microphone habits, etc... Having someone who is going to give you an accurate assessment of how you sound over the air (not the 9's rule, a friend who will tell you that you sound like Peewee Herman or Bob Dylan or something else unusual).
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KB7FSC
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Posts: 37




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« Reply #3 on: February 06, 2013, 05:03:35 PM »

Thanks for the feedback.  It is very much appreciated!  

I have noticed that it takes me two or three times to seem to find the maximum output of the amplifier.  I made a spreadsheet at various frequencies for what the plate and load settings are and what the maximum exciter power was. I also read on W8JI's website that once the amp is tuned for max output to increase the load setting slightly for a cleaner signal (I'm not sure why, but I'll roll with it).  I will experiment with the load setting after I'm tuned and see how it impacts Ig.  I'm looking forward to getting on the air this weekend and getting some audio reports- hopefully I sound more like Bob Dylan than Peewee  Roll Eyes

A couple of other questions I've thought of-

1- Is it safe to assume that if I tune my amplifier into my dummy load with a low SWR and if the antenna is tuned for low SWR with my antenna tuner, that I shouldn't have to re-tune over the airwaves with max output of the amplifier?

2- How far can I, or should I, move up and down the band without re-tuning the amplifier?

Thanks!

Wane - KB7FSC
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AA4HA
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« Reply #4 on: February 07, 2013, 12:30:51 AM »

A couple of other questions I've thought of-

1- Is it safe to assume that if I tune my amplifier into my dummy load with a low SWR and if the antenna is tuned for low SWR with my antenna tuner, that I shouldn't have to re-tune over the airwaves with max output of the amplifier?

2- How far can I, or should I, move up and down the band without re-tuning the amplifier?

You will always need to tweak it, no device is going to have the exact same tuning performance as an antenna simulator (dummy load). They will be minor tweaks but if you are paying attention it is something that you end up doing without even thinking. The same goes when you change frequencies more than a few tens of KHz, it is always a good idea to just touch the knobs to make sure you are still in the sweet spot on the controls. It is not quite the same as going through the entire set-up dance and you will quickly see how much "move" you can do on the tuning dial and what sort of minor change that makes to amp loading.

It will be a little different for each amp, for each band, for each antenna. It depends upon a bunch of factors, The Q of the antenna (bandwidth), what band you are on, how well your entire system (transmitter, amp, feedline, grounding, antenna) all plays together. This too becomes "the touch" that you will develop and when it becomes refined you will be able to tell if something just feels "off" with your setup. You may not know "why" it feels off but it is something you should pay attention to and try to remember what things that are different and what the end result was (like finding out that you had a bad trap in your antenna or water in a feedline). If you apply that knowledge right you will gain understanding of the nuances that make the difference between a good op and a great op.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
K2DC
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Posts: 1361


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« Reply #5 on: February 07, 2013, 03:44:38 AM »

Wane,

   One other item to add to AA4HA's list that also applies to a clean signal with or without an amplifier.  If you are not using a balun or choke on your antennas, you should consider it.  Common mode current on the feedline leads to RF in the shack, which can lead to distortion, splatter, and lots of other nasty stuff that will only get much worse at higher power levels.

73,

Don, K2DC
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KC9TNH
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Posts: 304




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« Reply #6 on: February 10, 2013, 07:32:59 AM »

I have owned it for about a week, and I have been playing around with tuning it into my dummy load.
Bless you.

The steps WB2WIK describes are for the same reason as the recommendation you found at W8JI's website. After tuning, and adding a little more LOAD, you may find you'll sacrifice a small number of watts no one will notice. But your grid current peaks will go down; amp is happy & operators too because signal is clean.

You can tune for the antenna (with the amp offline) at whatever power from the exciter gives you a true enough reading. Go to the dummy load to tune the amp. Then get on the air & enjoy.

FWIW, I did the same thing using a dummy load & keeping a simple spreadsheet (along with tuner settings for the Palstar). Very repeatable and takes one of the legs out from under Murphy.
Smiley Have fun.
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
K6AER
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Posts: 3515




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« Reply #7 on: February 15, 2013, 08:52:47 AM »

In addition to what others have said, Use only as mush grid current as necessary for your power output. Grid current in excess or near maximum specifications is what causes the tube to deteriorate in performance over time. Tubes like the 811’s do not much reserve grid current dissipation.
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