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Author Topic: Load values changed. al-811H  (Read 2555 times)
KF5ONT
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Posts: 8




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« on: December 18, 2012, 01:30:18 PM »

Hello, I'm a newb when it comes to amplifiers and have a question I really couldnt answer by googling.
I recently installed a lightning arrestor on the antenna outside which goes into a ground rod outside as well. After I did this my load settings seem to have dropped on the amp. Meaning, I decrease the load to peak out power from the amp from where I originally had the settings for a given freq. On 20 meters the setting for a given frequency has gone from 1.25 to 0.
This brings me to my question. What does this mean? I really don't know what is actually happening when I am peaking the load and plate settings in the amp.
Is it good/bad when my load settings change like this after an antenna mod? I guess I'm looking for a definition of Load when it pertains to a ham radio amp.
Thanks!
fyi, The antenna is a G5RV jr.
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W1QJ
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Posts: 1357




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« Reply #1 on: December 18, 2012, 02:01:51 PM »

What are you using for a tuner?
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N3QE
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Posts: 1868




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« Reply #2 on: December 18, 2012, 02:16:00 PM »

If the only change was the lightning arrestor, then you would not expect a change in matching. But if you added new cable lengths etc. esp to a G5RV then I would expect some change in feed impedance at the shack. Also... if you had an open or short circuit you would have a change in feed impedance too.

"Reasonable" changes in load impedance can be accomodated on a tube amp by retuning plate and load controls.

If the control is at a high or low limit... you might not be optimally matched.

Do you have an SWR meter, tuner after the amp? Can you ohm out your new cables and the lightning arrestor?

Tim.
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KF5ONT
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Posts: 8




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« Reply #3 on: December 18, 2012, 02:48:31 PM »

The tuner is a MFJ-962D. SWR is pretty much nil on all bands except 80. ~1.4 or less.

I do have a 259b analyser. Just got it last week. Barely got it out of the box. Can I ohm out the cables with this?

Thanks!
also, to the added ground outside I used about 30 feet fo solid copper. 18ga

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AD4U
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« Reply #4 on: December 18, 2012, 03:23:26 PM »

Not an answer to your question, but 30 ft of 18 ga copper wire running from your lightning arrestor to the ground is not much of a lightning ground.  Generally the "codes" specify 6 ga or larger.  Even though a lightning strike only lasts milli-seconds, the 18 ga wire may either burn in two or offer too much impedance to effectively dissipate (carry off) lightning strike energy.

Dick  AD4U
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KF5ONT
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Posts: 8




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« Reply #5 on: December 18, 2012, 03:39:51 PM »

Not an answer to your question, but 30 ft of 18 ga copper wire running from your lightning arrestor to the ground is not much of a lightning ground.  Generally the "codes" specify 6 ga or larger.  Even though a lightning strike only lasts milli-seconds, the 18 ga wire may either burn in two or offer too much impedance to effectively dissipate (carry off) lightning strike energy.

Dick  AD4U
My mistake.
Its 14ga solid copper.
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N4ATS
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Posts: 781




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« Reply #6 on: December 18, 2012, 03:56:37 PM »

Why not just be safe? Remove the lightning arrester and when not using the radio or when a storm is near, unplug the power and coax and be 100% safe?

Grounding can be a disaster when a storm comes as you are "really not" unhooked with grounds....

I get piles of lightning claims...First thing I ask , "was your station grounded?"

Typical responce "Oh yes and I had the power and antenna's unhooked"

Go figure....

« Last Edit: December 18, 2012, 04:09:02 PM by N4ATS » Logged
K7KBN
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Posts: 2697




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« Reply #7 on: December 18, 2012, 04:23:52 PM »

Not an answer to your question, but 30 ft of 18 ga copper wire running from your lightning arrestor to the ground is not much of a lightning ground.  Generally the "codes" specify 6 ga or larger.  Even though a lightning strike only lasts milli-seconds, the 18 ga wire may either burn in two or offer too much impedance to effectively dissipate (carry off) lightning strike energy.

Dick  AD4U
My mistake.
Its 14ga solid copper.


So it's still WAY smaller than the 6 gauge that it's supposed to be (not 16 - six).  "Solid" copper at 14 gauge will fuse far short of the tens of thousands of amperes in a typical lightning strike.

You can't "ohm out" wiring with a 259B.  I don't know what "ohming out" is; I suspect it's checking resistance, for which you need a multimeter.
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73
Pat K7KBN
CWO4 USNR Ret.
K4RVN
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Posts: 744




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« Reply #8 on: December 18, 2012, 05:18:25 PM »

Jeff
If you were referring to the load on the amp at 1.25 or zero, then you did not have the Al 811 H tuned properly in the first place.  I suppose that you are using the tuner after the amp and the watt meter in it, but you did not mention the hook up details.  If so try this on 20 meters if interested. Use 20 watts drive from your transceiver, set your load on the amp to 4  1/2 and the plate to 8 1/2. These are the AL 811H presets from the manual.
Peak the power out on the watt meter using the amp plate knob while keying the amp with the transceiver in the cw position.  Then peak the load for max power out. If this works for you, turn the drive up to about 70 watts and go through the process again for max power out. The load control should not be 1.25. If it is, then you have an amp, a connection, or antenna problem. Your amp readings for the plate and load settings should not vary greatly from the manual presets. Tune as quickly as possible so as not to overheat the tubes while out of tune.
On the MFJ 259B.  You can connect the antenna to it, and using the proper range settings, etc. it will give you the frequency and radiation resistance(impedance) of your antenna as you have it tuned with your tuner. In other words it will analyze your antenna. Don't worry if it does not read exactly 50 ohms, but it should be close. Good luck,

Frank





« Last Edit: December 18, 2012, 05:46:53 PM by K4RVN » Logged
KF7CG
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Posts: 724




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« Reply #9 on: December 21, 2012, 11:23:35 AM »

He may have hit the loose set-screw problem that I had with my 811-H amp. The amp was fine, but the set-screws on  the load control were loos and let the vernier drive slip on the load capacitor shaft. The load readings never were very close until I discovered the problem while activating 10 meters. Once fixed the amp tuned quiker (no slop) and the numbers were close to factory recommendations.

wn4zfx
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KF5ONT
Member

Posts: 8




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« Reply #10 on: December 27, 2012, 12:16:57 PM »

He may have hit the loose set-screw problem that I had with my 811-H amp. The amp was fine, but the set-screws on  the load control were loos and let the vernier drive slip on the load capacitor shaft. The load readings never were very close until I discovered the problem while activating 10 meters. Once fixed the amp tuned quiker (no slop) and the numbers were close to factory recommendations.

wn4zfx

This may very well be my issue. I'm going to give this a try asap. Given the change/symptoms, this makes perfect sense that my load knob may have slipped.
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WX7G
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Posts: 5683




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« Reply #11 on: December 27, 2012, 12:50:54 PM »

Why not just be safe? Remove the lightning arrester and when not using the radio or when a storm is near, unplug the power and coax and be 100% safe?

Grounding can be a disaster when a storm comes as you are "really not" unhooked with grounds....

I get piles of lightning claims...First thing I ask , "was your station grounded?"

Typical responce "Oh yes and I had the power and antenna's unhooked"

Go figure....



If you unhook the coax all of the coax must be place outside the house. Leaving it inside ungrounded is worse than grounding it. A VK on one of the forums had his shack burn down leaving the coax inside the shack but disconnected from the rig during a storm.
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WB6BYU
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Posts: 12760




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« Reply #12 on: December 27, 2012, 04:05:04 PM »

What is the SWR on the coax without the tuner?  Does it change at full power?

If the SWR is high enough and the arrestor happens to be at a high voltage point in the
feedline, it can arc over at full power, which certainly will change the load that the amp
sees.  You would know it if you were watching the SWR meter.

Because the SWR will change with power in that case, the tuner likely would not correct
for it, because the short wouldn't occur when tuning at low power.
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KH2G
Member

Posts: 219




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« Reply #13 on: January 02, 2013, 08:49:34 PM »

Consider the long ground run of 30 feet. That gets real close to the 20 meter band and is probably having some effect.
regards
KH2G
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