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Author Topic: Parasitic Problem 3-500ZG  (Read 1792 times)
KB0GU
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Posts: 130




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« on: March 21, 2005, 10:37:46 AM »

The short story, I have owned an amplifier running a pair of 3-500zg for many years and run it conservatively.  It is quite heavy duty but I kept having a problem with frying in the plate tune bandswitch area which I thought was antenna or operator problem.  Turns out it is the improper type of parasitic suppressors that were put in by the manufacturer.  These stock monsterous silver plated strap coils around a couple huge resistors installed by the manufacturer are impressive looking but I learned are virtually worthless against VHF parasite and the reason I have had to replace some expensive items on a couple of occasions in the plate tuning and bandswitch area of the amplifier.  Seems they are quite high Q for VHF instead of low Q.
I learned this reading an article by Richard Measures from October 1988 QST magazine.  I now am applying these ideas and hope to fix the frying and damage created by it altogether.
This was a helpful article even if I read it 17 years too late!
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KG6AMW
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Posts: 628




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« Reply #1 on: March 24, 2005, 06:52:02 AM »

I really don't know that much about amps with parasitic issues, but if this is such a common weakness, why doesn't Ten Tec, QRO and Ameritron modify their 3-500 amps to over come the problem?

KG6AMW
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KB0GU
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Posts: 130




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« Reply #2 on: March 24, 2005, 01:24:10 PM »

That is a good question, I am not familiar with those makes and models as I am with the one I am running here.  If you have not done so, I highly recommend a look at Richard Measures web site and a read of his article on parasitic suppressors.  I found it technically comprehensive and sound theory.  There is some notions and history advanced as to why this remains a problem in some amplifier designs.
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KE3HO
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Posts: 235




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« Reply #3 on: March 29, 2005, 04:19:23 AM »

The manufacturer and model of the amp would be helpful for us to offer help. Funny, but as soon as you said that you had learned that the manufacturer had installed the wrong kind of parasitic suppressors I immediately said to myself, "Oh, he's ready something by Richard Measures." Then a few lines later you confirmed it. I don't claim to be an expert, but I have read countless archives of old newsgroup and forum postings between Mr. Measures and a number of people who dissagreed with him. In my estimation, nothing Mr. Measures said about these suppressors stands up to the light of day. Several people performed tests on the suppressors that he called bad in the very amp models that he specified, and offered test data to show that there was nothing wrong with them. Some of them even went so far as to install the suppressors that Mr. Measures touted, and repeated the tests and found that in every way the amplifier was worse with his suppressors. When confronted with the data, he just vanished from the discussion and was not heard from again. Personally, I would look for something else in the amp before I bothered to change them. If the amp manufacturer is still in business, check with them to see if there is a knows problem/fix. If they are no longer in business, there is often a lot of good information on the internet if you do a google search. I would not put too much stock in the "improper parasitic suppressors" idea just yet.
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K7KBN
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Posts: 3693




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« Reply #4 on: March 29, 2005, 12:28:29 PM »

Measures' web site has moved to:

http://www.somis.org/

There are a lot of links to articles and other documents (probably including the discussion referred to above).  
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73
Pat K7KBN
CWO4 USNR Ret.
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