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Re: [Amps] IMD

Subject: Re: [Amps] IMD
From: "Roger (K8RI)" <>
Date: Thu, 29 Dec 2011 18:12:15 -0500
List-post: <">>
On 12/29/2011 2:09 PM, richard courtway wrote:
> Have been enjoying and learning from the discussions concerning IMD
> problems with SSB signals and their causes.
> Personally am a cw operator - and hear quit a few very wide and
> splattering cw signals .  Thinking that most of this is caused by
> operators trying to get a kw out of a 800 watt rig.  Overdriving - etc.
> Am approaching the time when I will have my first amp on line -  and
> wondering if any of you would care to comment on the causes of such
> lousy sounding cw signals.
Hi Dick,

First I'd suggest reading the thread on Linear Amplifier Tuning 
...Properly and The folly of using a TX tube beyond its ratings over on 
Ham Amps. It gets a bit colorful at times but there are many gems buried 
in both threads.

If you look at some of today's amps it's trying to get a KW out of a 500 
watt (or less) amp.

Then there is the lack of knowledge on how to set up the particular rig 
being used to drive the amp.
Some rigs, and particularly some of the Icoms have a problem with ALC 
overshoot which causes spikes on key up for every dot and dash.
If the exciter is clean then there is that problem of trying to get more 
out of an amp than it's rated for. Of course there are some amps out 
there that are rated for considerably more than the tubes in them, or 
they don't have a heavy enough power supply for good regulation.

Tetrodes with their very high gain are a bit more picky about tuning 
than many triodes used in grounded grid. Most tetrodes take very little 
power for drive which means backing off on the power out of the 
transceiver doing the driving. With some this also creates ALC 
overshoot.  As has been mentioned in other threads, it would be better 
to use an attenuator pad and run the exciter at its rated power in those 
cases. Of course triodes are not immune to being driven into non 
linearity, but they are less picky about tuning.  You just keep peaking 
the tune and load for max output while increasing drive without 
exceeding the maximum grid current.

Put a signal that's not the cleanest into a high gain amp and it's a 
really good example of a little garbage in and whole lot of garbage out.

We all,or most of us think we have the best rig and amp in the world, or 
at least won't admit to having purchased a lemon, so ham reviews 
*generally* tend to be more than a bit biased.

I'd read up on the amps you are interested in, then get out the tube 
data sheets and compare what the data sheet says and what the amp 
manufacturer says. Look at the power supply capabilities as well. I'd 
read the ham reviews and then compare a bunch of them for each amp.  
Each may be a bit biased, but when all are taken together they should 
indicate a general opinion, but still take with a grain of salt.

Ham Amps is also a good place to ask about any particular amp as most on 
here are not only strong on opinions and knowledge, many are quite 
willing to share their views <:-))  Just do your homework before 
starting to ask specific questions.

73 and good luck,

Roger (K8RI)
> Thankee - and HNY to you all.
> de Dick / aka N7RC
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