On Mon, 8 Sep 1997 23:47:30 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
To: <amps@contesting.com>
>> Date: Mon, 8 Sep 97 14:18:09 +0000
>If you look at N7WS's data, and his conclusions, the stock AL-80B
>suppressor is essentially equal to the nichrome model near
>the frequency where stability is a problem, although the stock
>suppressor does lower SYSTEM Q at VHF slightly more than the
>nichrome replacement.
That only holds true when the resistor used is new or undamaged by heat.
As it ages it becomes a liability, not a suppressor.
There would probably be less problems if we hams adopted some
preventitive medicine strategies. Changing the suppressor every few years
make some sense.
>The "advantage" of the nichrome really kicks in at HF, as evidenced
>by the heat produced and efficiency lost on upper HF when the
>nichrome suppressor is added.
What loss of efficiency? Just how much heat does it dissipate? Rich's
nichrome suppressor measures .38 Ohms DC resistance. It also appears to
be #22 gauge so I do question the RF amperage capability.
My version of a proper suppressor shows a DC resistance of .02 Ohms and
is constructed from .25" wide strap. It barely warms to the touch at 6M
much less 10M. It also appears to work equally well with all tube types I
have tried so far.
Perhaps Rich could have Wes conduct tests on numerous designs, including
overheated ones from various commercial amps. I would be most curious to
see the results.
>As measured by N7WS, the nichrome is about 103.3 ohms at 200 MHz with
>a resistance value of 78.7 ohms and an inductive reactance of 35.3
>ohms. The stock AL-80B suppressor, on the other hand, has a Rp of
>169.49 ohms at the same frequency and a resistance value of about 110
>ohms!
>
>Follow this simple series of facts........
>
>1.) Rich claims a "low Rp" is desirable, and we both agree his
>suppressor has lower VHF Rp than a stock suppressor.
>
>2.) A direct short has an Rp of zero ohms.
>
>3.) Removing turns from a suppressor LOWERS the suppressor's Rp
>value, because it makes the suppressor look more and more like a
>direct short.
>
>Now with that all in mind, ask yourself this very simple question.
>
>When you want to increase VHF stability, do you REDUCE the
>turns in the suppressor in the anode lead, or do you increase the
>turns in the suppressor??
This sounds more like typical double speak from Tom. He seems to have
forgotten that a few months ago he stated, "with authority", that the
number of turns depended upon the tube used. The 572B required more
turns than a 3-500 for instance due to the different parasitic
frequencies of the two tubes. It was also stated that the suppressor
resistor values differed.
>
>Is the best suppressor to use with an unstable tube a zero ohm
>resistor? Pretty obvious about the "low Rp" always being desirable,
>isn't it.
Is that not what is used in the AL-1500 Tom? Zero Ohms and a prayer it
would appear.
At VHF and above there is no such thing as a direct short, taken in the
context we are discussing here. That 1" or so of wire straight across the
resistor has a measurable inductance and therefore it will have an effect
at some frequency, probably in the UHF range. Its effect as a suppressor
depends upon the tube used and other circuit parameters. It may very well
work just fine with an 8877 or 3CX800 but will surely evaporate with a
572B or 811. The 8877 has poor feedthru isolation around 300MHz where
it is still capable of much gain.
73....Carl KM1H
>73, Tom W8JI
>
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