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[Amps] LK-500ZA hot

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Subject: [Amps] LK-500ZA hot
From: "Edwin Karl" <>
Date: Mon, 13 Aug 2007 15:43:18 -0500
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I own 2 LK-500 amps, one a "B" and the other a "C".

The "B" came without a high/low switch for the fan. Yes it runs quiet
in that position but does not cool the tubes when you are operating
for any length of time. The "C" model sort of cures the problem with the 
switch. You have to remember to operate it ...

The switch shorts out 2- 10 watt resistors in parallel (I forget the value) 
that are
in series with the fan motor. The normal voltage is about 55 volts on the 120 
volt van. So
you can do one of 2 things, a add the switch or install thermostats to have the 
amp do it
by itself. In my case I did the mods to the "B" (thermostats) I use 2, one 
the resistors out at about 160 degrees F. The other adds a third resistor 
across the first
2, again I have forgotten the value but the result brings the fan to 75 volts 
or so when the
temperature hits about 135 or 140 degrees. The 'sats are on a bracket between 
the tubes
and near the outer part of the chassis. The fan speed "stages" as the 
temperature dictates
The location insures the fan air is monitored after cooling the tubes. 
I am quite happy with the result.

Now your second question. First, the bandswitch has an extra set of unused 
contacts for
20, 15 and 10 meters. Add jumpers to the section of the wafer that is switching 
the coil taps.
This will take some of the current off the main contacts. You also could add an 
additional #14
wire or a bigger wire (#12) from switch output to the loading capacitor. his 
should reduce IR
losses (or IZ losses) in the tank circuit. 

The bandswitch "problems" are, I believe a result of high current through the 
switch on the upper
bands. Removing heat helps everything, tubes, switch, etc.

In operation the lower stat (in series with resistor #3) goes on almost 
immediately with first contact.
It will open up and the fan slows down if that's all the operation at that 
time. As things get going
ala a contest the fan goes full bore intermittently as needed.

Hope this helps ... Good Luck, we're all counting on you

ed K0KL
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