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[AMPS] More on LK550 cooling

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Subject: [AMPS] More on LK550 cooling
From: dick.green@valley.net (Dick Green)
Date: Mon, 23 Oct 2000 01:51:56 -0400
Some of you may remember my post of a few weeks ago asking how to reduce the
fan noise in an Amp International (nee Amp Supply) LK550-ZC. I only got two
or three responses, but those I got raised questions about the adequacy of
cooling in the stock LK550 at both idle and full output.

I called Billy Edwards at Omega Electronics and asked him about it. Billy
was the service manager for Amp Supply and Amp International before the
company went under. He bought the parts inventory and established Omega to
supply parts and repair several amp brands. Billy confirmed that cooling on
the earlier Amp Supply units was inadequate. But he said they later added a
bunch of holes to the tube deck and that this improved cooling to within
spec for the 3-500Zs. Since my LK-550 is an Amp International, it was one of
the last ones made and it has the extra cooling holes in the tube deck.

Using a thermocouple probe with my DMM, I took some temperature measurements
at several places under various conditions. I was able to check the
temperature at the filament pins (base seals), the tube glass on the side,
and the tube glass near the top. I was not able to measure the temperature
on the glass just below the anode cap (plate seal) because the presence of
the probe wire caused arcing (lost a $15 thermocouple probe that way). I
guess it's going to take temperature-sensitive paint to check that area. Can
anyone tell me where I can get some, what to get and how to use it?

In standby, the fans cycle from low speed to high speed and back. They run
for about 4-5 minutes at low speed, then kick into high gear. They run on
high speed for about a minute, then cycle back to low speed. At low speed,
temp at the filament pins rises from 101C up to 119C, at which point the
fans kick into high. After one minute, the seal temp drops back to 101C and
the fans go back to low speed. The sides of the tubes range from a low of
77C to high of 89C. The glass at the top of the tubes runs about 5 degrees
hotter than the sides.

Using contest software, I set up a continuous CQ at 30 WPM with a 3 second
interval between calls. I transmitted at 1500W into a dummy load on 14MHz.
The fans kick into high speed within a few seconds, and stay there until I
stop CQing. The temperature at the base seals quickly climbs to 123C before
the fans kick into high, then actually drop back to a range of 100C to 112C.
Once transmission stops, the filament pins drop down to 95C before the fans
cycle back to low speed. Temperature on the sides of the tubes goes up to
about 139C, where it stays as long as I CQ and the fans are on high speed.
After the CQ stops, temp drops to 77C before the fans switch back to low
speed. This takes 3-4 minutes. The glass at the top of the tube maxes out at
121C during the CQ, then after stopping the CQ drops down to 69C before the
fans switch back to low speed.

I also tried simulating intermittent "search-and-pounce" activity, which is
what the amp is used for. In this mode, the base seals never get above 118C
and the sides of the tubes never get above 103C.

Given Eimac's max of 200C for the base seals, I guess the readings are
within specs. The limit for the plate seal is 225C, but I can't measure that
one with my thermocouple probe. Given the base, side and top temperatures,
I'm optimistic that the plate seals are not running too hot.

My primary goal was to reduce the "freight train roar" of the fans when they
run at high speed. This is especially annoying during contests, when the
fans cycle into high speed every time I make a call. Even more annoying, the
fans kick into high speed every five minutes even when the amp is idling.

The LK-550 achieves low speed with a resistor in series with the fan supply.
A temperature-sensitive relay bypasses the resistor for high speed mode.
Billy indicated that it would be safe to insert another resistor in the line
from the relay, paralleling it with the existing resistor. He said the
existing low-speed resistance is 300 ohms, and thought an effective
resistance of 150-200 ohms would make the high speed quieter but still
adequate for cooling.

I was interested to find that my amp has a 430-ohm resistor for low speed
instead of a 300-ohm resistor. Just for fun, I tried paralleling a 1K
resistor with it, and found that at 300 ohms the fans run only slightly
louder on low speed. Better yet, the cycling from low to high in standby is
completely eliminated! Temperature measurements show that this speed keeps
the base seals at a steady temp of 107C, about the average between the low
and high points of the cycle that occurs with the 430-ohm resistor. The
sides stay at about 80C, and the top stays at about 76C. I would hope that
this is what the designer really wanted, not the cycling in standby.

I decided that by far the safest and most effective modification would be to
*increase* the *low* fan speed. As 300-ohms is probably the correct stock
value, so much the better. I wonder if some well-meaning previous owner
(there have been several) decided to make the fans quieter in idle mode?
There have been other strange mods in this amp (someone replaced the meter
calibration resistors with the wrong values.)

I also tried lowering the speed in fast mode by paralleling a 240-ohm
resistor with the 430/1K pair. This cut the high-speed noise considerably,
eliminating much of the screech. I found that maximum temperatures in CQ and
S&P mode were less than 10 degrees higher in all cases (some temps were only
2-3 degrees higher.) The sides and top of the tubes had the most increase
(+7C and +9C respectively.) Of course, the recovery time back to low speed
was slower. It took anywhere from 25% to 30% longer to return the tubes to
minimum temperature at the reduced high speed. So, there's a trade-off: the
fans run quieter, but longer.

I decided to install the speed-reduction resistor, but added a disable
switch so the fans can run at the stock high speed setting. My intention is
to use the higher setting for casual operating and CQing, and only switch to
the reduced high speed when using the amp for S&P in my SO2R station. A
little conservative, but why not?

BTW, for anyone wishing to make the same mods, 25W cement power resistors
are appropriate.

73, Dick WC1M


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