Hi Dick,
contrary to popular belief, it is the frequency and number of poles, not the
voltage that determines the motor speed. While adjusting the voltage may
have an effect on the speed, it can sometimes reduce the life of the motor.
You might try an adjustable fan speed switch.
Jim Smith, KQ6UV
----- Original Message -----
From: "Dick Green" <dick.green@valley.net>
To: "Amps" <amps@contesting.com>
Sent: Tuesday, August 29, 2000 10:02 PM
Subject: [AMPS] Reducing LK550-ZC Fan Noise
>
> Hello AMPers, long time no see.
>
> I've got an old Amp Supply (really Amp International) LK550-ZC that I
> rehabbed about 18 months ago. This is the three-hole 3-500Z model with a
> temperature controlled dual-speed fan system. The fans are very quiet in
the
> low-speed mode, but when they kick into high gear it sounds like a freight
> train is roaring through the shack. Very distracting.
>
> I'd like to reduce the fan speed in high gear as much as possible without
> risking damage to the tubes. Unfortunately, the schematics are rather
sparse
> for this part of the amp, so I'll have to follow some leads to figure out
> the exact wiring. There are four muffin fans, each rated 115V 9.5W
(Interfan
> PM040-115-3B.) The two upper fans and about 2/3 of the area of the two
lower
> fans are above the tube deck, leaving about 1/3 of the area of the two
lower
> fans below the tube deck to cool the pins. There's some sort of sealed
> temperature-sensing device (containing a thermistor?) on a post between
two
> of the tubes. I'm guessing that the fans normally run at 115VAC, and that
> when the temperature gets high enough, the temperture-sensing device
> switches the fan supply to 220VAC. If that's true, I believe the simplest
> approach would be to add a resistor to reduce the voltage in high-speed
> mode. I can measure the voltage and current draw in the high-speed mode to
> determine the required wattage rating.
>
> The real question is how much I can reduce the fan speed without risking
> tube damage (i.e., how big a resistor to use.) The Eimac specs say max
> temperature of the tube seals is 200 degrees C. I figure I should measure
> the seal temperature in low and high speed mode to get a baseline, then
try
> some different resistor values while closely monitoring the seal
> temperature. I don't have any temperature-sensitive paint, etc, but I do
> have a temperature probe for my DMM that goes up to 1000 degrees F. If I
use
> that to measure seal temperature, where's the best spot to place the
probe?
> What would be a reasonable operating temperature with suffcient safety
> margin?
>
> Has anyone out there done a mod like this to the LK550 or a similar Amp
> Supply model? If so, please save me some time and tell me how did you did
> it!
>
> 73, Dick WC1M
>
>
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>
>
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