[AMPS] Reducing LK550-ZC Fan Noise

[Phil Clements]

In several cases that I have seen, IMO, the current cooling system
in the LK-550-ZC is marginal even when running on high speed.
The thermostatic switches can fail with age, and the tubes will
over-heat before you notice.

My advice is to strap across the switch and let the fans run at full
speed all the time. If the noise bothers you, get a proper squirrel
cage blower and hose from W.W. Grainger and put the blower
in another room.

GL!
Phil, K5PC


> 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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