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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