Modification of Marshall EL34 100 + 100w Rack mount Valve Amplifier
Fault - The cooling fan is far to noisy and annoying loud clicks are heard every time the standby switches are turned on or off
This type of amplifier has an incredibly noisy cooling fan and as the customer was using it in a studio environment a modification was required to quieten things down, the amplifier has a single fan at the rear centre with quite a high velocity/flow rate, air exits through the side and top vents
See the above pictures, the amplifier has 8 EL34s and 4 smaller valves in quite a confined space, so sufficient cooling is required to avoid overheating, the amplifier has no over temperature sensors so any modification to the fan system would have to keep things cool.
The solution was to fit three low noise fans, the reason for fitting three is that the velocity/flow rate of these fans is les than the original, however the combined flow rate of all three new fans exceeds the air flow of original single fan, the new replacement centre fan fits in the original mounting position and is fastened with steel screws and nylock nuts, the two new outer fans are on the outside spaced of off the rear diamond cut cover to stop excessive wind noise that occurs if the fans were mounted directly to the cover, again steel screws and nylock nuts have be used.
The fans were connected to a new Molex plug/lead the original plug and fan have been saved if the amplifier needs to be returned to standard spec at a later date, the total current consumption of the three new fans is only slightly more than the single fan so does not stress the PSU,
The end result is a much quieter amplifier that actually runs cooler than the original set up, we also did a modification to the standby switches that eliminates the terrible clicks, this mod however is to remain a secret !
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