Metropoulos Forum

I noticed that some amp enthusiasts look down on cooling fans inside amps, is there a good reason for this?

Mostly because:
-They are not needed (as seen for the last 40 years of Marshall Tube amps)
-Hot tubes sound better
- AC fans will induce noise into an amp....

Chubs

Or because dummies hacked up the sides of their Marshall cabinets to install them (or just as bad, added holes to the chassis so it could sit on the chassis!!!) I had a '75 50-watt with a hole cut in the bottom of the cab for a fan, like it was going to draw any useful air from down there.

I'd think they are useful for hot outdoor gigs, but plan and implement carefully.

It's a designer's fault, to put too many output tubes into a too small case!

Output tubes in general don't like to be cooled by a fan, because this can increase the tension in the glass bulb and lead to cracks. The glass bulb won't break immediately, but the small cracks will cause vacuum lost over the time and finally the tube will die earlier. Also most of the old traditional tube manufacturers disapproved the use of cooling fans in their info sheets.

Larry

I guess I would respectively disagree with Larry on this. Fans are found on SVT's, high end audio, lots of classic lab equipment and they are not a problem, in fact they help a lot to prolong tube and associated component life. The problem is with hacking up a vintage amp to install one and we have all seen the wood-inducing classic Marshall that sounds killer and looks great except for a big-ass hole on the side where some stupid with a hole-saw decided to put a fan in. Kevin O'connor has some sensible recommendations on using fans at reduced speeds (without dimmer switches), but that doesn't make up for maintaining reasonable spacing between the tubes. As the plates are much hotter (and thus radiating much more heat), tubes radiating heat onto each other can be a problem. If I were designing an amp from the ground up, I would use one, but focus on making it quiet and unobtrusive.

Sparky wrote:I guess I would respectively disagree with Larry on this.

Not a disagreement in general, because it depends on how the fan is installed.

When you have a fan blowing high speed onto the output tube's glass from a distance of 2", then this would extremely stress the glass and it will crack some day.

But when you're mounting a fan at the opposite side of the output tube's location or anywhere farer away from it, who's 'gently' de-aerate the head's inside, then this will work fine - as long as the fan doesn't pull out 5 gallons of air every second.

I wrote it 'this way' above, because idiots often are so genious and I've already seen too many wrong placed fans in the past.

What stays is, that the necessity of a fan always is a designer's crutch and inevitable, when designers have tried to put too many heat dissipating components inside actually too less space.

Larry