Amp Fans

[kh15]

i just got a marshall dsl 40 amp and its amazing and worth every penny! but i have been reading online that some people are having over heating issues with the amps 4 tubes. i have yet to come across this problem and hope to never see it but just as a pre-caution should i by like some computer fans and put them in the back blowing air on the tubes to keep them cool? if so wouldent this cool the tubes down some and effect the tone cause i know tubes need to warm up to give you those legendary marshall tones but at the same time i dont need this amp breaking on me and solder joints loseing up and stuff?

[Jehle]

Where's Lovekraft when you need him?

If it aint' broke, don't fix it is my first thought. You are right that the tubes will become warm as you drive them. That's the way tubes work. They have to bake off electrons as part of the amplification process. The harder you drive them, the more electrons are baked off. That shouldn't really effect the heaters at all though.

What exactly is an overheating problem anyway? Do they catch fire or something? Or can you just fry an egg on top of them? Do they just run through tubes too fast?

There's a few ways to correct the problem, but make sure that you have a problem before you muck around with something that works fine the way it is.

[Paul Marossy]

I have put cooling fans in all of my DIY and commercially manufactured ones that didn't have one in them. Blowing a little air across the tubes will really help dissipate the heat produced by the tubes. It won't help too much if the tubes are improperly biased and are dissipating more watts than they should be. This is probably the source of the "overheating" problems some people have.

EDIT: BTW, in my opinion, it does not change the "tone of the tubes" to blow some air across them. The inside temperature of the tube will pretty much remain unchanged even though you are blowing air across the glass envelopes. What it does do is carry the heat away from the chassis, and that stays considerably cooler. It probably helps the life of the tube somewhat as well.

Edited August 27, 2005 by Paul Marossy

[lovekraft]

Well, my old Soldano/Yamaha had a pair of serious 8" muffin fans in it, so evidently somebody (probably Mike S.) thought that it might become an issue. That said, I tend to agree with Paul about the tubes themselves being designed to work at high internal temperatures (since they damn sure won't work without 'em), and since the interior is a vacuum, cooling the glass envelope isn't going to affect the inside temp very much anyway, so the only thing the fan needs to do is keep everything else cool enough to avoid problems. I can't see it affecting the sound at all, unless the fan motors induce electronic hash into the amp circuit, and that's unlikely.

A 120vac computer fan with a separate power line, well isolated from any audio circuitry, should cause no problems, and might well save wear and tear on the electrolytics and the printed circuit boards, both leading causes of premature failure in production tube amps. Go for it - even a little cooling should forestall a lot of problems, and it's not really a difficult mod, so it's probably well worth the trouble.

And like Paul said, it's absolutely crucial to keep the power tubes biased at a sane level - if they're trying to dissipate much more than they can handle, nothing short of liquid cooling is going to keep them from melting down and Chernobyling most of your power amp and psu.