# Cooling fan in tube amp..needed or not?



## kw_guitarguy (Apr 29, 2008)

Hello,

I have a Peavey Classic 50 head and it has a cooling fan in it. The fan is beginning to make some noise that is noticeable when playing.

My question is, for the hour or so at a time that I use the amp, is the fan necessary? If it matters, this is the tube compliment inside:

POWER AMPLIFIER
SECTION
4 - 6BQ5fEL84's with
12AX7 driver

PREAMP SECTION
2 - 12AXTs

Also, I have been reading about tube care and I wanted to double check that the minute or so that I leave the amp in standby before powering it off is sufficient?

Thanks.

~Andrew


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## Wild Bill (May 3, 2006)

I'd say you definitely need some kind of fan! I couldn't begin to count the number of Classic 50 and similar amps that have passed through my shop where the EL-84s had started to make rattling noises at the slightest mechanical vibration of the amp, due to heat damage. Especially JJs! JJs are a good quality tubes but these amps are constructed in such a way that the heat seems to be "cupped" inside, building up hotter and hotter.

So I would suggest you have the fan replaced but in the meantime there is a workaround that will help. In fact, if I were you I would use it all the time, internal fan or no fan!

Just hustle down to some thrift shop like Value Village and buy yourself on of those boxy floor fans. Run that behind your amp, blowing the air across the back. This will tend to suck the hot air out of the Peavey cab. You don't need a hurricane of air flow, just enough to keep slowly changing the air so the temperature is leveled off somewhat.

It's amazing how just a bit of air blowing on a tube will drastically extend its life! I would guess that with modern output tubes it could double or even triple how long they last!

Wild Bill


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## TWRC (Apr 22, 2011)

I'm sure I could have roasted a turkey over the top of my AC30. When I first got into AC30's I was really surprised by how hot they could get after a show etc and read horror stories of old AC30's catching on fire from regular usage (could be folklore). So to avoid this and to preserve my tubes, I had a fan installed at the bottom of the chassis and it made a huge difference on how physically hot the amp would get. As far as tube life is concerned, I'm not too sure if it makes a huge difference but I definitely feel better knowing that they're not glowing away in a convection oven.


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## kw_guitarguy (Apr 29, 2008)

Well, far be it from me to argue with experience!

I shall deal with the noise in the short term, until I can get in there and replace the fan.

Thanks!

~Andrew


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## Wild Bill (May 3, 2006)

TWRC said:


> I'm sure I could have roasted a turkey over the top of my AC30. When I first got into AC30's I was really surprised by how hot they could get after a show etc and read horror stories of old AC30's catching on fire from regular usage (could be folklore). So to avoid this and to preserve my tubes, I had a fan installed at the bottom of the chassis and it made a huge difference on how physically hot the amp would get. As far as tube life is concerned, I'm not too sure if it makes a huge difference but I definitely feel better knowing that they're not glowing away in a convection oven.


Actually TWRC, forced air across a tube's glass has been done ever since tubes were invented! It is standard practice with transmitting tubes, which can be very expensive in a broadcaster application.

Some tube data sheets will talk about using forced air cooling to boost ratings. In guitar amps, a true engineer would not use a fan but would make sure that the hot air was never allowed to collect around the tubes. The sides would all be open so that the warm air could easily drift out. AC 30's are perhaps the worst, since the hot air just jams up in the top of the cab and bakes, and bakes, and bakes...

It's not just tubes that can be damaged. I once had a Mesa Nomad come in with a blown power transformer. When I talked to the customer he told me it was the second time!

That perked my ears up. Once can be bad luck but transformers are rugged beasts, being essentially coils of wire around some iron. Something must have take the transformer out. However, when I crawled inside I couldn't find a blessed thing wrong with the rest of the amp! The transformer should never have blown.

Then I took another look. This amp cabinet was like the Classic 50, boxed in at the top so that the air would be stuck up there, around the entire chassis. I was holding the Power trannie in my hand and somehow, it felt a bit light in weight for the current those EL84s would need. The more current you demand of a trannie the more iron you have to use to make it and the more it weighs. The stock Boogie PT just felt too lameass light for the job!

Mesa Boogie of course would not likely hand out the specs and besides, they rarely let you talk to a tech type who knows anything, only salespeople. The specs of their transformer would be considered a proprietary secret, as if anyone with a reasonable knowledge of tube electronics couldn't figure it out anyway!

Getting a transformer from an amp manufacturer is almost always a long and expensive process. I said "screw that!" and dug out my Hammond transformer info to see if there was anything in the way of a stock catalogue item readily available that would fit the need.

I had to use two transformers, since the Nomad has some solid state stuff in it that needs a special low voltage supply. Still, there was no problem with fitting them in. Not only were the specs so much higher than what was needed that the chances are they would NEVER blow up but the combined price was much cheaper than getting a replacement Boogie trannie that would likely blow up just like the last one!

So the amp was repaired better than new and my customer was happy. A year or two later another Mesa Nomad comes into my shop for a different problem. It was a couple of years newer than the one I just described and one thing jumped out at me when I took the back off. There was a nice factory installation of a fan blowing right onto the top of the power transformer!

That was Boogie's way of handling the problem.

So some kind of fan to increase airflow is ALWAYS a good idea!

Wild Bill


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## greco (Jul 15, 2007)

Great story Wild Bill !! 

I think we often forget/underestimate the importance of air circulation in tube amps.

Andrew...I would be glad to help you put in the replacement fan, when you get it.

Cheers

Dave


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## allthumbs56 (Jul 24, 2006)

My DSL401 is one "Hot Mama" and would most surely benefit from a fan - after a gig you can't touch the faceplate for about ten minutes. I have read where some owners have installed pc fans, vented the tops and such. 

So far all I've don is mounted a heatsink on the bridge rectifier and I've been keeping my eyes open for some kind of a clip-on fan.


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