A fan in the amp! Hey all You amp wizards!

[Steven]

MJ Harnish, you said that it is a good idea to hang a fan behind all tube amps. Since I dont know much about how tube amps work, can You (or anyone else) explain why a fan is a good thing? I looked at the inventory at my local "Radio Shack" and found both AC and DC driven fans. Witch type is best to use? Is it possible to have a DC fan conected so that it gets its current somwhere inside the amp (I hate wall warts)? I use a Fender 63 Vibroverb RI.

[Tele295]

Keeps the tubes cool.

[MJ Harnish]

In a nutshell, cooler tubes last longer. Now, a tube can't be ice cold or otherwise it simply won't function since its magic is largely due to atoms flying around in a hot vacuum. However, tubes are rather inefficient in generating power and so a lot of energy is wasted in excess heat. In addition to causing tubes to wear out faster, the heat can potentially cause other problems (e.g., transformer failure due to overheating though that's not very likely at all).

Another benefit of a fan is that the chassis stays cool to the touch. My Carmen Ghia (and most other cathode biased amps) runs HOT; after a couple hours you could fry an egg on the chassis.

The easiest way to add a fan to any amp is just to take an ordinary 12VDC fan (like the ones used in computers) and hang it off the back of your amp, a few inches away from the power tubes, blowing cool air over them. How you power the fan is with a 9vdc wall-wart (yup); this causes the fan to turn slower (not a big deal since you don't need a high volume air to cool the tubes) and thus makes the whole thing pretty quiet.

You could rig a DC power supply for the fan tapped off your amp's power transformer but this would require a bit of modification including drilling a hole in the chassis. I wouldn't recommend doing this, especially in any sort of amp that's PCB based. The wall-wart is much simpler and cheaper, and avoids hooking a potentially electrically noisy fan into your amp's power supply. The wall-wart can be pretty small (I cannibalized one from an old answering machine); most 12vdc fans draw less than 200mA.

As far as how to hang the fan off the back; all you really need to do is make yourself an "L" shaped bracket from a couple strips of sheet metal. It just takes a little ingenuity to figure out a good way to hang it. My portable fan actually came from an old PC's power supply; I took the power box apart (they generally come in two pieces), leaving the fan attached to the grill and then just bent the casing into a "U" shape which hangs off the center brace on the back of my Carmen Ghia. It's not pretty but it works and was free.

[Steven]

Thank You so much! I´ll go for the 12 v DC fan running on 9 v. I guess I can live with the wall wart.

[MJ Harnish]

This particular wall-wart, at least for me, isn't so bothersome b/c I can plug it into the same strip the amp is plug into. It's the pedal related stuff that will drive you nuts b/c you can end up with 15 cords running to your pedal board.

[Dacious]

I personally wouldn't do this. Not to argue with MJ, but vacuum passes no heat, so the elements in the tube are not going to be affected one way or t'other by air flowing over the glass envelope. With a fan stuck randomly inside, cool air might not be flowing evenly across all the tubes, causing localised heating which is probably worse than none at all. Just blowing air inside an amp head is not likely to evenly cool without some sort of cooling fins/channels to distribute it.

The phenolic and/or ceramic tube base is not going to pass heat much via convection, especially given the localised mountings via rivets or screws. So the working core of the tube is not going to benefit much if at all. The potted and paper-wrapped transformer windings are not going to cop much either, inside their end-balls.

The other reason is - heat is power, one reason why amps seem to sound better after operating for a while. If tubes are operating in a specific range of voltage and current, they are working where they're designed and trying to alter that is not likely to improve anything. You might keep them out of 'the zone' although I doubt it.

Think of all the vacuum tube phone exchange equipment, radio transmitters etc (which still use enormous ceramic tubes) designed to operate continously around the clock with minimal maintenance. None of it had fans.

Computer gear and solidstate equipment is different as silicon chips and the components in a hard drive have much more critical temperature operating limits. Their outer surfaces are exposed solids. A 12vdc fan might help them - it's not going to help a set of red hot tubes much, but might coat them in dust sucked in from outside.

Just MHO.

[Steven]

In the early 90s I used a Boogie Mk 4 top and I think that it had a fan built in to it. Are You saying that that fan was just for cosmetical reasons? As you all know I´m a newbie when it comes to tube amp theory but isnt it a good thing to get rid of heat inside the amp chassi?

[Big Cookie]

Here is my case against fans in amps. Fans introduce dust into an amplifier's guts and dust causes a myriad of problems over time. If you live and or play in a dusty atmosphere to begin with, like the American West and much of the Midwest and South, you are begging for trouble. The only amps I would use a fan on in these areas are Voxes or any others that run notoriously hot.

Best Blues,
Cookie

[Dacious]

If Boogie put a fan in, then it was there for a reason. But tons of Fenders, Marshalls, Hiwatts and a plethora of others have been running 30, 40 or 50 years without them. If the maker thought it was desirable, they'd stick one in.

I am certainly not averse to sticking an external fan behind an amp to move air around on a hot day/night when there might be an exceptional build-up of heat. But a fan won't stop a bad component from failing due to heat. SS PA heads often have fans due to a lot of output chips in a small enclosed space. They are also often rack mounted or run in roadcases so air movement is limited. This is not the case with guitar amps.

[MJ Harnish]

Dacious,
No offense taken but of course here is my opinion. :D

vacuum passes no heat

If this were true the Earth would be as cold and dead as Pluto.

Vacuum passes no sound b/c sound is a mechanical force; heat as part of the EM spectrum travels just fine. Try touching the sides of the glass envelope of any operating tube and you'll have your proof. That's all being transferred by simple radiation. The base of a tube stays relatively cool compared to the envelope. In fact, the hottest part of the tubes is the seams on the plates, not the cathode.

Just because tubes generate heat doesn't mean they like to be hot. The cathode has to be at the correct operating temperature, but other than that and the power consumed by the filaments, every other heat source in a tube is detrimental to the tube's life expectancy.

Every other component in an amp, from the transformers to the resistors to the caps are rated to operate within a specific temperature. Operating over the temperature spec significantly shortens their life expectancy too and can lead to sudden failures in certain cases (e.g., electrolytic caps). One critical component which also benefits from cooling are cathode-bias resistors (if your amp happens to be cathode biased).

Unfortunately chassis design does not help the problem. Fenders and many others use upside down hanging tubes. We all know heat rises which means all of the heat is going right up into the chassis. If you don't believe me, crank up your Deluxe Reverb and play it for a few hours running hard... try touching the face plate of the amp. It's guaranteed to be warm and likely to be very hot. That heat is from the tubes and nowhere else. Touch the power transformer, carefully b/c it's also likely to be very hot to the touch: Unless you happen to be running a pair of 6L6 in there or the PT isn't functioning properly, all that heat is from the tubes. Worse still for heat dissipation are rack-mounted power amps... you own't find any reliable ones that don't use a fan (or two).

The idea about unequal cooling is also not of great importance. Why? Because if you measure the envelope of the tube very carefully you'd find that it isn't at a uniform temperature; it varies with the glass closest to the plate seams at the highest temperature. All tube manufacturers (at least the ones back in the 50's, 60's, etc.) specified how to orient the tubes so that this radiated heat was transferred outside the amp, rather than into other components (i.e., you want the plate seams pointed towards the back of the amp; unfortunately this also points the opposite side right at your OT in most cases). Aside from this, the critical item is the power tubes. Everything else stays cool if the power tubes are cool. The fan would not have any impact on the operating temperature of the cathode; the heater guarantees this stays hot and besides the fan is only helping remove the heat radiating from the envelope.

Ideally, the best place for a fan is probably on the front (grill cloth side) of the amp chassis sucking air in b/c then the airflow is more likely to get distributed a bit more evenly across the tubes. In most cases this isn't practical, particularly in a combo. So hanging the fan off the back is a compromise. Here you have two choices: 1) Have the fan blow cool air onto the power tubes or 2) have the fan help remove the heat from the chassis.
#2 in general doesn't work very well b/c the fan isn't set up to pull air out of the chassis where it's positioned. With the fan hanging several inches behind the tubes (the way it hangs there's quite a bit of space between it and the tubes) you get a much less focused airstream creating more generalized air circulation within the cabinet.

In my experience, hanging a fan on the back leads to significant cooling of the ENTIRE chassis. My Carmen Ghia chassis (which is aluminum and a great conductor of heat) stays cool to the touch. I've done this with numerous other amps, including a Bandmaster Reverb, & Deluxe Reverb all will similar results. Even my Champ's chassis gets hot to the touch after running a few hours.

As for the life of those big tubes: Keep in mind that what you're describing were fixed applications: The components were never moved, especially when they were still hot... that alone will contribute to a significant increase in tube life since you eliminate a lot of accidental mechanical damage related to heat.

[Dacious]

Try touching the sides of the glass envelope of any operating tube and you'll have your proof. That's all being transferred by simple radiation.

I have no beef with heat-caused radiation - I should have said heat transfer is impeded by vacuum, but I would suggest most of the heat in the glass is coming from where the metal components (the filaments and nodes ) are touching the sides of the glass envelopes inside the base (convection). I also presumed the proposed fan was being mounted inside the chassis - if it is hung in the speaker compartment under the chassis pointing at the tubes I would have less reason to object.

You're likely right in a sense that, cooling the glass will indirectly draw heat from the elements running through it. Given the small cross section/area of a thin circle of glass at that point, and I'd stand correction from an engineer, I'd say it would be minimally effective in cooling the incandescent bits over straigh radiation. I guess we'd need a subject tube with a temp probe inside it and measure with fan and without. I'd certainly say from the point of resistors and capacitors, they last longer run cooler. But that can be done within their design parameters.

My main point was, I guess, that unless all the tubes are getting an equal flow of cool, unwarmed air then it might be effective, or not. What's happening with the hot air - where is it going? Maybe the fan is just circulating warm air inside the amp? I don't know.

The transformers in the amps I own, even after some time operating, can still be touched with no discomfort. I wouldn't like to touch many of the resistors, though. I know some amps, the Vox AC30 in particular, runs pretty close to the wind tranny design-wise and is likely to be much warmer. As you said, nearly all the heat is from the tubes - but they need heat to work.

I also don't doubt that the chassis of the amps you mention run cooler with a fan. But cooling the chassis and the outside of the tubes may be merely a placebo. It may make diddly squat difference to what happens inside the tube. Whether that is a good thing in the context of the amp is another matter.

End of the day, the designer of the amp worked out if it needed cooling other than passive heat radiation. Most tube amps, radio, stereo, guitar, PA, non-audio apparently decided not. Personally I'm in the 'not broke, don't fix it camp'.

[MJ Harnish]

End of the day, the designer of the amp worked out if it needed cooling other than passive heat radiation. Most tube amps, radio, stereo, guitar, PA, non-audio apparently decided not. Personally I'm in the 'not broke, don't fix it camp'.

I'm actually in that crowd too. :D

However, for guitar stuff I would not rely on the designers to have made good decisions. Why? #1 - most of these designs were lifted straight out of tube books and built and then tweaked by non-engineers. Leo Fender and company never sat down and said "Let's build a reliable guitar amp from the ground up." A lot of the reliability in those old amps comes from the fact that they used over-rated parts and simple construction techniques. Old Vox amps (including the AC-30 though the AC-100 is the most notorious) have been known to burst into flames! The original designers never expected the heat build up would be anything like it actually was; reliability-wise those amps are a disaster (that's the main reason Mark Sampson, who was a well-known Vox tech, started Matchless amps... to build a reliable AC-30).

In case you're interested, a 50w push-pull fixed-bias amp with 3 12AX7's, a pair of 6L6's, and a GZ34 gives off a minimum of 27W in heat just from the heaters. If you take into account the amount lost from plates this jumps to a minimum of about100W! (you can find lots of details in K. O'Conner's TUT3 volume).

For reference, I think those old "easy bake ovens" used a smaller wattage light bulb to bake a cake. :?

Check out all of Kevin O'Conner's books: They're a bit pricey but contain lots of great info if you want to get into building (or just understanding) tube amplifiers.

[Paul G.]

I'm not concerned about cooling the tubes so much. The majority of amps hang the tubes below the chassis. Heat from the tubes rises and warms components, which are already warm from passing current. Some amps, such as Boogie Mk I through Mk IV run in very small cabinets which trap hot air.
This CAN cause excessive wear and premature failure.

Most amplifiers run in efficient A/B mode, and the tubes get plenty of rest, so they don't run too hot. Exceptions are amps with 4 or 6 power tubes and amps running closer to class A, and "compact" amps and heads..

The best thing to do is monitor your amp during and after a gig. Once the amp has been on for 1/2 hour or so, there should be no change in tone or volume. At the end of the night, the chassis and cab should be warm, but not hot.

If you notice symptoms of overheating, a fan running at half speed can be a big help. If not, I wouldn't bother.

P.

[Dacious]

Old Vox amps (including the AC-30 though the AC-100 is the most notorious) have been known to burst into flames!

This might be because Vox specced a 160ma transformer and biased the 4 tubes to draw nearly 50ma each in the AC30! In the case of the AC100, I've read that Vox believed that the US used dead on 110 volts, and allowed for no overvoltage. So if the bias is wrong, the design limits of the transformer can be exceeded, not to mention the wax and paper potting. So in their case (interesting thread on English electricals in Bad Dog Cafe) you are probably well and truly right!

[MJ Harnish]

I guess that would bring a whole new meaning the the phrase 'stage pyrotechnics.'

[Big Cookie]

Something in an overhead fire suppressor system might be more appropriate. Purple K is preferred in an electrical enviornment.

[Bill Ashton]

Got it real cheap as it ran red hot...enough to melt all the beeswax out of the power transformer!...and hence shorting problems...you could SEE the varnish on the copper wire windings boiling! A rewind from Mercury Magnetics was just to ticket to restore the best sounding amp I own. All this from only four tubes and about 3-4 watts output, far cry from an AC-30, but only two vents up top and three 1 inch" holes below...

[davidp158]

Interesting thread, and some great views for and against the idea of a fan with an amp. My EL84 amps run extremely hot, and I often think about using a fan to push some of that heat out of the amp. I would guess that vintage EL84 amps tend to have more heat related failures than other tube amps, but that's just a guess.

I tend to feel that a fan would do more good than harm, but Cookie's comment about blowing dust into an amp is very valid. Smokey club air could accumulate inside the amp, and the sticky residue would attract dust, which in turn would probably retain some of the heat you're try to expell. I guess a fan with a filter would be the best solution.

dave patterson

[MJ Harnish]

That's one solution. :) I'm not sure how much extra dust accumulation there is going to be though, especially inside the chassis since the air flow is not actually directed there. I mean, it's NOTHING like a fan in a computer which sucks air into a closed case where the dust then gets trapped. Hanging a fan off the back basically is increasing air flow throughout the cabinet, in particular the tubes, which is a fairly open space. While there may be some increase residue accumulation, I doubt it's going to be significant unless you're playing in a sandstorm. Having lived in Oklahoma for 10 years (which is not really as dry as most people think it is, though it is hot as hell ), I never experienced any more accumulation than what would be expected.

BC's comment does bring up a good point: You ought to vacuum out your amplifier(s) (and your computer!!!) at least once a year. You'd be amazed at what kinds of crude can accumulate over that time, particularly if you're playing out.

[feojack]

Very interesting topic, even though I understand about one third (at best) of the tech stuff. I'm more from the "heat bad, air good" school of thinking.I was told that heat and wet was the only real enemies of elecrtical components. ( now, as I learned, so is dust, barroom tobacco air, etc.)
What I've always done with my Fender heads (bassman & showman), is replace the front panel (the grill cloth, chip board part) with a nicely stained & finished wood panel that leaves about a 1" gap at the front/bottom of the head to allow air in to circulate. That, with the standard back panel, allows air to flow all over.
I do admit I was drawn to this discussion becuase I have also been toying with the idea of a small cooling fan mounted inside the amp head to move air about. I'm still undecided as to whether I'll do this or not.
Another thing I do is take the head off the cabinet & put it on the floor, sitting up, not unlike a computer tower. This allows the heat to not collect on the chassis. I play bass, and having the head off of my cabinet also keeps the bass vibes from rattling the inside of the amp. I always thought it would rattle the tubes too much. I don't know what someone would do with a combo. But that's my nickel's worth
Cheers, feo

[hawaiibuckeye]

I have a Mesa "Blue Angel" that uses fan cooling. It's a class A amp, It has 4 EL-84, 2 6V6, 5 12AX7 and
1 5AR4 (rectifier). With 12 tubes in a mid sized class A combo, ("mounted upside down"), there can be a lot of heat generated. I think I remember reading that Mesa put the fan in to keep the temps down to prolong tube life. It works well, makes very little noise (none through the amp).

[rickyralph]

tubes amps run hot.in 22 years of bar gigs 3 or 4 amps with tubes laid down.never happened on amps that i had a fan blowing air at the tubes. maybe its just the luck of the draw but ill always use a small fan? just call me a fan fan

[Phil Jacoby]

my Tone King 40a has a computer fan between the two output sections. It is designed to run on current production tubes at (I think) pretty high current. The TKC sounds fantastic, very lively and doesn't get as hot as my old Dlx Rvb used to.

Of course a tube rectifier can really add heat. I've been contemplating Weber Copper Caps for my beefy Princeton Rev for heat issues.