Suck or Blow?

Discussion in 'Amps and Cabs [BG]' started by Paul A, Jun 27, 2003.

  1. Paul A

    Paul A

    Dec 13, 1999
    Hertfordshire U.K!
    He He...... sorry 'bout the thread title but all will become apparent....

    I've been rebuilding a Selmer all Tube head.
    It's been comletely refurbed new (Modern!) audio grade caps, ofc wiring etc etc.
    It's running 4 JJ E34l's - these are basically KT77's in a strengthened EL34 envelope.
    It's putting out around 120 - 140 watts.
    The biasing is set pretty "hot" on these tubes and they run (temperature wise) a fair bit warmer than the originals.
    Soooo ..... to help with longer tube life I have a couple of really nice fans (from a Sun Computer power supply).
    Question is:

    Do you A) "Suck" the cooling air over the tubes and exhaust it out of the cab?


    B) "Blow" the cooling air onto the tubes from the outside as it were?

    Simple question but one that has me really confused - I think the answer is "suck" but I'm not sure!

    :confused: :confused: :confused:


    Dec 28, 2002
    i think guitarist has an old peavey mx ill check it..but i never have felt air blowing on me when in front of it
  3. You want to blow the air directly on the tubes, and just as importantly, the power transformer. You want to get as much air movement as possible, and if you put your hand in front of a fan, you feel a whole lot more air than you do when you put your hand behind a fan. Also, the air comes out of the front of the fan in a conical shape, not just a beaming straight line, so keep that in mind when picking an installation spot. I've even got an amp where the fan blows down onto the tubes and power transformer, which goes against the concept of heat rising, but works great because of the turbulence hitting all tubes and the power tranny. My same Mullard EL34's have been running full power for about 5 years now. Tubes last forever when kept cool and isolated from vibration.

    In the industrial world, you always want the cabinet to be "pressurized" from a fan blowing into a cabinet, instead of "rarified" from a fan sucking air out of a cabinet. This way all dust is kept out of the cabinet (assuming a filter is used in front of the fan). If the fan was sucking air out, then it would pull dust in through every crack and hole.

  4. Paul A

    Paul A

    Dec 13, 1999
    Hertfordshire U.K!
    I guess "Blow" it is then! :D

    Thanks for the tip re. keeping the tranny cool!

  5. BruceWane


    Oct 31, 2002
    Houston, TX
    Yeah, you want the fan to suck air from outside the amp into the amp. It's not that this is necessarily more effective in cooling than using the fan to suck hot air out of the amp, because either way you are circulating cooler air into the amp. It's just that the inside of the amp will stay a lot cleaner with the fan blowing in rather that sucking out.

    They call this "positive air pressure cooling" in the computer biz. The air pressure in the case is higher than outside the case. Keeps it A LOT cleaner. Older computers used the "exhaust fan" method and would get totally FILTHY. It really does make quite a difference.
  6. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania

    You're right with the cleanliness, but a negative pressure provides a more even flow, considering his amp will not be sealed around itself, I assume. Given the nature of this, it's highly probable that a fan 'sucking' out the back will pull air through the front and provide an even flow.

    As with computers, this has proven true by over a 3c temperature difference.

    As for the cleanliness, it's a difference but generally not all that significant, since instead of having a bit of dust drug through, you're actually pulling from an off-axis stance, the air through.
  7. Petebass


    Dec 22, 2002
    QLD Australia
    Swallow! :)

    I posted a similar question when I incorporated some fans into a rack mount case I was building.

    All the tech guys advised the blowing would work better. As Bob Lee put it, if you've been jogging on a hot day and wanted to cool off, would stand in front of a fan of behind it? The air movement in front of the fan is focused into a narrow beam, but the airflow behind the fan is being draw from all directions and is therefore less efficient.

    Just make sure you allow for the air to escape somehow.
  8. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania
    Pushing hot air around doesn't work too well, generally. The idea of air cooling is to get more ambient air in the area, not to push heated air around more.

    The problem in Bob's example is this: If you're jogging and want to cool off, there's a huge difference in the area of a for the most part-closed case and an open ended room with a little house-fan.
  9. Piezoman


    Nov 29, 2002
    Bronx, New York
    Since I have no cloo wud U guyz are talking about, I jus gotta say this sounds like a dimented forum withall this sucking and blowing. I like it.:D
  10. My Eden VT300A Sucks the air through the front vents and blows it out the back. I own several SS power amps and some suck and others blow. My QSC USA850 blows but my QSC USA1310 sucks. Go figure. My guess is that it really doesn't matter as long as the internal hot air is being exchanged with cooler outside air.
  11. Trevorus


    Oct 18, 2002
    Urbana, IL
    do both. Put a fan blowing in on one side, and another blowing out on the other. BOB W.