Running a fan to cool amplifier any ill effects?

Try to prevent the fans from touching the chassis .
The fans could induce a low level vibration and a subsonic that the Amp will carry. Perhaps making your amp work harder.
If you can, put the fans on a stand blowing on the Amp.

I have two mono blocks that are fan cooled. Because the design was for rack mounting, and the close proximity of the 8 el34's per amp, this was a must. The fans blow directly across the tubes. The original fans were ac driving, and I replaced them with very quiet dc driven types with their own supply system. Each fan is on a rubber mounting system to help isolate their vibration(which is minimal)from the chassis. I believe that this prolongs the life not only of the tubes, but other components inside the chassis. These amps are now 45 years old! I believe if you check around, you will find that most "experts" will tell you that excess heat will age a tube about as fast as anything(ok over biased etc). Some complain of the noise, but I have been able to screen this and slow the fans such that it doesn't bother me........

My amp has 12 6550's in it and while the tube cage doesn't get hot to the touch, I cool it down with an external fan. In between playings of an hour or so, I will just cool the amp down for a minute or so and than continue to listen to more music. Once I shut down the amp, I also cool it again until the there is no heat at all emanating from the amp. I have an open rack which makes this procedure easy. I have experienced no ill effects from this and if it does extend the life of the tubes, that's a bonus.

I hooked up the two whisper fans above the housing on the VAC 140's....about 5 inches above....the fans move a good amount of air and are 34db quiet according to specs. The 140's have a "tube sentry" which monitors the tubes. When they are initially powered up the 4 lights on the front turn green...which is good....as the tubes reach the correct bias the lights go off(normal). When the lights went from green to off...I then turned the fans on...in about 20 seconds a few of the lights went green again...on one amp a red light came on. I replaced that pair and powered the amps on again with no problems...but I did not use the fans.
I going to call Kevin and see why this happened. I could use a variac on the fans and reduce the speed to lower the cfm. I would like to cool these...but maybe the sentry looks at the tube temps or bias is affected by the tempature on the 300B....things are not as easy as they seem.

I contacted Kevin Hayes this morning and his suggestion was to run the fan air across the tubes. The air currently is cooling the resistors below the brass plate....thereby affecting the sentry circuit....makes sense. So I will configure the fans another way and direct the air away from the brass vent.