Ampeg B2R fan question

[audiomitch]

I just picked up a very used Ampeg B2R off Craigslist. It works great, SLM, Made in the USA, 1998 (according to the serial number). It's a little beat - missing a knob and 2 EQ sliders. Various scratches and dents.

Anyhow, the fan is amazingly loud. The guy I bought it from said he didn't think it was the original fan. I took the top off and it looks like the fan is probably a replacement as the power leads are spliced in. The thing definitely works, and it pushes so much air it creates turbulence noise around the vents.

Here's the info I got on the current fan:

Comair Rotron Sprint - ST24A3

Electrical Specifications
Operating Voltage: 24 Volts
Voltage Range: 12 - 28 Volts
Power Supply: 3.3 Watts
Line Current: 0.14 Amps
Locked Rotor Current 0.44 Amps
Nominal Speed: 3440 RPM
Mechanical Specifications
Dimensions: 3.1 x 3.1 x 1.3 in
Weight 6.35 oz
Max Airflow @ 0 in H2O 34.0 CFM
Max Pressure @ 0 CFM 0.225 in H2O
Noise 40.9 dBA

And here's what I'm thinking of replacing it with -

http://www.cwc-group.com/ad0824mba70gl.html

ADDA AD0824MB-A70GL 80mm 24V Fan

# AD0824MB-A70GL
# Ball Bearing
# DC 24V, 0.10A, 2.4W
# Speed: 2440 RPM
# Air Flow: 31.4 CFM
# Noise: 28.3 dBA
# 2-Lead Wire

My question is if anybody knows how much air needs to be pushed for the B2R to safely stay cool? The new fan would be quieter, but pushing less air (34 cfm versus 31.4 cfm).

Thanks

[Jerrold Tiers]

That may be the original fan, or at least fan type. That sounds like an Ampeg fan number as far as I recall, probably a 24-200-01. I'd have to dig up a bill of materials to see if it is.

BUT..... Your fan should be running slow until the amp heats up. It is supposed to be temperature controlled.

if that is NOT happening, then something is wrong with the fan driver, which is pretty simple, but is usually quite effective.

[audiomitch]

Thanks, Jerrold. I read all of your posts with great interest.

The fan is blasting away from the moment the amp is switched on. Not sure if this head has gone through any mods. The inside looked fairly clean and orderly, but I don't have a schematic or anything. How would I test the fan driver?

[Jerrold Tiers]

if you are reasonably "techish", you can measure fan voltage. If you need to know details of how to do that, you probably shouldn't do it.

When amp is cold, it should be well below 24V...... and fan should be slow. It should not be near full speed until the heatsink gets hot.

There are test point voltages on the schematic (which I have in PDF if you want it) but unfortunately none for the fan..... IIRC it is something like 10 to 12 V cold, but I don't recall exactly...... it was that for the same fan in another product.

[audiomitch]

"Techish" describes me pretty well I wonder if the fan driver failed and somebody found another source of 24 volts as a quick and cheap fix. Again, I have no idea.

I guess it's better to have it stuck on high than not coming on at all.

I'll PM you for the schematic. Thanks!

[Bongolation]

Thanks for the explanation of how than fan actually works. I was told is was off/on, not voltage-regulated, which didn't seem right as it always is on somewhat.

Yes, it is the noisiest fan in Christendom.

I also have never seen one of these with all its fader tips!

[audiomitch]

Originally Posted by Bongolation

I also have never seen one of these with all its fader tips!

I found a source for these short throw faders. I ordered two of them, they cost all of $11. Here's the link -

No Affiliation

[audiomitch]

Update:

I replaced the fan with the one in the link. It is definitely quieter, but more important the fan driver works now with this one. It slows down when cool. I'm guessing that the other fan only ran at one speed. I think that's a "feature" in some applications, but obviously not here.

I'm going to hook the head up to my THD Hot Plate and get it real warm to see what happens.

Edit: Ran the head through a THD Hot Plate with the gain at 3:00, Bass at 3:00, Master at 3:00, bottom two EQ fader about half way up. The limiter light was pretty solid the whole time. I recorded and looped a bassline in Pro Tools, then reamped it to the B2R for a solid hour. The fan never sped up, the head was warm but not hot, no thermal shut down or smoke. I'm thinking I solved my obnoxiously loud fan issue.

I also replaced the two broken EQ sliders with parts from the second link. Works and looks great now.