[Amps] Here's a new one for me that I'd like to share

[w2cqm at juno.com]

After some preliminary tests,  I decided that a dual 8877 hombrew  amplifier project  would not be cooled  adequately with a previously installed  low RPM tandem blower. Checked the "junk box" for another candidate with higher speed and more volume and thanks to a long forgotten   hamfest  purchase, from yesteryear(s),  I found a suitable replacement.  The squirrel cage blower was larger and  the manufacturer's  label indicated TWO speeds. Imprinted on the motor were  specifications including  a 115VAC  operating voltage and a schematic showing  the three wire  hookup sequence.  Two color coded wires indicated line voltage input and the third wire (black)   depicted the installation of an  AC  starting capacitor  wired from that lead  across  to one of the color coded  AC leads.  NO VALUE WAS GIVEN  TO THE CAPACITOR.  I initially utilized a 1uFd cap for testing. I was disappointed  since there were no other wires that could be used  for speed selection.  To test for suitability,  I nevertheless increased the opening  for the larger blower mounting flange and  installed it on the rear of the amp enclosure. It started to work; but nowhere near that  potentially higher value of rotation stenciled on the motor body.  I was a bit stymied at that point!  Not certain what possessed me, but I decided to reach in and placed  my hand over the blower output  to restrict flow.  Mysteriously, the fan substantially increased its speed.  Removed  the back pressure  restriction caused by  my hand and it slowed down.  Not certain if I can explain the phenomenon , but I did experiment  with much higher  AC capacitors values  and found the fan speed increasing   but  that  resulted  in the buildup of  abnormal heat around the motor.  That was unacceptable.  I settled on  the one and  only  low value  1 uFd  capacitor I had since the amplifier cooling system  was now  operating  well with adequate cooling volume.  After replacing the tubes in the sockets and sealing off the pressurized lower cabinet, the fan increased under the two tube cooling requirement  restrictions and the blower motor  heat build up is no longer an issue. Obviously,  the automatic speed control  has something to do with the value of the starting capacitor, the normal back pressure of cooling, and to some extent the resultant additional current draw.   It's safe to assume that it's not a ghost nor is there an air flow control. within the fan body!  Hopefully, someone can  explain what's happening and share it with me and the other  AMPS subscribers?  Thanks, Ron W2CQM/3  Note! you can view some early digitals of  this  project with annotations on my QRZ site. Click on SHUTTERFLY  at the bottom of the page.