Phoenix Engineering Falcon and Eagle

BP....the way you describe the symptoms of varying speed on one of your VPI's sound like belt stretch to me.  Try boiling the belt/installing a new one/switching to rim drive.  Just trying to be helpful.

on the point of having manufacturers (or even stores) keep in touch with their valued customers...nope - has NEVER happened in the 50 years or so I've nuddled around with this stuff...  However, if ever I had an issue/question/comment,... any need I had was quickly addressed with VPI (Harry), Vandersteen (Richard himself, and John Rutan of Audio Connection) or Ayre (Michael Weidemeir).  No ...not even one complaint the way I've been treated.

 Reducing the voltage to the motor after start-up decreases the magnitude of the pulses. On my table, that results in a lower noise floor, blacker background, improved soundstage, tighter imaging and cleaner presentation. Not subtle differences.

That has been my experience too. One issue a variac will not address is distortion on the AC mains. Many things connected to the grid "suck off" power at the peak of the AC cycle distorting the waveform by flattening the tops. This does impact the functioning of an AC synchronous motor.

Another design aspect with AC synchronous motors with two field coils (most) is the need for the voltage to the second field coil to be shifted by 90 degrees. This is usually accomplished with a series capacitor. The other option is to use a motor controller with two outputs (requires rewiring the motor and removing the cap.), providing a separate phase shifted output for each field coil. Controllers of this type allow the user to "tweak" the phase angle to the motor to minimize vibration. My experience has been that controlling the voltage is the more important of the two. Although, tweaking the phase does provide small improvements.

Stringreen - I don't have speed variation issues.  What I notice is the pulses from the motor being transmitted through the belt to the platter.  Reducing the voltage reduces the magnitude of the pulses which reduces the noise.  The speed holds quite steady even below 70v.  I use 70v as my motor operating voltage once motor startup is complete. 

I was frustrated by a Minneapolis high-end manufacturer that has been around over 40 years.  But I have always had great support from VPI and exception support from Herron Audio.   Never a complaint about them.  There are some great people in high-end audio.  There is also a large share of turds that foul the punch bowl.