Tuesday, November 10, 2015

Stancor Ultralinear Amplifier

The Stancor original Williamson and Williamson Ultralinear amplifiers were kits marketed by Stancor to showcase the performance of their transformers.  They used a separate power supply chassis connected by a 4 conductor cable. A pair of these makes a great sounding stereo power amplifier.  The Ultralinear is much preferred because the power output is much greater. 25 watts vs. 8 watts. 

I rebuilt a set of these, and can attest to their fine performance. 
The number one problem with any old electronic equipment is the electrolytic capacitors. I removed the metal can capacitors and replaced them with modern ones.  The modern ones are much smaller than the originals, and are designed for mounting on a printed circuit board.

I cut pieces of circuit board material into the shape of the original capacitor bases, and drilled holes for the new capacitors.
At Right:  The new next to the old.

Below: An amplifier chassis with the new capacitors.
One of the originals was a dual capacitor, so my replacement has 2 capacitors, too. 
 The power supply chassis has 3.   The 3 capacitors combined with one of Stancor's chokes do a great job of filtering hum from the 440 volt supply.
 I made new cables to connect the power supplies to the amplifiers.  The plugs and sockets are the same as early 4 pin tubes, like the type 80 rectifier.  I needed a replacement plug, so I took the base off an old Philco 80 tube, and made an aluminum cap for it.
The chassis had no bottoms, but they have threaded holes to attach bottom covers.
I made covers from sheet steel and attached rubber feet to them
I modernized the chassis by adding power sockets and replaced the original 2 wire lamp cord with 3 wire grounded cords.
 The modern cords and the steel bottom covers make these amplifiers much safer.
The amplifier schematic.
  Ultra-linear circuits are easy to identify by the number of transformer leads going to the output tubes.  Ultra-linear circuits have 2 wires to each tube, while other circuits have just one.  If you find a Stancor chassis with the labels missing, this is how you can tell which version you have.

The amplifier has sockets to measure and balance the plate currents on the 807 output tubes.  This is important for two reasons. One, balanced current is important to achieve the lowest distortion. Another good reason is to verify that the tubes are not using too much current.  This happened to me. C4 was bad.  If C4 or C5 are leaky, the grid bias voltage will go positive, and cause the tubes to draw excess current.  Drawing only a little too much will cause the plates to glow red to an excessive degree..
In this picture, the tube in the foreground is drawing too much plate current.  The one in the background is ok. Its plate is slightly red, that is acceptable.  The blue glow on the glass is acceptable, too.  A gassy tube has a glow inside the tube.  These are factory new Raytheon tubes.

The power supply is simple.  The supplies are only large enough for a single amplifier, and will overheat if two amps are connected to one power supply.