[Amps] Alpha 87A Power Supply Question

[Paul Christensen]

>"If I correctly understand what you are saying, this is exactly what the Alpha 76 and my old B&W PT-2500A have done. They have dual 120 volts primaries that they put in series for 240 operation. 120 volts for the blower is derived from one side of a 120 primary and the "center tap" between the two 120 volt primaries."

Yes, that's correct.  

The Alpha 70/77 series doesn’t allow for that connectivity while retaining the air flow boost benefit because the blower is in series with the transformer's HV primary winding.  By design, as the load increases with output power, so does blower speed.  There's two options to consider for safe wiring of these amps:

1) Forego the blower boost and connect the blower between L1 and the LV CT winding; or

2) Run a 4-wire branch circuit to the shack and upgrade the amp power cord to 4-wire.  Then, the chassis/neutral connection as wired by the factory can be separated in the Cinch-Jones power plug.  No mods needed inside the amp.  Ground lead to chassis, and neutral to the isolated blower return.  After the mod, neutral and the grounding conductor never come in contact.  I chose this option for all my 70/77 series amps. 

The above said, these amps have been working fine for decades for hundreds, or perhaps thousands of users when using the ground lead as a neutral.  That was often the way products were designed up until the early '70s.  Even today, I'll bet that just about every Alpha 70/77 owner has his or her amp wired this way.  Although terribly wrong, provided there's a tight, low resistance grounding conductor path to the service panel, all is well.  Of course, if the grounding conductor becomes loose or lifted, the chassis becomes even "hotter" with current and  the blower will absolutely find a return path back to the service panel through the chassis-mounted connectors and cabling.  

Paul, W9AC