Identifying Output transformer secondary wires

multichamp12

AK Subscriber
Subscriber
Hey everyone. So I recently picked up a pair of Stromberg AP-50 amplifiers, which have a bunch of different OT secondary taps:

0ohm - Black
8ohm - Yellow
25v - (Green or Orange?)
16ohm - (Green or Orange?)
32ohm - Black/Yellow
70v - Black/Red
100v - Black/White

When I got the amp, the 16ohm solder terminal had nothing going to it, so it was disconnected at some point (also where the NFB is suppose to be) and the 25v tap had the NFB going to it. So I am trying to determine if someone moved the 16ohm tap and the NFB wire to the 25v point, or it was wired incorrectly and the 16ohm tap is indeed the one disconnected.

The 2 color wires that I am unsure of are Green and Orange, so one is the 16ohm tap, and one is the 25v tap, but they are so close when measuring the resistance with a multimeter, that I dont know which is which. According to the sams manual, the 25v should be .4 ohms, and the 16ohm should be .43ohms.

Is there another way I can determine which tap is which?
 
Feed AC voltage across the primary and measure on the secondary. The higher the voltage output, the higher the output impedance. I'd probably run enough across the primary to get about 1v on the 8 ohm tap and go from there. Isolation transformer and a variac make a fine source.
 
Im at my bench now and used a function generator across the primary. When measuring the secondary, the AC voltage on the orange lead is slightly higher than the green. So the Orange is the 16ohm, and the green is the 25v. So whoever wired the NFB did it incorrectly. Thanks!
 
The 25v tap should be right between the 8 and 16, so it probably wasn't off by that much. With a 50w amp it should be 12.5 ohms. If it should be on the 16 and was on the 25v it got a little less feedback than it should have had originally.
 
Yea so not that much of a difference, but glad to figure it out and put it back to the original state. Thanks gadget.
 
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