AC/DC Adapter for Camera Flash Mod
The problem is that your "AC adapter" can't supply enough current.  It is only rated for 300mA, which way to wussy.  I don't know about the SB-26 stobe in particular, but other strobes I have looked at will happily draw 1 A or more after a strong flash.

Get a proper power supply rated for at least 1A.  Don't play games and experiment with the voltage if you value your strobe.  You can safely apply about 1.5 V times the number of AA cells the strobe is intended to take.  If it's two cells, then 3 V.  If 4 cells, then don't exceed 6V.  1 A may not be enough for short periods right after a flash, but is probably enough to get thru the tough charging phase a little slower to where it will draw less current after a few seconds.  A little voltage sag is OK since the strobe has to be designed to at least not get hurt by low batteries.  I would probably get a 2 A 6 V power supply (assuming 4 AA batteries) to get good quick recharge time.

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