Saga of the SB200 amp (KG5HOK)

Keith , KG5HOK has been diligently working on an non-working SB200, which he plans to donate to the club via an SK estate from his friend’s father.  The SB200 is a well known workhorse using a pair of 872’s to output close to 700 W out.  Keith has rolled up his sleeves and replaced the aged High Voltage power supply with a new bank of modern electrolytica. 

During his troubleshooting adventure, he discovered that the previous owner had miswired the output relay circuitry.  The unit is in pristine condition,  and it’s possible that the amp never worked!  with slow and careful work, Keith made the modifications back to spec.  Ultimately, he got the amp singing wiht about 700-800W output on an external wattmeter. As we all know, Murphy only came by *after* he reinstalled the chassis back into the cabinet.  Upon another final checkout, he heard the dreaded “POP” and the acrid smell of burning components.  So its back to the drawing board.  It’s now speculated that the 572B arced and took out the grid resistor.  Reviewing online, there a huge amount of posts about poorly designed 572Bs that have been a well known industry wide problem.  Good luck to Keith and his efforts to nurse the SB200 back to operating status.

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