[AMPS] SB-220 failure

[Pete Smith]

This morning, while tuning up my SB-220 on 15 meters, I did the
flash-bang-bad smell number, followed by a sudden flare of white light from
one of the 3-500Zs in my SB-220.  As long-time readers may recall, this amp
has the AG6K parasitic suppressors.

After the event, the amp showed about 1/2 normal resting plate current and,
when driven, approximately half normal output.  

Opening the case, I found that the low-value resistor in series with the
anode of the other tube (the one that did not "light up") is shattered.

As I understand it, the resistors are there as a fuse against plate
glitches.  My post-glitch experience suggests that the other tube is OK -
normal resting current and output.  Should I just replace the series
resistor and try the amp again, or should I assume that the 3-500 on the
damaged side is kaput?  I ask because I'd rather not kill the remaining
tube, if there's any risk of doing so.

Also, any idea what caused the episode?  The SWR seen by the amp was quite
low (about 1.3:1).  Loading was (I think) set just on the high side of
maximum output, and tuning was at resonance.  I was in CW position, driving
the amplifier with full output from my transceiver (about 110 w.).  Output
power seemed abnormally low, which is why I was tweaking it when it blew.

Any advice can only help this amplifier neophyte.

73, Pete Smith N4ZR
n4zr@contesting.com 


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