[Amps] SB-220 troubleshooting

[Pete Smith N4ZR]

My venerable SB-220 is displaying some odd symptoms.  On 10M, for 
example, the Tune setting for maximum output has changed substantially, 
and the output power is down about 150 watts.  There are smaller 
changes, progressively less serious, as I step down through the bands.  
Is it likely that the current pair of 3-500s is failing, or should I be 
looking somewhere else in the amp?

-- 

73, Pete N4ZR
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