[Amps] SB-220 WARC problem

[Tim]

Hi all:

I have an SB-220 I have been using successfully on 17M for a few months now.  On the 15M bandswitch position I got a better match than on 20M and it's been working well.  I limited the output to 500W or less to give it a break here.  Yesterday it didn't want to load anymore- it "sputters" pretty loudly and the meters jump around when the output reaches a couple of hundred watts.  I wasn't able to see into the enclosure to look for arcs, and I didn't hear any arcing type noises but figured that must be the case and gave up.   I'm using a doublet fed with 450 ohm ladder line and tuner and the SWR was very good.  It still loads normally on 40M and 20M and gives full output.  I'll try using a dummy load today and verify that it is load independent.  I'll also inspect the coupling cap for arcing.  Anything that I should suspect?

Any ideas why it would quit without any changes to the system? 

Thanks- ' 73

Tim