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Discussion Starter #1
I have an AC30 I pulled out of storage after a couple years of non use. It goes into a loud feedback at idle with no instrument plugged in an volume at zero. Goes away when I switch from pentode to triode mode. Any ideas?I guess my first thing to try will be changing the power tubes. Seems strange that it happens with volume off though.
 

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What he said ^^^. Sounds like at least one of the power tubes has gone microphonic, not uncommon for EL84's. You could try pressing on/holding one at a time with a rag or removing one at a time to try and isolate in case it's just one or two. Then at least you can keep the others for use or as spares.
 

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Discussion Starter #4
I played it a bit more last night in triode mode and after warming it up a while switched to pentode and it seemed better but still had a bit of a "plinky" sound at times. Ordered a quad of JJ's from the tubestore. Hopefully that's all it needs.
 
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