Motorboating after tube change


I posted this on Audioasylum tubes, but I'd like to increase the knowledge base, so here I am.

I own an Aikido preamp that was built for me in 2007. Since I don't use it that much, I haven't replace the tubes till now. After I changed them out, there was clearly audible motorboating emanating from both speakers. I returned each old tube to the unit but the motorboating didn't stop. The only screwup was that when initially replacing the tubes, I accidentally installed a 5751 into the spot for a 5755.

Any ideas?

Thx,

GP
128x128garibaldi
From what I saw from pin locations there could be some protection involved and maybe some resistor is blown that provides voltage drop for the offset.