gear question stromberg carlson pa amp

[Joel Fritz <jfritz666@xxxxxxxxxxx>]

Just picked up a Stromberg Carlson Signet 33 amp on ebay.  Tube 
complement is (2x) 12ax7,  6u8, 5u4gb, and (2x) 6l6gc. Is anyone 
familiar with this amp?

It's pretty loud.  I've been fooling around with different tubes in the 
12az7 sockets to minimize feedback.  What does the 6u8 do?  From what 
I've read it was mostly used in tv sets.  Which one is the phase inverter?

I plan to get a cap job, put on a three prong power cord, and replace 
the tubes, but it sounds pretty good as is.  Have  I missed something 
obvious (or subtle) in turning it into a harp amp?

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