Since I have solder the subminiture tube on the SE mu-stage preamp, it do not allow easy swapping of tubes and thus lack lots of fun. Thus I have found some 9-pin extention sockets and find a way to put a submini tube onto it. I solder the leads of submini tube onto a pin head with 1mm diameter size. The pin head can be inserted into the 9-pin socket. Thus now I can swap tubes easily again!
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In picture, the tube is 6021 to replace the 6DJ8 tube on my SE mu-stage preamp. I put the two triodes in parallel and thus there will be only 5 connections from submini tube onto socket.
