For 9 pin tubes, I think Amperex 6DJ8 is very good performer in mu-stage pre-amp. Thus many other triodes are evaluated in Tube post. Now here is some photos of the mu-stage with subminiature tubes. The way is to solder the small tube on the bottom of the tube socket and the two triodes in one tube is put in parallel. Insulation shrink tubes is used to prevent short circuit of leads.
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I have purchased one pair Sylvania 6021 and 4 pcs Raytheon 6111 tubes (non marking type). The pair of 6021 give very good matching for the gain but only 2 out of 6111 tube are with good gain match. The worst 6111 tube is about 1 db lower gain than the other 3 pcs.
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Note that the mu-stage is also upgraded with KBG and Telfon capacitors!
