tube upgrade for first sound


Has anyone been able to improve on the stock tubes in the First Sound preamps? I was thinking about trying the Russian 6H23. Any thoughts? thanks
84audio
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tvad - mine are same as yours. PQ white lettering, nt pinched waist. I'd like to also get some pinch waists, but have yet to see any for sale.....
To calrify again - my PQ 6922's are the "D getter" version - early 1960's production.
ok - so two more simple tweak considerations for First Sound owners. Maybe this has been covered already, but it bears repeating, i think...

1) Aurios isolation/decoupling bearings - clean up and solidify the sound very nicely. Emmanuel recommended them, and as usual, he is right on the money....

2) straight into the wall. I found my Hydra robbed a bit of the inner sparkle and life.
Hello Guys

Please excuse my ignorance, but where can the above mentioned tubes be purchased? What would I ask for? I'm just getting into tubes so this is a whole new world for me. :-)

Thanks

Paul