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I have basically "given up" on buying any more 6B4- type tubes. If you have a reliable source, then good, don't tell anyone. So I started using Soviet 6C4C ST shape tubes in their place. Not much more affordable, but at least the 6C4C tubes are new (NOS). So far, every one I paid for is a winner. Regardless, does anyone have any knowledge of just, "How Close" a 6C4C tube is to a 6B4-? They are certainly drop in replacements in most cases. I have not found any information on anything that should be "Tweaked" when replacing a 6B4- with a Soviet 6C4C. I know there was a fair amount of copying, tube designs, so maybe they are virtually identical. The 6C4C tubes look very well made, and in many cases, are not really THAT old by NOS vacuum tube standards. Yes, but I am getting old, so I may not hear all of the nuances. They sound good to me. In comparison to a 6AV5GA, I can't say which is my favorite. I really like the DHT sound, but the 6AV5GA is a bit quieter (at least in my G-S-G).

6C4C and 6B4G are absolutely equivalents. I would recommend buying 6C4C produced before 1970. These lamps are very high quality.

Thank you for the reply. Sometimes a small change needs to be made to get the last little bit of performance, when you do a substitution. I have not seen any recommendations for a 6B4G to 6C4C swap.