[Boatanchors] National VHF Receive Converters

[Brian Harris]

I have some National VHF receive converters which have me puzzled. I have two 2 meter versions (NC-300C2) and have no questions about them. However, I have three 6 meter versions (NC-300C6) and two of them have two tubes (6CB6/6U8) whereas a third one has four tubes (6BZ7/6AK5/6AK5/6U8) just like the 2 meter versions.  A friend also has one of the four tube 6 meter versions.  Information is available for the two tube versions but not the four tube version. My questions are: 
1. Which version came first?
2. Which version has the best performance?
3.  Any clue why there was a change?

Thanks for your time. 

Brian WA5UEK 
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