Roc:
Were the tubes supplied by AR?
If your USA BB's are Richardson Electronics tubes (Richardson purchased the Amperex namesake quite some time ago and therefore can rebrand any tube "Amperex") they could be most anything.
I have yet to see a vintage Amperex "USA" 6dj8/6922 with the BB logo (all have been of Euro manufacture).
Maybe ask AR (if applicable) who manufactured the tubes in question as they should know what it is they are buying/supplying.
Obtaining a positive ID on your current tubes should allow for knowledgable comparisons as Richardson does not always rebrand the same tubes (same manufacture) from what I have seen.
If "USA" does designate that the tubes were manufactured in the US, then they are most likely Philips ECG's or Sylvanias.
Neither have an overall great reputation for sound quality, but having tried some specialy tested/selected Sylvania Green labels I do know that they can sound quite good in this case (meaning pretty damned good when compared to some of the Holy Grail 6922 types).
Were the tubes supplied by AR?
If your USA BB's are Richardson Electronics tubes (Richardson purchased the Amperex namesake quite some time ago and therefore can rebrand any tube "Amperex") they could be most anything.
I have yet to see a vintage Amperex "USA" 6dj8/6922 with the BB logo (all have been of Euro manufacture).
Maybe ask AR (if applicable) who manufactured the tubes in question as they should know what it is they are buying/supplying.
Obtaining a positive ID on your current tubes should allow for knowledgable comparisons as Richardson does not always rebrand the same tubes (same manufacture) from what I have seen.
If "USA" does designate that the tubes were manufactured in the US, then they are most likely Philips ECG's or Sylvanias.
Neither have an overall great reputation for sound quality, but having tried some specialy tested/selected Sylvania Green labels I do know that they can sound quite good in this case (meaning pretty damned good when compared to some of the Holy Grail 6922 types).