I picked up a NOS few tubes today that do not have clear markings as to whether they are 6L6 or El34? One is RCA and the other is marked Great Britain, with B3A4 near the base. They look so similar in construction, I cannot tell what they are. The price was good enough for unboxed NOS tubes that I couldn't pass them up. Is there a way to positively I.D. them as one or the other?