[Johnson] Tube Designation

[David C. Hallam]

Here's a question for the experts.  In the Johnson Pacemaker, several
12AT7's are used.  On the schematic the phase inverter is designated as a
12AT7 but the balanced modulators are designated as ECC-81.  I always
thought the ECC-81 was just the European designation for a 12AT7.  Why did
Johnson use a different designation for these?  Is there a difference?

David
KC2JD/4